View source for Biology:Evolutionary history of life

{{short description|The processes by which organisms evolved on Earth}}
{{Evolutionary biology}}
The '''evolutionary history of life''' on [[Astronomy:Earth|Earth]] traces the processes by which living and [[Biology:Fossil|fossil]] organisms evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years ([[Year|Ga]]) ago and evidence suggests life emerged prior to 3.7 Ga.<ref name="Pearce 343–364">{{Cite journal|last1=Pearce|first1=Ben K.D.|last2=Tupper|first2=Andrew S.|last3=Pudritz|first3=Ralph E.|last4=Higgs|first4=Paul G.|date=2018-03-01|title=Constraining the Time Interval for the Origin of Life on Earth|journal=Astrobiology|language=en|volume=18|issue=3|pages=343–364|doi=10.1089/ast.2017.1674|pmid=29570409|bibcode=2018AsBio..18..343P|arxiv=1808.09460}}</ref><ref name="Rosing 674–676">{{Cite journal|last=Rosing|first=Minik T.|date=1999-01-29|title=13C-Depleted Carbon Microparticles in &gt;3700-Ma Sea-Floor Sedimentary Rocks from West Greenland|journal=Science|language=en|volume=283|issue=5402|pages=674–676|doi=10.1126/science.283.5402.674|issn=0036-8075|pmid=9924024|bibcode=1999Sci...283..674R}}</ref><ref name="Ohtomo 25–28">{{Cite journal|last1=Ohtomo|first1=Yoko|last2=Kakegawa|first2=Takeshi|last3=Ishida|first3=Akizumi|last4=Nagase|first4=Toshiro|last5=Rosing|first5=Minik T.|date=January 2014|title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks|journal=Nature Geoscience|language=En|volume=7|issue=1|pages=25–28|doi=10.1038/ngeo2025|issn=1752-0908|bibcode=2014NatGe...7...25O}}</ref> (Although there is some evidence of life as early as 4.1 to 4.28 Ga, it remains controversial due to the possible non-biological formation of the purported fossils.<ref name="Pearce 343–364"/><ref>{{Cite journal|last1=Papineau|first1=D.|last2=Gregorio|first2=B. T. De|last3=Cody|first3=G. D.|last4=O’Neil|first4=J.|last5=Steele|first5=A.|last6=Stroud|first6=R. M.|last7=Fogel|first7=M. L.|date=June 2011|title=Young poorly crystalline graphite in the >3.8-Gyr-old Nuvvuagittuq banded iron formation|journal=Nature Geoscience|language=En|volume=4|issue=6|pages=376–379|doi=10.1038/ngeo1155|issn=1752-0908|bibcode=2011NatGe...4..376P}}</ref><ref>{{cite web|date=October 19, 2015|title=Life on Earth likely started 4.1 billion years ago—much earlier than scientists thought|url=http://phys.org/news/2015-10-life-earth-billion-years-agomuch.html}}</ref><ref>{{cite journal|last1=Bell|first1=Elizabeth A.|last2=Boehnke|first2=Patrick|last3=Harrison|first3=T. Mark|last4=Mao|first4=Wendy L.|title=Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon|journal=Proceedings of the National Academy of Sciences|date=24 November 2015|volume=112|issue=47|pages=14518–14521|doi=10.1073/pnas.1517557112|pmid=26483481|bibcode=2015PNAS..11214518B|pmc=4664351}}</ref><ref>{{Cite journal|last1=Nemchin|first1=Alexander A.|last2=Whitehouse|first2=Martin J.|last3=Menneken|first3=Martina|last4=Geisler|first4=Thorsten|last5=Pidgeon|first5=Robert T.|last6=Wilde|first6=Simon A.|date=July 2008|title=A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills|journal=Nature|language=En|volume=454|issue=7200|pages=92–95|doi=10.1038/nature07102|pmid=18596808|issn=1476-4687|bibcode=2008Natur.454...92N}}</ref>) The similarities among all known present-day [[Biology:Species|species]] indicate that they have diverged through the process of [[Biology:Evolution|evolution]] from a common ancestor.<ref>{{harvnb|Futuyma|2005}}</ref> Approximately 1 trillion species currently live on Earth<ref name="NSF-2016002">{{cite news |author=Staff |title=Researchers find that Earth may be home to 1 trillion species |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446 |date=2 May 2016 |work=[[Organization:National Science Foundation|National Science Foundation]] |accessdate=11 December 2016 }}</ref> of which only 1.75–1.8 million have been named<ref name="Chapman2009">{{cite book |last=Chapman |first=Arthur D. |year=2009 |title=Numbers of Living Species in Australia and the World. 2nd edition |location=Canberra |publisher=Australian Biological Resources Study |isbn=978-0-642-56860-1 |url=https://www.environment.gov.au/science/abrs/publications/other/numbers-living-species/ |accessdate=2016-11-06 }}</ref><ref name="NYT-20141108-MJN">{{cite news |last=Novacek |first=Michael J. |date=November 8, 2014 |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |newspaper={{wipe|The New York Times}} |location=New York |publisher=[[Company:The New York Times Company|The New York Times Company]] |accessdate=2014-12-25}}</ref> and 1.6 million documented in a central database.<ref name="col2016">{{cite web |url=http://www.catalogueoflife.org/annual-checklist/2016/info/ac |title=Catalogue of Life: 2016 Annual Checklist |year=2016 |accessdate=2016-11-06}}</ref> These currently living species represent less than one percent of all species that have ever lived on earth.<ref name="Book-Biology">{{harvnb|McKinney|1997|p=[https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110&lpg=PA110&dq#v=onepage&q&f=false 110]}}</ref><ref name="StearnsStearns2000">{{cite book |last1=Stearns |first1=Beverly Peterson |last2=Stearns |first2=S. C. |last3=Stearns |first3=Stephen C. |title=Watching, from the Edge of Extinction |url=https://books.google.com/books?id=0BHeC-tXIB4C&q=99%20percent#v=onepage |year=2000 |publisher=Yale University Press |isbn=978-0-300-08469-6|page=preface x |accessdate=30 May 2017 }}</ref>
{{Life timeline}}
The earliest evidence of life comes from [[Earth:Biogenic substance|biogenic]] [[Biology:Δ13C|carbon signatures]]<ref name="Rosing 674–676"/><ref name="Ohtomo 25–28"/> and [[Earth:Stromatolite|stromatolite]] fossils<ref>{{Cite journal|last1=Nutman|first1=Allen P.|last2=Bennett|first2=Vickie C.|last3=Friend|first3=Clark R. L.|last4=Kranendonk|first4=Martin J. Van|last5=Chivas|first5=Allan R.|date=September 2016|title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures|journal=Nature|language=En|volume=537|issue=7621|pages=535–538|doi=10.1038/nature19355|pmid=27580034|issn=1476-4687|bibcode=2016Natur.537..535N|url=http://ro.uow.edu.au/cgi/viewcontent.cgi?article=5181&context=smhpapers}}</ref> discovered in 3.7 billion-year-old metasedimentary rocks from western [[Place:Greenland|Greenland]]. In 2015, possible "remains of [[Chemistry:Biotic material|biotic life]]" were found in 4.1 billion-year-old rocks in Western Australia.<ref name="AP-20151019">{{cite news |last=Borenstein |first=Seth |title=Hints of life on what was thought to be desolate early Earth |url=http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |date=19 October 2015 |work=[[Company:Excite|Excite]] |location=Yonkers, NY |publisher=Mindspark Interactive Network |agency=Associated Press |url-status=dead |archiveurl=https://web.archive.org/web/20151023200248/http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |archivedate=23 October 2015 |accessdate=8 October 2018}}</ref><ref name="PNAS-20151014-pdf">{{cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnike |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |display-authors=3 |date=19 October 2015 |title=Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon |url=http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf |journal=Proc. Natl. Acad. Sci. U.S.A. |doi=10.1073/pnas.1517557112 |accessdate=2015-10-20 |pmid=26483481 |pmc=4664351 |volume=112 |issue=47 |pages=14518–21|bibcode=2015PNAS..11214518B }} Early edition, published online before print.</ref> In March 2017, putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized [[Biology:Microorganism|microorganism]]s discovered in [[Earth:Hydrothermal vent|hydrothermal vent]] precipitates in the Nuvvuagittuq Belt of Quebec, Canada, that may have lived as early as 4.28 billion years ago, not long after the [[Earth:Origin of water on Earth#Water in the development of Earth|oceans formed]] 4.4 billion years ago, and not long after the [[Earth:Age of the Earth|formation of the Earth]] 4.54 billion years ago.<ref name="NAT-20170301"/><ref name="NYT-20170301">{{cite news |last=Zimmer |first=Carl |title=Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest |url=https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |date=1 March 2017 |work={{wipe|The New York Times}} |accessdate=2 March 2017 }}</ref>

Microbial mats of coexisting [[Biology:Bacteria|bacteria]] and [[Biology:Archaea|archaea]] were the dominant form of life in the early [[Earth:Archean|Archean]] Epoch and many of the major steps in early evolution are thought to have taken place in this environment.<ref name="NisbetFowler1999ArchaeanMetabolicEvolution">{{cite journal |last1=Nisbet |first1=Euan G. |last2=Fowler |first2=C. M. R. |date=December 7, 1999 |title=Archaean metabolic evolution of microbial mats |journal=[[Organization:Proceedings of the Royal Society#Proceedings of the Royal Society B|Proceedings of the Royal Society B]] |volume=266 |issue=1436 |pages=2375–2382 |doi=10.1098/rspb.1999.0934 |pmc=1690475 }}</ref> The evolution of  [[Earth:Photosynthesis|photosynthesis]], around 3.5 Ga, eventually led to a buildup of its waste product, [[Chemistry:Oxygen|oxygen]], in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga.<ref name="Anbar2007">{{cite journal |last1=Anbar |first1=Ariel D. |author2=Yun Duan |last3=Lyons |first3=Timothy W. |last4=Arnold |first4=Gail L. |last5=Kendall |first5=Brian |last6=Creaser |first6=Robert A. |last7=Kaufman |first7=Alan J. |last8=Gordon |first8=Gwyneth W. |last9=Scott |first9=Clinton |last10=Garvin |first10=Jessica |last11=Buick |first11=Roger |display-authors=3 |date=September 28, 2007 |title=A Whiff of Oxygen Before the Great Oxidation Event? |journal=Science |volume=317 |issue=5846 |pages=1903–1906 |bibcode=2007Sci...317.1903A |doi=10.1126/science.1140325 |pmid=17901330}}</ref> The earliest evidence of [[Biology:Eukaryote|eukaryote]]s (complex [[Biology:Cell|cell]]s with [[Biology:Organelle|organelle]]s) dates from 1.85 Ga,<ref>{{cite journal |last1=Knoll |first1=Andrew H. |last2=Javaux |first2=Emmanuelle J. |last3=Hewitt |first3=David |last4=Cohen |first4=Phoebe |title=Eukaryotic organisms in Proterozoic oceans |date=June 29, 2006 |journal=[[Organization:Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=1023–1038 |doi=10.1098/rstb.2006.1843 |pmc=1578724 |pmid=16754612 }}</ref><ref name="Fedonkin2003OriginOfMetazoa" /> and while they may have been present earlier, their diversification accelerated when they started using oxygen in their [[Biology:Metabolism|metabolism]]. Later, around 1.7 Ga, [[Biology:Multicellular organism|multicellular organism]]s began to appear, with [[Biology:Cellular differentiation|differentiated cell]]s performing specialised functions.<ref name="Bonner1999OriginsOfMulticellularity" /> [[Biology:Sexual reproduction|Sexual reproduction]], which involves the fusion of male and female reproductive cells ([[Biology:Gamete|gamete]]s) to create a [[Biology:Zygote|zygote]] in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including almost all eukaryotes (which includes [[Biology:Animal|animal]]s and [[Biology:Plant|plant]]s).<ref>{{cite journal |last1=Otto |first1=Sarah P. |authorlink1=Sarah Otto |last2=Lenormand |first2=Thomas |date=April 2002 |title=Evolution of sex: Resolving the paradox of sex and recombination |journal=Nature Reviews Genetics |volume=3 |issue=4 |pages=252–261 |doi=10.1038/nrg761 |pmid=11967550}}</ref> However the origin and [[Biology:Evolution of sexual reproduction|evolution of sexual reproduction]] remain a puzzle for biologists though it did evolve from a common ancestor that was a single celled eukaryotic species.<ref name="Letunic I and Bork P">{{cite web |url=http://itol.embl.de/ |title=iTOL: Interactive Tree of Life |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |publisher=[[Organization:European Molecular Biology Laboratory|European Molecular Biology Laboratory]] |location=Heidelberg, Germany |accessdate=2015-07-21}}
*{{cite journal |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |date=January 1, 2007 |title=Interactive Tree Of Life (iTOL): an online tool for phylogenetic tree display and annotation |url=http://itol.embl.de/help/17050570.pdf |journal=Bioinformatics |volume=23 |issue=1 |pages=127–128 |doi=10.1093/bioinformatics/btl529 |pmid=17050570 |accessdate=2015-07-21}}
*{{cite journal |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |date=July 1, 2011 |title=Interactive Tree Of Life v2: online annotation and display of phylogenetic trees made easy |url=http://itol.embl.de/help/gkr201.pdf |journal=[[Chemistry:Nucleic Acids Research|Nucleic Acids Research]] |volume=39 |issue=Suppl. 2 |pages=W475–W478 |doi=10.1093/nar/gkr201 |pmc=3125724 |pmid=21470960 |accessdate=2015-07-21}}</ref> [[Biology:Bilateria|Bilateria]], animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago).<ref>{{cite journal |last1=Fedonkin |first1=Mikhail A. |last2=Simonetta |first2=Alberto |last3=Ivantsov |first3=Andrei Yu. |year=2007 |title=New data on ''Kimberella'', the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications |url=http://www.monash.edu.au/science/about/schools/geosciences/precsite/docs/workshop/prato04/abstracts/fedonkin2.pdf |journal=Geological Society Special Publication  |volume=286 |issue=1 |pages=157–179 |bibcode=2007GSLSP.286..157F |doi=10.1144/SP286.12 |accessdate=2013-05-16 }}</ref>

The earliest complex land plants date back to around 850 Ma,<ref>{{cite journal | title= The late Precambrian greening of the Earth | volume= 460 | issue= 7256 | pages= 728–32 | journal= Nature | doi= 10.1038/nature08213 | pmid= 19587681 | year= 2009 | last1= Knauth | first1= L. Paul | last2= Kennedy | first2= Martin J. |bibcode = 2009Natur.460..728K}}</ref> from carbon isotopes in Precambrian rocks, while algae-like multicellular land plants are dated back even to about 1 billion years ago,<ref>{{cite journal |last1=Strother |first1=Paul K. |last2=Battison |first2=Leila |last3=Brasier |first3=Martin D. |last4=Wellman |first4=Charles H. |display-authors=3 |date=May 26, 2011 |title=Earth's earliest non-marine eukaryotes |url=https://www.nature.com/articles/nature09943 |journal=Nature |volume=473 |issue=7348 |pages=505–509 |doi=10.1038/nature09943 |access-date=10 April 2018 |pmid=21490597|bibcode=2011Natur.473..505S }}</ref> although evidence suggests that [[Biology:Microorganism|microorganism]]s formed the earliest [[Earth:Terrestrial ecosystem|terrestrial ecosystem]]s, at least 2.7 Ga.<ref>{{cite journal |last=Beraldi-Campesi |first=Hugo |date=February 23, 2013 |title=Early life on land and the first terrestrial ecosystems |journal=Ecological Processes |volume=2 |issue=1 |page=4 |doi=10.1186/2192-1709-2-1 |doi-access=free }}</ref> Microorganisms are thought to have paved the way for the inception of land plants in the [[Earth:Ordovician|Ordovician]]. Land plants were so successful that they are thought to have contributed to the [[Earth:Late Devonian extinction|Late Devonian extinction event]].<ref name="AlgeoScheckler1998errestrialMarineTeleconnectionsInDevonian" /> (The long causal chain implied seems to involve the success of early tree [[Biology:Archaeopteris|archaeopteris]] (1) drew down CO<sub>2</sub> levels, leading to [[Earth:Global cooling|global cooling]] and lowered sea levels, (2) roots of archeopteris fostered soil development which ''increased'' rock weathering, and the subsequent nutrient run-off may have triggered [[Biology:Algal bloom|algal bloom]]s resulting in [[Earth:Anoxic event|anoxic event]]s which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.)

Ediacara biota appear during the [[Earth:Ediacaran|Ediacaran]] period,<ref>{{cite journal |author1=Jun-Yuan Chen |last2=Oliveri |first2=Paola |author3=Chia-Wei Li |author4=Gui-Qing Zhou |author5=Feng Gao |last6=Hagadorn |first6=James W. |last7=Peterson |first7=Kevin J. |last8=Davidson |first8=Eric H. |date=April 25, 2000 |title=Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=9 |pages=4457–4462 |bibcode=2000PNAS...97.4457C |doi=10.1073/pnas.97.9.4457 |pmid=10781044 |pmc=18256 |display-authors=3 }}</ref> while [[Biology:Vertebrate|vertebrate]]s, along with most other modern [[Biology:Phylum|phyla]] originated about {{ma|525|Ma|Cambrian}} during the [[Earth:Cambrian explosion|Cambrian explosion]].<ref name="Shu et al. 1999">{{cite journal |author1=D-G. Shu |author2=H-L. Luo |last3=Conway Morris |first3=Simon |author4=X-L. Zhang |author5=S-X. Hu |author6=L. Chen |author7=J. Han |author8=M. Zhu |author9=Y. Li |author10=L-Z. Chen |date=November 4, 1999 |title=Lower Cambrian vertebrates from south China |url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |journal=Nature |volume=402 |pages=42–46 |doi=10.1038/46965 |issue=6757 |bibcode=1999Natur.402...42S |display-authors=3 |archiveurl=https://web.archive.org/web/20090226122732/http://www.bios.niu.edu/davis/bios458/Shu1.pdf |archivedate=2009-02-26 |accessdate=2015-01-22 }}</ref> During the [[Earth:Permian|Permian]] period, [[Biology:Synapsid|synapsid]]s, including the ancestors of [[Biology:Mammal|mammal]]s, dominated the land,<ref>{{cite web |url=http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |title=Synapsid Reptiles |last=Hoyt |first=Donald F. |date=February 17, 1997 |website=ZOO 138 Vertebrate Zoology |publisher=[[Organization:California State Polytechnic University, Pomona|California State Polytechnic University, Pomona]] |location=Pomona, CA |type=Lecture |archiveurl=https://web.archive.org/web/20090520072737/http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |archivedate=2009-05-20 |accessdate=2015-01-22}}</ref> but most of this group became extinct in the [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]] {{ma|252|Ma}}.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |title=The Great Dying |last=Barry |first=Patrick L. |date=January 28, 2002 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=Science and Technology Directorate, [[Organization:Marshall Space Flight Center|Marshall Space Flight Center]] |accessdate=2015-01-22}}</ref> During the recovery from this catastrophe, [[Biology:Archosaur|archosaur]]s became the most abundant land vertebrates;<ref name="TannerLucas">{{cite journal |last1=Tanner |first1=Lawrence H. |last2=Lucas |first2=Spencer G. |last3=Chapman |first3=Mary G. |date=March 2004 |title=Assessing the record and causes of Late Triassic extinctions |url=http://nmnaturalhistory.org/pdf_files/TJB.pdf |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=65 |issue=1–2 |pages=103–139 |bibcode=2004ESRv...65..103T |doi=10.1016/S0012-8252(03)00082-5 |archiveurl=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archivedate=2007-10-25 |accessdate=2007-10-22 }}</ref> one archosaur group, the [[Biology:Dinosaur|dinosaur]]s, dominated the [[Earth:Jurassic|Jurassic]] and [[Earth:Cretaceous|Cretaceous]] periods.<ref name="BentonVertebratePaleontology">{{harvnb|Benton|1997}}</ref> After the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] {{ma|Paleogene|Ma}} killed off the non-avian dinosaurs,<ref>{{cite journal |last1=Fastovsky |first1=David E. |last2=Sheehan |first2=Peter M. |date=March 2005 |title=The Extinction of the Dinosaurs in North America |url=https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |journal=GSA Today |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 |accessdate=2015-01-23 }}</ref> mammals increased rapidly in size and diversity.<ref>{{cite news |last=Roach |first=John |date=June 20, 2007 |title=Dinosaur Extinction Spurred Rise of Modern Mammals |url=http://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |work=National Geographic News |location=Washington, D.C. |publisher=[[Organization:National Geographic Society|National Geographic Society]] |accessdate=2009-03-08}}</ref> Such [[Biology:Extinction event|mass extinction]]s may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<ref name="Van Valkenburgh 1999 463–493">{{cite journal |last=Van Valkenburgh |first=Blaire |date=May 1999 |title=Major patterns in the history of carnivorous mammals |journal=[[Earth:Annual Review of Earth and Planetary Sciences|Annual Review of Earth and Planetary Sciences]] |volume=27 |pages=463–493 |bibcode=1999AREPS..27..463V |doi=10.1146/annurev.earth.27.1.463 |url=https://zenodo.org/record/890156 }}</ref>

== Earliest history of Earth ==
{{Graphical timeline

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 | period1=Hadean
 | period1-text=[[Earth:Hadean|Hadean]]
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 | period2-text=[[Earth:Archean|Archean]]
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 | period3-text=[[Earth:Proterozoic|Protero<br />-zoic]]
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 | period5-text = [[Earth:Eoarchean|Eo]]
 | period6      = Paleoarchean
 | period6-text = [[Earth:Paleoarchean|Paleo]]
 | period7      = Mesoarchean
 | period7-text = [[Earth:Mesoarchean|Meso]]
 | period8      = Neoarchean
 | period8-text = [[Earth:Neoarchean|Neo]]

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 | period9-text  = [[Earth:Paleoproterozoic|Paleo]]
 | period10      = Mesoproterozoic
 | period10-text = [[Earth:Mesoproterozoic|Meso]]
 | period11      = Neoproterozoic
 | period11-text = [[Earth:Neoproterozoic|Neo]]

 | period12      = Paleozoic
 | period12-text = [[Earth:Paleozoic|Paleo]]
 | period13      = Mesozoic
 | period13-text = [[Earth:Mesozoic|Meso]]
 | period14      = Cenozoic
 | period14-text = [[Earth:Cenozoic|Ceno]]

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 | bar2-from=-{{period start|eoarchean}}

<!-- Annotations, & occasional bar-->
|note1=[[Astronomy:Earth|Earth]] and [[Astronomy:Solar System|Solar System]] formed
|note1-at=-4550
|note1-nudge-left=5.5
|note1-nudge-down=0.0 <!--- adj from 0.7 to 0.0 -- seems better --->
|note2= <!--- Impact formed [[Astronomy:Moon|Moon]] -- overlaps other titles --->
|note2-at=-4510
|note2-nudge-left=5.5
|note2-nudge-down=0.3
|note3=Cool surface, oceans, atmosphere
|note3-at=-4450
|note3-nudge-left=5.5
|note3-nudge-up=0.0
|note4=[[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]]
|note4-at=-3900
|note4-nudge-left=5.5
|note4-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->
| bar4-colour=gray
| bar4-left=0.555
| bar4-right = 0.60
| bar4-from=-4000
| bar4-to=-3800
|note5=[[Biology:Abiogenesis|Earliest evidence of life (-4100)]]
|note5-at=-4100
|note5-nudge-left=5.5
|note5-nudge-down=0.0
|note6=Oxygenation of atmosphere
|note6-at=-2400
|note6-nudge-left=5.5
|note6-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->
|note7-at=-0800
|note7-nudge-left=5.5
|note7-nudge-down=0.0
|note8= <!--- Earliest known [[Biology:Fungi|fungi]] -- overlaps other titles  --->
|note8-at=-0430
|note8-nudge-left=5.5
|note8-nudge-down=0.3
|note9= <!--- Earliest known [[Biology:Cnidaria|cnidaria]]ns -- overlaps other titles --->
|note9-at=-580
|note9-nudge-left=5.5
|note9-nudge-down=0.7
|note10=[[Earth:Cambrian explosion|Cambrian explosion]]
|note10-at=-530
|note10-nudge-left=5.5
|note10-nudge-down=0.4
|note11=Earliest land [[Biology:Invertebrate|invertebrate]]s and [[Biology:Plant|plant]]s
|note11-at=-470
|note11-nudge-left=5.5
|note11-nudge-down=0.1
|note12=Earliest land [[Biology:Vertebrate|vertebrate]]s
|note12-at=-345
|note12-nudge-left=5.5
|note12-nudge-down=0.1
|note13=Earliest known [[Biology:Dinosaur|dinosaur]]
|note13-at=-225
|note13-nudge-left=5.5
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|note14=Extinction of non-avian dinosaurs
|note14-at=-65
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|note14-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->

 | caption=Scale: [[Megaannum|Ma]] (Millions of years)

}}
The oldest [[Astronomy:Meteorite|meteorite]] fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient [[Chemistry:Lead|lead]] deposits, has put the estimated age of Earth at around that time.<ref name="ageOfEarth">{{harvnb|Dalrymple|1991}}
*{{harvnb|Newman|2007}}
*{{cite journal |last=Dalrymple |first=G. Brent |year=2001 |title=The age of the Earth in the twentieth century: a problem (mostly) solved |url=http://sp.lyellcollection.org/content/190/1/205.abstract |journal=Geological Society Special Publication |volume=190 |issue=1 |pages=205–221 |bibcode=2001GSLSP.190..205D |doi=10.1144/GSL.SP.2001.190.01.14 |accessdate=2015-01-23 }}</ref> The [[Astronomy:Moon|Moon]] has the same composition as Earth's [[Earth:Crust (geology)|crust]] but does not contain an [[Chemistry:Iron|iron]]-rich [[Astronomy:Planetary core|core]] like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing into orbit crust material that formed the Moon. Another [[Hypothesis|hypothesis]] is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger [[Physics:Gravity|gravity]] than the early Moon, attracted almost all the iron particles in the area.<ref>{{cite journal |last1=Galimov |first1=Erik M. |last2=Krivtsov |first2=Anton M. |date=December 2005 |title=Origin of the Earth—Moon system |url=http://www.ias.ac.in/jessci/dec2005/ilc-3.pdf |journal=[[Earth:Journal of Earth System Science|Journal of Earth System Science]] |volume=114 |issue=6 |pages=593–600 |bibcode=2005JESS..114..593G |doi=10.1007/BF02715942 |accessdate=2015-01-23 |citeseerx=10.1.1.502.314 }}</ref>

Until 2001, the oldest rocks found on Earth were about 3.8 billion years old,<ref>{{cite news |last=Thompson |first=Andrea |date=September 25, 2008 |title=Oldest Rocks on Earth Found |url=http://www.livescience.com/2896-oldest-rocks-earth.html |work=LiveScience |publisher=Imaginova |location=Watsonville, CA |accessdate=2015-01-23}}</ref><ref>{{harvnb|Dalrymple|1991}}</ref><ref>{{harvnb|Newman|2007}}</ref><ref name="Dalrymple_2001">{{cite journal |last=Dalrymple |first=G. Brent |year=2001 |title=The age of the Earth in the twentieth century: a problem (mostly) solved |url=http://sp.lyellcollection.org/content/190/1/205.abstract |journal=Geological Society Special Publication |volume=190 |issue=1 |pages=205–221 |bibcode=2001GSLSP.190..205D |doi=10.1144/GSL.SP.2001.190.01.14 |accessdate=2015-01-23 }}</ref> leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the [[Earth:Hadean|Hadean]].<ref name="sciencemag.org">{{cite journal |last1=Cohen |first1=Barbara A. |last2=Swindle |first2=Timothy D. |last3=Kring |first3=David A. |date=December 1, 2000 |title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages |journal=Science |volume=290 |issue=5497 |pages=1754–1756 |bibcode=2000Sci...290.1754C |doi=10.1126/science.290.5497.1754 |pmid=11099411 }}</ref> However, analysis of [[Chemistry:Zircon|zircon]]s formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an [[Astronomy:Atmosphere|atmosphere]], which may have been capable of supporting life.<ref>{{cite press release |author=<!--Staff writer(s); no by-line.--> |title=Early Earth Likely Had Continents And Was Habitable |url=http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |location=Boulder, CO |publisher=[[Organization:University of Colorado|University of Colorado]] |date=November 17, 2005 |accessdate=2015-01-23 |archive-url=https://web.archive.org/web/20150124005824/http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |archive-date=January 24, 2015 |url-status=dead }}</ref><ref>{{cite journal |last1=Cavosie |first1=Aaron J. |last2=Valley |first2=John W. |last3=Wilde |first3=Simon A. |author4=Edinburgh Ion Microprobe Facility |date=July 15, 2005 |title=Magmatic δ<sup>18</sup>O in 4400–3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |volume=235 |issue=3–4 |pages=663–681 |bibcode=2005E&PSL.235..663C |doi=10.1016/j.epsl.2005.04.028 }}</ref><ref name="PiP">{{cite journal |last=Garwood |first=Russell J. |year=2012 |title=Patterns In Palaeontology: The first 3 billion years of evolution |url=http://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ |journal=Palaeontology Online |volume=2 |issue=11 |pages=1–14 |accessdate=2015-06-25}}</ref>

Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a [[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]] by debris that was left over from the formation of the [[Astronomy:Solar System|Solar System]], and the Earth should have experienced an even heavier bombardment due to its stronger gravity.<ref name="sciencemag.org" /><ref name="space.com-bombardment">{{cite news |last=Britt |first=Robert Roy |date=July 24, 2002 |title=Evidence for Ancient Bombardment of Earth |url=http://www.space.com/scienceastronomy/planetearth/earth_bombarded_020724.html |work=Space.com |location=New York |publisher=Space Holding Corp. |archiveurl=https://web.archive.org/web/20060415193738/http://space.com/scienceastronomy/planetearth/earth_bombarded_020724.html |archivedate=2006-04-15 |accessdate=2015-01-23}}</ref> While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment.<ref>{{cite journal |last1=Valley |first1=John W. |last2=Peck |first2=William H. |last3=King |first3=Elizabeth M. |last4=Wilde |first4=Simon A. |date=April 2002 |title=A cool early Earth |url=http://www.geology.wisc.edu/zircon/Valley2002Cool_Early_Earth.pdf |journal=Geology |volume=30 |issue=4 |pages=351–354 |bibcode=2002Geo....30..351V |doi=10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2 |accessdate=2008-09-13 }}</ref> This event may well have stripped away any previous atmosphere and oceans; in this case [[Physics:Gas|gas]]es and water from [[Astronomy:Comet|comet]] impacts may have contributed to their replacement, although [[Physics:Outgassing|outgassing]] from [[Earth:Volcano|volcano]]es on Earth would have supplied at least half.<ref>{{cite journal |last1=Dauphas |first1=Nicolas |last2=Robert |first2=François |last3=Marty |first3=Bernard |date=December 2000 |title=The Late Asteroidal and Cometary Bombardment of Earth as Recorded in Water Deuterium to Protium Ratio |journal=Icarus |volume=148 |issue=2 |pages=508–512 |bibcode=2000Icar..148..508D |doi=10.1006/icar.2000.6489 }}</ref> However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.<ref>{{cite web |url=https://astrobiology.nasa.gov/articles/2009/5/20/microbial-habitability-during-the-late-heavy-bombardment/ |archive-url=https://archive.today/20150124225202/https://astrobiology.nasa.gov/articles/2009/5/20/microbial-habitability-during-the-late-heavy-bombardment/ |url-status=dead |archive-date=January 24, 2015 |title=Microbial Habitability During the Late Heavy Bombardment |last=Scalice |first=Daniella |editor-last=Fletcher |editor-first=Julie |date=May 20, 2009 |website=Astrobiology |publisher=[[Organization:NASA Astrobiology Institute#Program|NASA Astrobiology Program]] |location=Mountain View, CA |accessdate=2015-01-23 }}</ref>

== Earliest evidence for life on Earth ==
{{main|Biology:Earliest known life forms}}
The earliest identified organisms were minute and relatively featureless, and their fossils look like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga.<ref name="BrasierMcLoughlinEtAl2006FreshLook" /> Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria,<ref>{{cite journal |last=Schopf |first=J. William |date=April 30, 1993 |title=Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life |journal=Science |volume=260 |issue=5108 |pages=640–646 |bibcode=1993Sci...260..640S |doi=10.1126/science.260.5108.640 |pmid=11539831 }}
* {{cite journal |last1=Altermann |first1=Wladyslaw |last2=Kazmierczak |first2=Józef |date=November 2003 |title=Archean microfossils: a reappraisal of early life on Earth |journal=Research in Microbiology |volume=154 |issue=9 |pages=611–617 |doi=10.1016/j.resmic.2003.08.006 |pmid=14596897 }}</ref> with [[Chemistry:Geochemistry|geochemical]] evidence also seeming to show the presence of life 3.8 Ga.<ref>{{cite journal |last1=Mojzsis |first1=Stephen J. |last2=Arrhenius |first2=Gustaf |last3=McKeegan |first3=Kevin D. |last4=Harrison |first4=T. Mark |last5=Nutman |first5=Allen P. |last6=Friend |first6=Clark R. L. |date=November 1996 |title=Evidence for life on Earth before 3,800 million years ago |journal=Nature |volume=384 |issue=6604 |pages=55–59 |bibcode=1996Natur.384...55M |doi=10.1038/384055a0 |pmid=8900275 |display-authors=3 |hdl=2060/19980037618 |hdl-access=free }}</ref> However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.<ref name="GrotzingerRothman1996AbioticModelForStromatolites">{{cite journal |last1=Grotzinger |first1=John P. |last2=Rothman |first2=Daniel H. |date=October 3, 1996 |title=An abiotic model for stromatolite morphogenesis  |journal=Nature |volume=383 |issue=6599 |pages=423–425 |bibcode=1996Natur.383..423G |doi=10.1038/383423a0 }}</ref><ref>{{cite journal |last1=Fedo |first1=Christopher M. |last2=Whitehouse |first2=Martin J. |date=May 24, 2002 |title=Metasomatic Origin of Quartz-Pyroxene Rock, Akilia, Greenland, and Implications for Earth's Earliest Life |journal=Science |volume=296 |issue=5572 |pages=1448–1452 |bibcode=2002Sci...296.1448F |doi=10.1126/science.1070336 |pmid=12029129 }}
* {{cite journal |last1=Lepland |first1=Aivo |last2=van Zuilen |first2=Mark A. |last3=Arrhenius |first3=Gustaf |last4=Whitehouse |first4=Martin J. |last5=Fedo |first5=Christopher M. |date=January 2005 |title=Questioning the evidence for Earth's earliest life—Akilia revisited |journal=Geology |volume=33 |issue=1 |pages=77–79 |bibcode=2005Geo....33...77L |doi=10.1130/G20890.1 |display-authors=3 }}</ref> While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life,<ref name="BrasierMcLoughlinEtAl2006FreshLook">{{cite journal |last1=Brasier |first1=Martin |last2=McLoughlin |first2=Nicola |last3=Green |first3=Owen |last4=Wacey |first4=David |date=June 2006 |title=A fresh look at the fossil evidence for early Archaean cellular life |url=http://physwww.mcmaster.ca/~higgsp/3D03/BrasierArchaeanFossils.pdf |journal=Philosophical Transactions of the Royal Society B |volume=361 |issue=1470 |pages=887–902 |doi=10.1098/rstb.2006.1835 |pmc=1578727 |pmid=16754605 |accessdate=2008-08-30 }}</ref><ref>{{cite journal |last=Schopf |first=J. William |date=June 29, 2006 |title=Fossil evidence of Archaean life |journal=Philosophical Transactions of the Royal Society B |volume=361 |issue=1470 |pages=869–885 |doi=10.1098/rstb.2006.1834 |pmid=16754604 |pmc=1578735 }}</ref> although these statements have not been thoroughly examined by critics.

Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old was found in the Nuvvuagittuq Greenstone Belt in Quebec, Canada,<ref name="NAT-20170301">{{cite journal |author=Dodd, Matthew S. |author2=Papineau, Dominic |author3=Grenne, Tor |author4=slack, John F. |author5=Rittner, Martin |author6=Pirajno, Franco |author7=O'Neil, Jonathan |author8=Little, Crispin T. S. |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates|journal=Nature |volume=543 |issue=7643 |pages=60–64 |date=2 March 2017 |doi=10.1038/nature21377|pmid=28252057 |bibcode=2017Natur.543...60D |url=http://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf }}</ref> although the evidence is disputed as inconclusive.<ref>{{Cite news|url=http://news.nationalgeographic.com/2017/03/oldest-life-earth-iron-fossils-canada-vents-science/|title=This May Be the Oldest Known Sign of Life on Earth|date=2017-03-01|access-date=2017-03-02}}</ref>

== Origins of life on Earth ==
{{PhylomapA|size=300px|caption=[[Biology:Phylogenetic tree|Evolutionary tree]] showing the divergence of modern species from their common ancestor in the center.<ref name="Ciccarelli">{{cite journal |last1=Ciccarelli |first1=Francesca D. |last2=Doerks |first2=Tobias |last3=von Mering |first3=Christian |last4=Creevey |first4=Christopher J. |last5=Snel |first5=Berend |last6=Bork |first6=Peer |date=March 3, 2006 |title=Toward Automatic Reconstruction of a Highly Resolved Tree of Life |journal=Science |volume=311 |issue=5765 |pages=1283–1287 |bibcode=2006Sci...311.1283C |doi=10.1126/science.1123061 |pmid=16513982 |display-authors=3 |url=http://bioinformatics.bio.uu.nl/pdf/Ciccarelli.s06-311.pdf |type=Submitted manuscript |citeseerx=10.1.1.381.9514 }}</ref> The three domains are colored, with [[Biology:Bacteria|bacteria]] blue, [[Biology:Archaea|archaea]] green, and [[Biology:Eukaryote|eukaryote]]s red.}}
{{further|Biology:Evidence of common descent|Biology:Common descent|Biology:Homology}}
[[Biologist]]s reason that all living organisms on Earth must share a single last universal ancestor, because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms.<ref>{{cite journal |last=Mason |first=Stephen F. |title=Origins of biomolecular handedness |journal=Nature |volume=311 |issue=5981 |pages=19–23 |year=1984 |pmid=6472461 |doi=10.1038/311019a0 |bibcode=1984Natur.311...19M }}</ref><ref>{{cite journal |last=Orgel |first=Leslie E. |date=October 1994 |title=The Origin of Life on the Earth |url=http://courses.washington.edu/biol354/The%20Origin%20of%20Life%20on%20Earth.pdf |journal=Scientific American |volume=271 |issue=4 |pages=76–83 |doi=10.1038/scientificamerican1094-76 |pmid=7524147 |accessdate=2008-08-30 |archive-url=https://archive.today/20010124054500/http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html|archive-date=January 24, 2001|bibcode=1994SciAm.271d..76O }}</ref><!-- **** "After all, it is next to impossible for such universal traits to have evolved separately"- Orgel, cited **** -->

=== Independent emergence on Earth ===
{{main|Biology:Abiogenesis}}
Life on Earth is based on [[Chemistry:Carbon|carbon]] and [[Chemistry:Water|water]]. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from [[Chemistry:Carbon dioxide|carbon dioxide]].<ref name="PiP" /> There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; [[Chemistry:Silicon|silicon]], the element directly below carbon on the [[Chemistry:Periodic table|periodic table]], does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because [[Chemistry:Silicon dioxide|silicon dioxide]] is a hard and abrasive solid in contrast to [[Chemistry:Carbon dioxide|carbon dioxide]] at temperatures associated with living things, it would be more difficult for organisms to extract. The elements [[Chemistry:Boron|boron]] and [[Chemistry:Phosphorus|phosphorus]] have more complex chemistries, but suffer from other limitations relative to carbon. Water is an excellent [[Physics:Solvent|solvent]] and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have [[Physics:Electric charge|electrically negative]] and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as [[Chemistry:Ammonia|ammonia]], are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages.<ref>{{harvnb|Bennett|2008|pp=[https://archive.org/details/beyondufossearch00benn/page/84 82–85]}}</ref> Organisms based on [[Unsolved:Hypothetical types of biochemistry|alternative biochemistry]] may, however, be possible on other planets.<ref>{{cite journal |last1=Schulze-Makuch |first1=Dirk |last2=Irwin |first2=Louis N. |date=April 2006 |title=The prospect of alien life in exotic forms on other worlds |journal=Naturwissenschaften |volume=93 |issue=4 |pages=155–72 |bibcode=2006NW.....93..155S |doi=10.1007/s00114-005-0078-6 |pmid=16525788 }}</ref>

Research on how life might have emerged from non-living chemicals focuses on three possible starting points: self-replication, an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external [[Biology:Cell membrane|cell membrane]]s, which allow food to enter and waste products to leave, but exclude unwanted substances.<ref>{{cite journal |last=Peretó |first=Juli |year=2005 |title=Controversies on the origin of life |url=http://www.im.microbios.org/0801/0801023.pdf |journal=[[Biology:International Microbiology|International Microbiology]] |volume=8 |issue=1 |pages=23–31 |pmid=15906258 |accessdate=2007-10-07 }}</ref> Research on abiogenesis still has a long way to go, since theoretical and [[Empirical evidence|empirical]] approaches are only beginning to make contact with each other.<ref>{{cite journal |last=Szathmáry |first=Eörs |date=February 3, 2005 |title=Life: In search of the simplest cell |journal=Nature |volume=433 |issue=7025 |pages=469–470 |bibcode=2005Natur.433..469S |doi=10.1038/433469a |pmid=15690023 }}</ref><ref>{{cite journal |last1=Luisi |first1=Pier Luigi |authorlink1=Pier Luigi Luisi |last2=Ferri |first2=Francesca |last3=Stano |first3=Pasquale |date=January 2006 |title=Approaches to semi-synthetic minimal cells: a review |journal=Naturwissenschaften |volume=93 |issue=1 |pages=1–13 |bibcode=2006NW.....93....1L |doi=10.1007/s00114-005-0056-z |pmid=16292523 }}</ref>

==== Replication first: RNA world ====
Even the simplest members of the [[Biology:Three-domain system|three modern domains]] of life use [[Biology:DNA|DNA]] to record their "recipes" and a complex array of [[Biology:RNA|RNA]] and [[Biology:Protein|protein]] molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can [[Chemistry:Catalysis|catalyze]] both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA.<ref>{{cite journal |last=Joyce |first=Gerald F. |date=July 11, 2002 |title=The antiquity of RNA-based evolution |journal=Nature |volume=418 |issue=6894 |pages=214–221 |bibcode=2002Natur.418..214J |doi=10.1038/418214a |pmid=12110897 }}</ref> These [[Biology:Ribozyme|ribozyme]]s could have formed an RNA world in which there were individuals but no species, as [[Biology:Mutation|mutation]]s and [[Biology:Horizontal gene transfer|horizontal gene transfer]]s would have meant that the offspring in each generation were quite likely to have different [[Biology:Genome|genome]]s from those that their parents started with.<ref name="Hoenigsberg2003SelectionWithoutSpeciation">{{cite journal |last=Hoenigsberg |first=Hugo |date=December 30, 2003 |title=Evolution without speciation but with selection: LUCA, the Last Universal Common Ancestor in Gilbert's RNA world |url=http://www.funpecrp.com.br/gmr/year2003/vol4-2/gmr0070_full_text.htm |journal=Genetics and Molecular Research |volume=2 |issue=4 |pages=366–375 |pmid=15011140 |accessdate=2008-08-30 }}</ref> RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have.<ref name="Hoenigsberg2003SelectionWithoutSpeciation" /><ref>{{cite journal |last1=Trevors |first1=Jack T. |last2=Abel |first2=David L. |date=November 2004 |title=Chance and necessity do not explain the origin of life |journal=[[Biology:Cell Biology International|Cell Biology International]] |volume=28 |issue=11 |pages=729–739 |doi=10.1016/j.cellbi.2004.06.006 |pmid=15563395 }}</ref><ref>{{cite journal |last1=Forterre |first1=Patrick |last2=Benachenhou-Lahfa |first2=Nadia |last3=Confalonieri |first3=Fabrice |last4=Duguet |first4=Michel |last5=Elie |first5=Christiane |last6=Labedan |first6=Bernard |year=1992 |title=The nature of the last universal ancestor and the root of the tree of life, still open questions |journal=[[Biology:BioSystems|BioSystems]] |volume=28 |issue=1–3 |pages=15–32 |doi=10.1016/0303-2647(92)90004-I |pmid=1337989 |display-authors=3 }}</ref> Ribozymes remain as the main components of [[Biology:Ribosome|ribosome]]s, modern cells' "protein factories."<ref>{{cite journal |last=Cech |first=Thomas R. |authorlink=Thomas Cech |date=August 11, 2000 |title=The Ribosome Is a Ribozyme |journal=Science |volume=289 |issue=5481 |pages=878–879 |doi=10.1126/science.289.5481.878 |pmid=10960319 }}</ref> Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga.<ref>{{Cite journal|last1=Pearce|first1=Ben K. D.|last2=Pudritz|first2=Ralph E.|last3=Semenov|first3=Dmitry A.|last4=Henning|first4=Thomas K.|date=2017-10-24|title=Origin of the RNA world: The fate of nucleobases in warm little ponds|journal=Proceedings of the National Academy of Sciences|language=en|volume=114|issue=43|pages=11327–11332|doi=10.1073/pnas.1710339114|issn=0027-8424|pmid=28973920|pmc=5664528|arxiv=1710.00434|bibcode=2017PNAS..11411327P}}</ref>

Although short self-replicating RNA molecules have been artificially produced in laboratories,<ref>{{cite journal |last1=Johnston |first1=Wendy K. |last2=Unrau |first2=Peter J. |last3=Lawrence |first3=Michael S. |last4=Glasner |first4=Margaret E. |last5=Bartel |first5=David P. |date=May 18, 2001 |title=RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension |journal=Science |volume=292 |issue=5520 |pages=1319–1325 |bibcode=2001Sci...292.1319J |doi=10.1126/science.1060786 |pmid=11358999 |display-authors=3 |url=http://www.dna.caltech.edu/courses/cs191/paperscs191/Science(292)1319.pdf |type=Submitted manuscript |citeseerx=10.1.1.70.5439 }}</ref> doubts have been raised about whether natural non-biological synthesis of RNA is possible.<ref name="RNANaturalSynthesisProblems">{{cite journal |last1=Levy |first1=Matthew |last2=Miller |first2=Stanley L. |date=July 7, 1998 |title=The stability of the RNA bases: Implications for the origin of life |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=95 |issue=14 |pages=7933–7938 |bibcode=1998PNAS...95.7933L |doi=10.1073/pnas.95.14.7933 |pmc=20907 |pmid=9653118 }}
* {{cite journal |last1=Larralde |first1=Rosa |last2=Robertson |first2=Michael P. |last3=Miller |first3=Stanley L. |date=August 29, 1995 |title=Rates of decomposition of ribose and other sugars: Implications for chemical evolution |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=92 |issue=18 |pages=8158–8160 |bibcode=1995PNAS...92.8158L |doi=10.1073/pnas.92.18.8158 |pmc=41115 |pmid=7667262 }}
* {{cite journal |last=Lindahl |first=Tomas |date=April 22, 1993 |title=Instability and decay of the primary structure of DNA |journal=Nature |volume=362 |issue=6422 |pages=709–715 |bibcode=1993Natur.362..709L |doi=10.1038/362709a0 |pmid=8469282 }}</ref> The earliest "ribozymes" may have been formed of simpler [[Biology:Nucleic acid|nucleic acid]]s such as [[Biology:Peptide nucleic acid|PNA]], [[Chemistry:Threose nucleic acid|TNA]] or [[Chemistry:Glycol nucleic acid|GNA]], which would have been replaced later by RNA.<ref>{{cite journal |last=Orgel |first=Leslie |date=November 17, 2000 |title=A Simpler Nucleic Acid |journal=Science |volume=290 |issue=5495 |pages=1306–1307 |doi=10.1126/science.290.5495.1306 |pmid=11185405 }}</ref><ref>{{cite journal |last1=Nelson |first1=Kevin E. |last2=Levy |first2=Matthew |last3=Miller |first3=Stanley L. |date=April 11, 2000 |title=Peptide nucleic acids rather than RNA may have been the first genetic molecule |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=8 |pages=3868–3871 |bibcode=2000PNAS...97.3868N |doi=10.1073/pnas.97.8.3868 |pmc=18108 |pmid=10760258 }}</ref>

In 2003, it was proposed that porous metal sulfide [[Chemistry:Precipitation|precipitates]] would assist RNA synthesis at about {{convert|100|°C|°F}} and ocean-bottom pressures near [[Earth:Hydrothermal vent|hydrothermal vent]]s. Under this hypothesis, [[Chemistry:Lipid|lipid]] membranes would be the last major cell components to appear and, until then, the [[Biology:Protocell|protocell]]s would be confined to the pores.<ref>{{cite journal |last1=Martin |first1=William |last2=Russell |first2=Michael J. |date=January 29, 2003 |title=On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells |journal=Philosophical Transactions of the Royal Society B |volume=358 |issue=1429 |pages=59–85 |doi=10.1098/rstb.2002.1183 |pmc=1693102 |pmid=12594918 }}</ref>

==== Metabolism first: Iron–sulfur world ====
A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including [[Chemistry:Carbon monoxide|carbon monoxide]] and [[Chemistry:Hydrogen sulfide|hydrogen sulfide]] could be achieved by using [[Chemistry:Iron(II) sulfide|iron sulfide]] and [[Chemistry:Nickel sulfide|nickel sulfide]] as [[Chemistry:Catalyst|catalyst]]s. Most of the steps required temperatures of about {{convert|100|°C|°F}} and moderate pressures, although one stage required {{convert|250|°C|°F}} and a pressure equivalent to that found under {{convert|7|km|mi}} of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents.<ref>{{cite journal |last=Wächtershäuser |first=Günter |date=August 25, 2000 |title=Life as We Don't Know It |journal=Science |volume=289 |issue=5483 |pages=1307–1308 |doi=10.1126/science.289.5483.1307 |pmid=10979855 }}</ref>
{{clear}}

==== Membranes first: Lipid world ====
{{Annotated image | caption=Cross-section through a [[Biology:Liposome|liposome]] | image=Liposome cross section.png | width=250 | height=160 | image-width = 125 | image-left=0 | image-top=0
| annotations =
{{Annotation|150|10|{{Color box|white|border=darkgray}} {{=}} water-attracting heads of [[Chemistry:Lipid|lipid]] molecules}}
{{Annotation|150|90|{{Color box|#cd9922|border=darkgray}} {{=}} water-repellent tails}}
}}
It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.<ref>{{cite journal |last1=Trevors |first1=Jack T. |last2=Psenner |first2=Roland |date=December 2001 |title=From self-assembly of life to present-day bacteria: a possible role for nanocells |journal=FEMS Microbiology Reviews |volume=25 |issue=5 |pages=573–582 |doi=10.1111/j.1574-6976.2001.tb00592.x |pmid=11742692 |doi-access=free }}</ref> Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form [[Biology:Liposome|liposome]]s, double-walled "bubbles," and then reproduce themselves.<ref name="PiP" /> Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to [[Biology:Natural selection|natural selection]] for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside.<ref>{{cite journal |last1=Segré |first1=Daniel |last2=Ben-Eli |first2=Dafna |last3=Deamer |first3=David W. |last4=Lancet |first4=Doron |date=February 2001 |title=The Lipid World |url=http://wws.weizmann.ac.il/molgen/Lancet/sites/molgen.Lancet/files/uploads/segre_lipid_world.pdf |journal=[[Biology:Origins of Life and Evolution of Biospheres|Origins of Life and Evolution of Biospheres]] |volume=31 |issue=1–2 |pages=119–145 |doi=10.1023/A:1006746807104 |pmid=11296516 |accessdate=2015-01-26 |bibcode=2001OLEB...31..119S }}</ref>

==== The clay hypothesis ====
RNA is complex and there are doubts about whether it can be produced non-biologically in the wild.<ref name="RNANaturalSynthesisProblems" /> Some [[Earth:Clay|clay]]s, notably [[Chemistry:Montmorillonite|montmorillonite]], have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their [[Earth:Crystal|crystal]]line pattern; they are subject to an analog of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules.<ref>{{harvnb|Cairns-Smith|1968|pp=57–66}}</ref> Although this idea has not become the scientific consensus, it still has active supporters.<ref>{{cite journal |last=Ferris |first=James P. |date=June 1999 |title=Prebiotic Synthesis on Minerals: Bridging the Prebiotic and RNA Worlds |journal=[[Biology:The Biological Bulletin|The Biological Bulletin]] |volume=196 |issue=3 |pages=311–314 |doi=10.2307/1542957 |jstor=1542957 |pmid=10390828 }} "This paper was originally presented at a workshop titled ''Evolution: A Molecular Point of View''."</ref>

Research in 2003 reported that montmorillonite could also accelerate the conversion of [[Chemistry:Fatty acid|fatty acid]]s into "bubbles," and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes.<ref>{{cite journal |last1=Hanczyc |first1=Martin M. |last2=Fujikawa |first2=Shelly M. |last3=Szostak |first3=Jack W. |date=October 24, 2003 |title=Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division |journal=Science |volume=302 |issue=5645 |pages=618–622 |bibcode=2003Sci...302..618H |doi=10.1126/science.1089904 |pmid=14576428 |pmc=4484575 }}</ref>

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of [[Biology:Nucleotide|nucleotide]]s, lipids and [[Chemistry:Amino acid|amino acid]]s.<ref>{{cite journal |last=Hartman |first=Hyman |date=October 1998 |title=Photosynthesis and the Origin of Life |journal=Origins of Life and Evolution of Biospheres |volume=28 |issue=4–6 |pages=512–521 |bibcode=1998OLEB...28..515H |doi=10.1023/A:1006548904157 |pmid=11536891}}</ref>

=== Life "seeded" from elsewhere ===
{{main|Biology:Panspermia}}
The Panspermia hypothesis does not explain how life arose in the first place, but simply examines the possibility of it coming from somewhere other than the Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher [[Biography:Anaximander|Anaximander]] in the sixth century [[Common Era|BCE]].<ref>{{harvnb|O'Leary|2008}}</ref> In the twentieth century it was proposed by the [[Chemistry:Physical chemistry|physical chemist]] [[Biography:Svante Arrhenius|Svante Arrhenius]],<ref name="Arrhenius1903">{{harvnb|Arrhenius|1980|p=32}}</ref> by the [[Astronomy:Astronomer|astronomer]]s [[Biography:Fred Hoyle|Fred Hoyle]] and [[Biography:Chandra Wickramasinghe|Chandra Wickramasinghe]],<ref name="HoyleWickramasinghe1979">{{cite journal|last2=Wickramasinghe|first2=Nalin C.|date=November 1979|title=On the Nature of Interstellar Grains|journal=[[Physics:Astrophysics and Space Science|Astrophysics and Space Science]] |volume=66|issue=1|pages=77–90|bibcode=1979Ap&SS..66...77H|doi=10.1007/BF00648361|last1=Hoyle|first1=Fred|authorlink2=Chandra Wickramasinghe}}</ref> and by [[Biology:Molecular biology|molecular biologist]] [[Biography:Francis Crick|Francis Crick]] and chemist Leslie Orgel.<ref name="CrickOrgel1973DirectedPanspermia">{{cite journal|last2=Orgel|first2=Leslie E|date=July 1973|title=Directed Panspermia|journal=Icarus|volume=19|issue=3|pages=341–348|bibcode=1973Icar...19..341C|doi=10.1016/0019-1035(73)90110-3|last1=Crick|first1=Francis H.|authorlink1=Francis Crick}}</ref>

There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large [[Astronomy:Meteoroid#Meteor|meteor]] impact, in which case the most credible sources are [[Astronomy:Mars|Mars]]<ref name="Scientific-American-panspermia">{{cite journal|last2=Weiss|first2=Benjamin|date=November 2005|title=Did Life Come From Another World?|journal=Scientific American|volume=293|issue=5|pages=64–71|doi=10.1038/scientificamerican1105-64|pmid=16318028|last1=Warmflash|first1=David|bibcode=2005SciAm.293e..64W}}</ref> and [[Astronomy:Venus|Venus]];<ref>{{cite journal|last2=Wickramasinghe|first2=Janaki T.|date=September 2008|title=On the possibility of microbiota transfer from Venus to Earth|journal=Astrophysics and Space Science |volume=317|issue=1–2|pages=133–137|bibcode=2008Ap&SS.317..133W|doi=10.1007/s10509-008-9851-2|last1=Wickramasinghe|first1=Nalin C.|citeseerx=10.1.1.470.2347}}</ref> by [[Unsolved:Directed panspermia|alien visitors]], possibly as a result of accidental [[Astronomy:Interplanetary contamination|contamination]] by microorganisms that they brought with them;<ref name="CrickOrgel1973DirectedPanspermia" /> and from outside the Solar System but by natural means.<ref name="Arrhenius1903" /><ref name="Scientific-American-panspermia" />

Experiments in low Earth orbit, such as [[Organization:Long Duration Exposure Facility#EXOSTACK|EXOSTACK]], demonstrated that some microorganism [[Biology:Spore|spore]]s can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years.<ref name="Clancy">{{harvnb|Clancy|Brack|Horneck|2005}}</ref><ref name="Horneck">{{cite journal|last2=Klaus|first2=David M.|last3=Mancinelli|first3=Rocco L.|date=March 2010|title=Space Microbiology|journal=[[Biology:Microbiology and Molecular Biology Reviews|Microbiology and Molecular Biology Reviews]]|volume=74|issue=1|pages=121–156|doi=10.1128/mmbr.00016-09|last1=Horneck|first1=Gerda|pmid=20197502|pmc=2832349|bibcode=2010MMBR...74..121H}}</ref> Scientists are divided over the likelihood of life arising independently on Mars,<ref>{{cite news|url=http://www.space.com/4267-claim-martian-life-called-bogus.html|title=Claim of Martian Life Called 'Bogus'|last=Than|first=Ker|date=August 23, 2007|work=Space.com|publisher=Imaginova|location=Watsonville, CA|accessdate=2015-01-25}}</ref> or on other planets in our [[Astronomy:Galaxy|galaxy]].<ref name="Scientific-American-panspermia" />

== Environmental and evolutionary impact of microbial mats ==
{{main|Biology:Microbial mat|Earth:Great Oxygenation Event}}
[[File:Stromatolites in Sharkbay.jpg|right|thumb|Modern stromatolites in Shark Bay, Western Australia]]
Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms.<ref name="KrumbeinBrehmEtAl2003BiofilmBiodictyonBiomatEtc">{{harvnb|Krumbein|Brehm|Gerdes|Gorbushina|2003|pp=1–28}}</ref> To some extent each mat forms its own food chain, as the by-products of each group of microorganisms generally serve as "food" for adjacent groups.<ref name="RisattiEtAl1994MatCommunityStructure">{{cite journal |last1=Risatti |first1=J. Bruno |last2=Capman |first2=William C. |last3=Stahl |first3=David A. |date=October 11, 1994 |title=Community structure of a microbial mat: The phylogenetic dimension |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=91 |issue=21 |pages=10173–10177 |bibcode=1994PNAS...9110173R |doi=10.1073/pnas.91.21.10173 |pmc=44980 |pmid=7937858 }}</ref>

[[Earth:Stromatolite|Stromatolite]]s are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.<ref name="KrumbeinBrehmEtAl2003BiofilmBiodictyonBiomatEtc" /> There has been vigorous debate about the validity of alleged fossils from before 3 Ga,<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=June 8, 2006 |title=Biodiversity rocks |url=http://www.nature.com/nature/journal/v441/n7094/edsumm/e060608-01.html |journal=Nature |type=Editor's summary |volume=441 |issue=7094 |accessdate=2009-01-10}}</ref> with critics arguing that so-called stromatolites could have been formed by non-biological processes.<ref name="GrotzingerRothman1996AbioticModelForStromatolites" /> In 2006, another find of stromatolites was reported from the same part of Australia as previous ones, in rocks dated to 3.5 Ga.<ref>{{cite journal |last1=Allwood |first1=Abigail C. |last2=Walter |first2=Malcolm R. |last3=Kamber |first3=Balz S. |last4=Marshall |first4=Craig P. |last5=Burch |first5=Ian W. |date=June 8, 2006 |title=Stromatolite reef from the Early Archaean era of Australia |journal=Nature |volume=441 |pages=714–718 |bibcode=2006Natur.441..714A |doi=10.1038/nature04764 |issue=7094 |pmid=16760969 |display-authors=3 }}</ref><!-- previous version **** 
[[Earth:Stromatolite|Stromatolite]]s are stubby pillars built as microbes in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.<ref name="KrumbeinBrehmEtAl2003BiofilmBiodictyonBiomatEtc" /> Although earlier reports of fossilized stromatolites from about 3.5 Ga were criticized on the grounds that the structures in the rocks could have been produced by non-biological processes,<ref name="GrotzingerRothman1996AbioticModelForStromatolites" /> in 2006 another find of stromatolites was reported from the same part of Australia, in rocks also dated to 3.5 Ga.<ref>{{cite journal 
| author=Allwood, A.C., Walter, M.R., Kamber, B.S., Marshall1, C.P. and Burch, I.W.
| title= Stromatolite reef from the Early Archaean era of Australia
| journal=Nature | volume=441 | pages=714–718 | date=June 2006 | doi=10.1038/nature04764 
| url=http://www.nature.com/nature/journal/v441/n7094/abs/nature04764.html | accessdate=2008-08-31 
| pmid=16760969 
| issue=7094 
|bibcode = 2006Natur.441..714A }}</ref> **** -->

In modern underwater mats the top layer often consists of photosynthesizing [[Biology:Cyanobacteria|cyanobacteria]] which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there.<ref name="RisattiEtAl1994MatCommunityStructure" /> It is estimated that the appearance of [[Chemistry:Oxygen#Photosynthesis and respiration|oxygenic photosynthesis]] by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The [[Chemistry:Reducing agent|reducing agent]] used by oxygenic photosynthesis is water, which is much more plentiful than the geologically produced reducing agents required by the earlier non-oxygenic photosynthesis.<ref name="Blankenship2001EvolutionOfPhotosynthesis">{{cite journal |last=Blankenship |first=Robert E. |date=January 1, 2001 |title=Molecular evidence for the evolution of photosynthesis |url=  |journal=Trends in Plant Science |volume=6 |issue=1 |pages=4–6 |doi=10.1016/S1360-1385(00)01831-8 |pmid=11164357 }}</ref> From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes.<ref name="HoehlerBeboutMarais2001MicrobialMatsInProductionOfReducedGasesOnEarlyEarth">{{cite journal |last1=Hoehler |first1=Tori M. |last2=Bebout |first2=Brad M. |last3=Des Marais |first3=David J. |date=July 19, 2001 |title=The role of microbial mats in the production of reduced gases on the early Earth |journal=Nature |volume=412 |issue=6844 |pages=324–327 |doi=10.1038/35085554 |pmid=11460161 |bibcode=2001Natur.412..324H }}</ref> Oxygen is toxic to organisms that are not adapted to it, but greatly increases the [[Biology:Metabolism|metabolic]] efficiency of oxygen-adapted organisms.<ref name="Abele2002ToxicOxygen">{{cite journal |last=Abele |first=Doris |date=November 7, 2002 |title=Toxic oxygen: The radical life-giver |journal=Nature |volume=420 |issue=6911 |page=27 |bibcode=2002Natur.420...27A |doi=10.1038/420027a |pmid=12422197 |url=https://epic.awi.de/id/eprint/6249/1/Abe2002b.pdf }}</ref><ref>{{cite web |url=http://trc.ucdavis.edu/biosci10v/bis10v/week3/06aerobicrespirintro.html |title=Introduction to Aerobic Respiration |last=Westerdahl |first=Becky B. |year=2007 |website=Biological Science 10V |publisher=[[Organization:University of California, Davis|University of California, Davis]] |location=Davis, CA |type=Lecture |archiveurl=https://web.archive.org/web/20071029120120/http://trc.ucdavis.edu/biosci10v/bis10v/week3/06aerobicrespirintro.html |archivedate=2007-10-29 |url-status=dead |accessdate=2008-07-14}}
* Textbook used for lecture: ''Biology Today and Tomorrow With Physiology'' (2007), {{ISBN|0-495-01654-3}}.</ref> Oxygen became a significant component of Earth's atmosphere about 2.4 Ga.<ref>{{cite journal |last1=Goldblatt |first1=Colin |last2=Lenton |first2=Timothy M. |last3=Watson |first3=Andrew J. |authorlink3=Andrew Watson (scientist) |year=2006 |title=The Great Oxidation at ~2.4 Ga as a bistability in atmospheric oxygen due to UV shielding by ozone |url=http://www.cosis.net/abstracts/EGU06/00770/EGU06-J-00770.pdf |journal=Geophysical Research Abstracts |volume=8 |issue=770 |id=SRef-ID: 1607-7962/gra/EGU06-A-00770 |accessdate=2008-09-01 }}</ref> Although eukaryotes may have been present much earlier,<ref name="GlansdorffXuLabedan2008LUCA">{{cite journal |last1=Glansdorff |first1=Nicolas |author2=Ying Xu |last3=Labedan |first3=Bernard |date=July 9, 2008 |title=The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner |journal=[[Biology:Biology Direct|Biology Direct]] |volume=3 |issue=29 |page=29 |doi=10.1186/1745-6150-3-29 |pmc=2478661 |pmid=18613974 }}</ref><ref name="BrocksLoganEtAl1999RiseOfEukaryotes">{{cite journal |last1=Brocks |first1=Jochen J. |last2=Logan |first2=Graham A. |last3=Buick |first3=Roger |last4=Summons |first4=Roger E. |authorlink4=Roger Everett Summons |date=August 13, 1999 |title=Archean Molecular Fossils and the Early Rise of Eukaryotes |journal=Science |volume=285 |issue=5430 |pages=1033–1036 |doi=10.1126/science.285.5430.1033 |pmid=10446042 |citeseerx=10.1.1.516.9123 }}</ref> the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built.<ref name="HedgesBlairEtAl2004molecularTimescaleOfEukaryoteEvolution">{{cite journal |last1=Hedges |first1=S. Blair |authorlink1=Stephen Blair Hedges |last2=Blair |first2=Jaime E. |last3=Venturi |first3=Maria L. |last4=Shoe |first4=Jason L. |date=January 28, 2004 |title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life |journal=BMC Evolutionary Biology |volume=4 |page=2 |doi=10.1186/1471-2148-4-2 |pmc=341452 |pmid=15005799 }}</ref> The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created [[Biology:Evolutionary pressure|selection pressure]] for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via [[Biology:Endosymbiont|endosymbiosis]], where one organism lives inside another and both of them benefit from their association.<ref name="NisbetFowler1999ArchaeanMetabolicEvolution" />

Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient of the mat organisms and were well-adapted to strike out on their own both as floating mats and as the first of the [[Earth:Phytoplankton|phytoplankton]], providing the basis of most marine food chains.<ref name="NisbetFowler1999ArchaeanMetabolicEvolution" />

== Diversification of eukaryotes ==
<div style="width:40%; border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
| style= font-size:100%; line-height:130%
| label1=Eukarya
| 1={{clade
| label1=[[Biology:Diaphoretickes|Diaphoretickes]]
| 1={{clade
| label1=
| 1=[[Biology:Archaeplastida|Archaeplastida]] ([[Biology:Embryophyte|Land plant]]s, [[Biology:Green algae|green algae]], [[Biology:Red algae|red algae]], and [[Biology:Glaucophyte|glaucophyte]]s)
| label2=
| 2=[[Biology:Hacrobia|Hacrobia]]
| label3=
| 3=[[Biology:SAR supergroup|SAR]] (Stramenopiles, Alveolata, and [[Biology:Rhizaria|Rhizaria]])
}}
| label2=
| 2=[[Biology:Excavata|Excavata]]
| label3=[[Biology:Amorphea|Amorphea]]
| 3={{clade
| 1=[[Biology:Amoebozoa|Amoebozoa]]
| label2 =
| 2=[[Biology:Apusozoa|Sulcozoa]]
| label3=Opisthokonta
| 3={{clade
| label1=
| 1=Metazoa ([[Biology:Animal|Animal]]s)
| label2=
| 2=[[Biology:Fungi|Fungi]]
 }}
}} }} }}
</div>One possible family tree of eukaryotes<ref name="Adl2012">{{cite journal | last1 = Adl | first1 = Sina M. | display-authors = etal   | date = September 2012 | title = The revised classification of eukaryotes | url = http://www.paru.cas.cz/docs/documents/93-Adl-JEM-2012.pdf | journal = Journal of Eukaryotic Microbiology | volume = 59 | issue = 5| pages = 429–514 | doi=10.1111/j.1550-7408.2012.00644.x | pmid=23020233 | pmc=3483872}}</ref><ref name="burki2014">{{cite journal|author = Burki, F.|year = 2014|title = The eukaryotic tree of life from a global phylogenomic perspective|journal = Cold Spring Harbor Perspectives in Biology|pages = 1–17|doi = 10.1101/cshperspect.a016147|pmid = 24789819|volume = 6|issue = 5|pmc = 3996474}}</ref></div>
{{main|Biology:Eukaryote}}

===Chromatin, nucleus, endomembrane system, and mitochondria===
Eukaryotes may have been present long before the oxygenation of the atmosphere,<ref name="GlansdorffXuLabedan2008LUCA" /> but most modern eukaryotes require oxygen, which their [[Biology:Mitochondrion|mitochondria]] use to fuel the production of [[Chemistry:Adenosine triphosphate|ATP]], the internal energy supply of all known cells.<ref name="HedgesBlairEtAl2004molecularTimescaleOfEukaryoteEvolution" /> In the 1970s it was proposed and, after much debate, widely accepted that eukaryotes emerged as a result of a sequence of endosymbiosis between [[Biology:Prokaryote|prokaryote]]s. For example: a predatory microorganism invaded a large prokaryote, probably an [[Biology:Archaea|archaea]]n, but the attack was neutralized, and the attacker took up residence and evolved into the first of the mitochondria; one of these [[Biology:Chimera (genetics)|chimera]]s later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of [[Biology:Plant|plant]]s; and so on. After each endosymbiosis began, the partners would have eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them.<ref>{{harvnb|Margulis|1981}}</ref><ref>{{cite journal |last1=Vellai |first1=Tibor |last2=Vida |first2=Gábor |date=August 7, 1999 |title=The origin of eukaryotes: the difference between prokaryotic and eukaryotic cells |journal=Proceedings of the Royal Society B |volume=266 |issue=1428 |pages=1571–1577 |doi=10.1098/rspb.1999.0817 |pmc=1690172 |pmid=10467746 }}</ref><ref>{{cite journal |last1=Selosse |first1=Marc-André |last2=Abert |first2=Béatrice |last3=Godelle |first3=Bernard |date=March 1, 2001 |title=Reducing the genome size of organelles favours gene transfer to the nucleus |url=  |journal=Trends in Ecology & Evolution |volume=16 |issue=3 |pages=135–141 |doi=10.1016/S0169-5347(00)02084-X |pmid=11179577 }}</ref> Another hypothesis proposes that mitochondria were originally [[Chemistry:Sulfur|sulfur]]- or [[Chemistry:Hydrogen|hydrogen]]-metabolising endosymbionts, and became oxygen-consumers later.<ref>{{cite journal |last1=Pisani |first1=Davide |last2=Cotton |first2=James A. |last3=McInerney |first3=James O. |date=August 2007 |title=Supertrees Disentangle the Chimerical Origin of Eukaryotic Genomes |journal=[[Biology:Molecular Biology and Evolution|Molecular Biology and Evolution]] |volume=24 |issue=8 |pages=1752–1760 |doi=10.1093/molbev/msm095 |pmid=17504772 |doi-access=free }}</ref> On the other hand, mitochondria might have been part of eukaryotes' original equipment.<ref>{{cite journal |last1=Gray |first1=Michael W. |last2=Burger |first2=Gertraud |last3=Lang |first3=B. Franz |date=March 5, 1999 |title=Mitochondrial Evolution |journal=Science |volume=283 |issue=5407 |pages=1476–1481 |bibcode=1999Sci...283.1476G |doi=10.1126/science.283.5407.1476 |pmid=10066161 }}</ref>

There is a debate about when eukaryotes first appeared: the presence of [[Chemistry:Sterane|sterane]]s in Australian [[Chemistry:Shale|shale]]s may indicate that eukaryotes were present 2.7 Ga;<ref name="BrocksLoganEtAl1999RiseOfEukaryotes" /> however, an analysis in 2008 concluded that these chemicals infiltrated the rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes.<ref>{{cite journal |last1=Rasmussen |first1=Birger |last2=Fletcher |first2=Ian R. |last3=Brocks |first3=Jochen J. |last4=Kilburn |first4=Matt R. |date=October 23, 2008 |title=Reassessing the first appearance of eukaryotes and cyanobacteria |journal=Nature |volume=455 |issue=7216 |pages=1101–1104 |bibcode=2008Natur.455.1101R |doi=10.1038/nature07381 |pmid=18948954 }}</ref> Fossils of the [[Biology:Algae|algae]] ''[[Biology:Grypania|Grypania]]'' have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised<ref name="Fedonkin2003OriginOfMetazoa" />), and indicates that eukaryotes with organelles had already evolved.<ref>{{cite journal |author1=Tsu-Ming Han |last2=Runnegar |first2=Bruce |date=July 10, 1992 |title=Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan |journal=Science |volume=257 |issue=5067 |pages=232–235 |bibcode=1992Sci...257..232H |doi=10.1126/science.1631544 |pmid=1631544 }}</ref> A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga.<ref>{{cite journal |last1=Javaux |first1=Emmanuelle J. |last2=Knoll |first2=Andrew H. |last3=Walter |first3=Malcolm R. |date=July 2004 |title=TEM evidence for eukaryotic diversity in mid-Proterozoic oceans |journal=Geobiology |volume=2 |issue=3 |pages=121–132 |doi=10.1111/j.1472-4677.2004.00027.x }}</ref> The earliest known fossils of [[Biology:Fungus|fungi]] date from 1.43 Ga.<ref name="Butterfield2005ProterozoicFungi">{{cite journal |last=Butterfield |first=Nicholas J. |date=Winter 2005 |title=Probable Proterozoic fungi |url=http://paleobiol.geoscienceworld.org/content/31/1/165.abstract |journal=Paleobiology |volume=31 |issue=1 |pages=165–182 |doi=10.1666/0094-8373(2005)031<0165:PPF>2.0.CO;2 |accessdate=2015-01-30 }}</ref>

=== Plastids ===
[[Biology:Plastid|Plastid]]s, the superclass of [[Biology:Organelle|organelle]]s of which [[Biology:Chloroplast|chloroplast]]s are the best-known exemplar, are thought to have originated from endosymbiotic cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out [[Earth:Carbon fixation#Oxygenic photosynthesis|oxygenic photosynthesis]].<ref name="HedgesBlairEtAl2004molecularTimescaleOfEukaryoteEvolution" /> Three evolutionary lineages have since emerged in which the plastids are named differently: chloroplasts in [[Biology:Green algae|green algae]] and plants, rhodoplasts in [[Biology:Red algae|red algae]] and cyanelles in the [[Biology:Glaucophyte|glaucophyte]]s.

== Sexual reproduction and multicellular organisms ==

=== Evolution of sexual reproduction ===
{{Main|Biology:Evolution of sexual reproduction|Biology:Sexual reproduction}}
The defining characteristics of [[Biology:Sexual reproduction|sexual reproduction]] in eukaryotes are [[Biology:Meiosis|meiosis]] and fertilization. There is much [[Biology:Genetic recombination|genetic recombination]] in this kind of reproduction, in which offspring receive 50% of their genes from each parent,<ref name="Jokela2001SexAdvantage" /> in contrast with asexual reproduction, in which there is no recombination. Bacteria also exchange DNA by [[Biology:Bacterial conjugation|bacterial conjugation]], the benefits of which include resistance to antibiotics and other [[Biology:Toxin|toxin]]s, and the ability to utilize new [[Biology:Metabolite|metabolite]]s.<ref>{{harvnb|Holmes|Jobling|1996}}</ref> However, conjugation is not a means of reproduction, and is not limited to members of the same species&nbsp;– there are cases where bacteria transfer DNA to plants and animals.<ref name="Christie, P. J. 2001 294–305">{{cite journal |last=Christie |first=Peter J. |date=April 2001 |title=Type IV secretion: intercellular transfer of macromolecules by systems ancestrally related to conjugation machines |journal=Molecular Microbiology |volume=40 |issue=2 |pages=294–305 |doi=10.1046/j.1365-2958.2001.02302.x |pmid=11309113 |pmc=3922410 }}</ref>

On the other hand, bacterial transformation is clearly an adaptation for transfer of DNA between bacteria of the same species. Bacterial transformation is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex.<ref>{{cite journal |last1=Michod |first1=Richard E. |last2=Bernstein |first2=Harris |last3=Nedelcu |first3=Aurora M. |date=May 2008 |title=Adaptive value of sex in microbial pathogens |url=http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf |journal=[[Biology:Infection, Genetics and Evolution|Infection, Genetics and Evolution]] |volume=8 |issue=3 |pages=267–285 |doi=10.1016/j.meegid.2008.01.002 |pmid=18295550 }}</ref><ref>{{cite journal |last1=Bernstein |first1=Harris |last2=Bernstein |first2=Carol |date=July 2010|title=Evolutionary Origin of Recombination during Meiosis |journal=[[Biology:BioScience|BioScience]] |volume=60 |issue=7 |pages=498–505 |doi=10.1525/bio.2010.60.7.5  }}</ref> This process occurs naturally in at least 67 prokaryotic species (in seven different phyla).<ref>{{cite journal |last1=Johnsborg |first1=Ola |last2=Eldholm |first2=Vegard |last3=Håvarstein |first3=Leiv Sigve |date=December 2007 |title=Natural genetic transformation: prevalence, mechanisms and function |journal=Research in Microbiology |volume=158 |issue=10 |pages=767–778 |doi=10.1016/j.resmic.2007.09.004 |pmid=17997281 }}</ref> Sexual reproduction in eukaryotes may have evolved from bacterial transformation.<ref name="novapublishers.com">{{harvnb|Bernstein|Bernstein|Michod|2012|pp=1–50}}</ref> (Also see [[Biology:Evolution of sexual reproduction#Origin of sexual reproduction|Evolution of sexual reproduction]].)

The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect.<ref name="Jokela2001SexAdvantage">{{cite encyclopedia |last1=Neiman |first1=Maurine |title=Encyclopedia of Life Sciences |last2=Jokela |first2=Jukka |encyclopedia=[[Biology:Encyclopedia of Life Sciences|Encyclopedia of Life Sciences]] |year=2010 |publisher={{wipe|John Wiley & Sons}} |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1002/9780470015902.a0001716.pub2|chapter=Sex: Advantage }}</ref> Nevertheless, the great majority of animals, plants, fungi and [[Biology:Protist|protist]]s reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then.<ref>{{cite journal |last1=Ramesh |first1=Marilee A. |last2=Malik |first2=Shehre-Banoo |last3=Logsdon |first3=John M., Jr. |date=January 26, 2005 |title=A Phylogenomic Inventory of Meiotic Genes: Evidence for Sex in ''Giardia'' and an Early Eukaryotic Origin of Meiosis |url=http://euplotes.biology.uiowa.edu/web/jmlpubls/rml05.pdf |archive-url=https://web.archive.org/web/20050226071107/http://euplotes.biology.uiowa.edu/web/jmlpubls/rml05.pdf |url-status=dead |archive-date=February 26, 2005 |journal=[[Biology:Current Biology|Current Biology]] |volume=15 |issue=2 |pages=185–191 |doi=10.1016/j.cub.2005.01.003 |pmid=15668177 |accessdate=2008-12-22 |bibcode= }}</ref> How sexual reproduction evolved and survived is an unsolved puzzle.<ref name="OttoGerstein2006WhySex">{{cite journal |last1=Otto |first1=Sarah P. |last2=Gerstein |first2=Aleeza C. |date=August 2006 |title=Why have sex? The population genetics of sex and recombination |journal=[[Organization:Biochemical Society Transactions|Biochemical Society Transactions]] |volume=34 |issue=Pt 4 |pages=519–522 |doi=10.1042/BST0340519 |pmid=16856849 }}</ref>
[[File:Horodyskia per Fedonkin 2003.png|thumb|right|''Horodyskia'' may have been an early metazoan,<ref name="Fedonkin2003OriginOfMetazoa" /> or a colonial foraminiferan.<ref name="DongEtAl2008SilicifiedHorodyskia" /> It apparently re-arranged itself into fewer but larger main masses as the sediment grew deeper round its base.<ref name="Fedonkin2003OriginOfMetazoa" />]]
The [[Biology:Red Queen hypothesis|Red Queen hypothesis]] suggests that sexual reproduction provides protection against [[Biology:Parasite|parasite]]s, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical [[Biology:Cloning|clone]]s than those of sexual species that present moving targets, and there is some experimental evidence for this. However, there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species.<ref name="Jokela2001SexAdvantage" /> Furthermore, contrary to the expectations of the Red Queen hypothesis, Kathryn A. Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.<ref>{{cite journal |last1=Hanley |first1=Kathryn A. |last2=Fisher |first2=Robert N. |last3=Case |first3=Ted J. |date=June 1995 |title=Lower Mite Infestations in an Asexual Gecko Compared With Its Sexual Ancestors |journal=Evolution |volume=49 |issue=3 |pages=418–426 |doi=10.2307/2410266 |jstor=2410266 |pmid=28565091 }}</ref> In addition, biologist Matthew Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host.<ref>{{cite journal |last1=Parker |first1=Matthew A. |date=September 1994 |title=Pathogens and sex in plants |journal=Evolutionary Ecology |volume=8 |issue=5 |pages=560–584 |doi=10.1007/bf01238258 }}</ref>

[[Biography:Alexey Kondrashov|Alexey Kondrashov]]'s ''deterministic mutation hypothesis'' (DMH) assumes that each organism has more than one harmful mutation and the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the [[Biology:Gene pool|gene pool]] by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However, the evidence suggests that the DMH's assumptions are shaky because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of [[Biology:Synergy|synergy]] between harmful mutations.<ref name="Jokela2001SexAdvantage" /> (Further criticisms of this hypothesis are discussed in the article [[Biology:Evolution of sexual reproduction#Removal of deleterious genes|Evolution of sexual reproduction]])

The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This [[Biology:Genetic drift|genetic drift]] is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction.<ref name="OttoGerstein2006WhySex" /> Other combinations of hypotheses that are inadequate on their own are also being examined.<ref name="Jokela2001SexAdvantage" />
<!--
The following hypotheses attempt to explain how and why sex evolved:
*It may have enabled organisms to repair genetic damage.<ref name="dna-repair">{{cite journal
| author=Bernstein, H., Byerly, H., Hopf, F. and Michod, R. | title = Origin of sex | year = 1984
| journal = Journal of Theoretical Biology | volume=110 | issue=3 | pages = 323–351
| pmid = 6209512 | doi = 10.1016/S0022-5193(84)80178-2
}}</ref> The most primitive form of sex may have been one organism repairing damaged DNA by replicating an undamaged strand from a similar organism.<ref name="TatianaSexAdvice">{{cite book | author = Judson, O. | title = Dr. Tatiana's sex advice to all creation | url = https://archive.org/details/drtatianassexadv00juds | url-access = registration | publisher = Metropolitan Books | location = New York | year = 2002 | isbn = 0-8050-6331-5 | pages =[https://archive.org/details/drtatianassexadv00juds/page/233 233–4]}}</ref>
*Sexual reproduction may have originated from selfish parasitic genetic elements propagating themselves by transfer to new hosts.<ref name="hickey">{{cite journal | author = Hickey, D. | title = Selfish DNA: A Sexually-Transmitted Nuclear Parasite | journal = Genetics | volume = 101 | issue = 3–4 | pages = 519–31 | year = 1982 | pmid = 6293914 | pmc = 1201875}}</ref>
*It may have evolved from [[Biology:Cannibalism|cannibalism]], where some of the victim's DNA was incorporated into the cannibal organism.<ref name="TatianaSexAdvice"/>
*Sexual reproduction may have evolved from ancient [[Biology:Haloarchaea|haloarchaea]] through a combination of jumping genes, and swapping [[Biology:Plasmid|plasmid]]s.<ref>{{cite journal | title=Extreme Microbes | first1=S. | last1=DasSarma | doi = 10.1511/2007.65.1024 | journal=American Scientist | year=2007 | volume=95 |issue=3| pages= 224–231 }}</ref>
*Or it may have evolved as a form of [[Biology:Vaccination|vaccination]] in which infected hosts exchanged weakened [[Earth:Symbiosis|symbiotic]] copies of parasitic DNA as protection against more virulent versions. The [[Biology:Meiosis|meiosis]] stage of sexual reproduction may then have evolved as a way of removing the symbiotes.<ref name="Sterrer">{{cite journal | author = Sterrer, W. | title = On the origin of sex as vaccination | journal = Journal of Theoretical Biology | volume = 216 | pages = 387–96 | year = 2002 | doi = 10.1006/jtbi.2002.3008
| pmid = 12151256 | issue = 4
}}</ref>
[[Biology:Bacteria|Bacteria]] also exchange [[Biology:DNA|DNA]] by [[Biology:Bacterial conjugation|bacterial conjugation]], the benefits of which include resistance to antibiotics and other [[Biology:Toxin|toxin]]s, and the ability to utilize new [[Biology:Metabolite|metabolite]]s.<ref>{{cite book |last1=Holmes |first1=Randall K. |last2=Jobling |first2=Michael G. |year=1996 |chapter=Genetics |chapterurl=https://www.ncbi.nlm.nih.gov/books/NBK7908/ |editor-last=Baron |editor-first=Samuel |title=Medical Microbiology |edition=4th |location=Galveston, TX |publisher=[[Organization:University of Texas Medical Branch|University of Texas Medical Branch]] |at=Exchange of Genetic Information |isbn=0-9631172-1-1 |lccn=95050499 |oclc=33838234 |pmid=21413277 }}</ref> However, conjugation is not a means of reproduction and is not limited to members of the same species, and there are cases where bacteria transfer DNA to plants and animals.<ref name="Christie, P. J. 2001 294–305"/> Nevertheless, it may be an example of the "selfish genetic element" hypothesis, as it transfers DNA by means of such a "selfish gene," the [[Biology:F-plasmid|F-plasmid]].<ref name="TatianaSexAdvice"/>
-->

The adaptive function of sex today remains a major unresolved issue in biology. The competing models to explain the adaptive function of sex were reviewed by John A. Birdsell and [[Biography:Christopher Wills|Christopher Wills]].<ref>{{harvnb|Birdsell|Wills|2003|pp=27–137}}</ref> The hypotheses discussed above all depend on the possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose and is maintained, as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct.<ref name="novapublishers.com" /><ref>{{harvnb|Bernstein|Hopf|Michod|1987|pp=323–370}}</ref>

=== Multicellularity ===
{{main|Biology:Multicellular organism }}
The simplest definitions of "multicellular," for example "having multiple cells," could include [[Biology:Colony|colonial]] cyanobacteria like ''[[Biology:Nostoc|Nostoc]]''. Even a technical definition such as "having the same genome but different types of cell" would still include some [[Biology:Genus|genera]] of the green algae [[Biology:Volvox|Volvox]], which have cells that specialize in reproduction.<ref>{{cite journal |last1=Bell |first1=Graham |last2=Mooers |first2=Arne O. |year=1997 |title=Size and complexity among multicellular organisms |url=http://biology.mcgill.ca/faculty/bell/articles/70.BellMooers_1997_BJLS60.pdf |journal=[[Organization:Biological Journal of the Linnean Society|Biological Journal of the Linnean Society]] |volume=60 |issue=3|pages=345–363 |doi=10.1111/j.1095-8312.1997.tb01500.x |accessdate=2015-02-02 |ref=hav}}</ref> Multicellularity evolved independently in organisms as diverse as [[Biology:Sponge|sponge]]s and other animals, fungi, plants, [[Biology:Brown algae|brown algae]], cyanobacteria, [[Biology:Slime mold|slime mold]]s and [[Biology:Myxobacteria|myxobacteria]].<ref name="Fedonkin2003OriginOfMetazoa">{{cite journal |last=Fedonkin |first=Mikhail A. |authorlink=Mikhail Fedonkin |date=March 31, 2003 |title=The origin of the Metazoa in the light of the Proterozoic fossil record |url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |journal=Paleontological Research |volume=7 |issue=1 |pages=9–41 |doi=10.2517/prpsj.7.9 |accessdate=2008-09-02 |url-status=dead |archiveurl=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |archivedate=February 26, 2009 }}</ref><ref>{{cite journal |last=Kaiser |first=Dale |authorlink=A. Dale Kaiser |date=December 2001 |title=Building a multicellular organism |journal=Annual Review of Genetics |volume=35 |pages=103–123 |doi=10.1146/annurev.genet.35.102401.090145 |pmid=11700279 }}</ref> For the sake of brevity, this article focuses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the [[Biology:Evolution of biological complexity|evolution of biological complexity]] could be regarded as "rather [[Philosophy:Anthropocentrism|anthropocentric]]."<ref name="Bonner1999OriginsOfMulticellularity">{{cite journal |last=Bonner |first=John Tyler |authorlink=John Tyler Bonner |year=1998 |title=The origins of multicellularity |journal=[[Biology:Integrative Biology|Integrative Biology]] |volume=1 |issue=1 |pages=27–36 |doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 }}</ref>

[[File:Slime mold solves maze.png|thumb|right|A slime mold solves a maze. The mold (yellow) explored and filled the maze (left). When the researchers placed sugar (red) at two separate points, the mold concentrated most of its mass there and left only the most efficient connection between the two points (right).<ref name="NakagakiYamadaTóth2000Intelligence" />]]
The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell,<ref name="Koschwanezetal2011PLoS">{{cite journal |last1=Koschwanez |first1=John H. |last2=Foster |first2=Kevin R. |last3=Murray |first3=Andrew W. |date=August 9, 2011 |title=Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity |journal=PLOS Biology |volume=9 |issue=8 |pages=e1001122 |doi=10.1371/journal.pbio.1001122 |pmid=21857801 |pmc=3153487 }}</ref> increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis;<ref name="Butterfield2000Bangiomorpha" /> the ability to create an internal environment that gives protection against the external one;<ref name="Bonner1999OriginsOfMulticellularity" /> and even the opportunity for a group of cells to behave "intelligently" by sharing information.<ref name="NakagakiYamadaTóth2000Intelligence">{{cite journal |last1=Nakagaki |first1=Toshiyuki |last2=Yamada |first2=Hiroyasu |last3=Tóth |first3=Ágota |date=September 28, 2000 |title=Intelligence: Maze-solving by an amoeboid organism |journal=Nature |volume=407 |issue=6803 |pages=470 |doi=10.1038/35035159 |pmid=11028990 |bibcode=2000Natur.407..470N }}</ref> These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.<ref name="Butterfield2000Bangiomorpha" />

Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity.<ref name="Butterfield2000Bangiomorpha" />

The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eukaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function.<ref name="Butterfield2000Bangiomorpha" />

By comparing the composition of transcription factor families and regulatory network motifs between unicellular organisms and multicellular organisms, scientists found there are many novel transcription factor families and three novel types of regulatory network motifs in multicellular organisms, and novel family transcription factors are preferentially wired into these novel network motifs which are essential for multicullular development. These results propose a plausible mechanism for the contribution of novel-family transcription factors and novel network motifs to the origin of multicellular organisms at transcriptional regulatory level.<ref name="MBE_1767">{{cite journal |last1=Jin JP|display-authors=0| title= An Arabidopsis transcriptional regulatory map reveals distinct functional and evolutionary features of novel transcription factors|journal= Molecular Biology and Evolution | date=July 2015 |volume=32 |issue=7|doi=10.1093/molbev/msv058 |url=http://mbe.oxfordjournals.org/content/32/7/1767.full |pmid=25750178 |pmc=4476157 |pages=1767–1773}}</ref>

=== Fossil evidence ===
The [[Earth:Francevillian biota|Francevillian biota]] fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular.<ref name="albani"/> They may have had differentiated cells.<ref name="dickey">{{cite journal |last=Dickey |first=Gwyneth |date=July 31, 2010 |title=Evidence for earlier multicellular life |url=http://www.sciencenewsdigital.org/sciencenews/20100731?pg=19#pg19 |format=PNG |journal=Science News |volume=178 |issue=3 |page=17 |accessdate=2015-02-02 |doi=10.1002/scin.5591780322}}</ref> Another early multicellular fossil, ''Qingshania'', dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called ''[[Biology:Bangiomorpha|Bangiomorpha]]'', dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism.<ref name="Butterfield2000Bangiomorpha">{{cite journal |last=Butterfield |first=Nicholas J. |date=Summer 2000 |title=''Bangiomorpha pubescens'' n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |url=http://paleobiol.geoscienceworld.org/content/26/3/386.abstract |journal=Paleobiology |volume=26 |issue=3 |pages=386–404 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |accessdate=2015-02-01 }}</ref> The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells.<ref name="Butterfield2005ProterozoicFungi" /> The "string of beads" organism ''[[Biology:Horodyskia|Horodyskia]]'', found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan;<ref name="Fedonkin2003OriginOfMetazoa" /> however, it has also been interpreted as a colonial [[Biology:Foraminifera|foraminifera]]n.<ref name="DongEtAl2008SilicifiedHorodyskia">{{cite journal |author1=Lin Dong |author2=Shuhai Xiao |author3=Bing Shen |author4=Chuanming Zhou |date=January 2008 |title=Silicified ''Horodyskia'' and ''Palaeopascichnus'' from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis |url=http://jgs.geoscienceworld.org/content/165/1/367 |journal=Journal of the Geological Society |volume=165 |issue=1 |pages=367–378 |doi=10.1144/0016-76492007-074 |accessdate=2015-02-01 |bibcode=2008JGSoc.165..367D }}</ref>
{{clear}}

== Emergence of animals ==
<div style="width:40%; border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=&nbsp;&nbsp;&nbsp;
  |1={{clade
      |label1=&nbsp;&nbsp;&nbsp;
    |1={{clade
        |label1=&nbsp;&nbsp;&nbsp;
        |1={{clade
              |label1=&nbsp;&nbsp;&nbsp;
              |1={{clade
                  |label1=[[Biology:Bilateria|Bilateria]]ns
                  |1={{clade
                      |label1=&nbsp;&nbsp;&nbsp;
                      |1={{clade
                           |1=[[Biology:Deuterostome|Deuterostome]]s ([[Biology:Chordate|chordate]]s, [[Biology:Hemichordate|hemichordate]]s, [[Biology:Echinoderm|echinoderm]]s)
                           |label2=[[Biology:Protostome|Protostome]]s
                           |2={{clade
                               |label1=&nbsp;&nbsp;&nbsp;
                               |1=[[Biology:Ecdysozoa|Ecdysozoa]] ([[Biology:Arthropod|arthropod]]s, [[Biology:Nematode|nematode]]s, [[Biology:Tardigrade|tardigrade]]s, etc.)
                               |label2=&nbsp;&nbsp;&nbsp;
                               |2=[[Biology:Lophotrochozoa|Lophotrochozoa]] ([[Biology:Mollusca|mollusc]]s, [[Biology:Annelid|annelid]]s, [[Biology:Brachiopod|brachiopod]]s, etc.)
                              }}
                         }}
                      |label2=&nbsp;&nbsp;&nbsp;
                      |2=Acoelomorpha
                     }}
                  |2=[[Biology:Cnidaria|Cnidaria]] ([[Biology:Jellyfish|jellyfish]], [[Biology:Sea anemone|sea anemone]]s, [[Biology:Coral|coral]]s)
                  |3=[[Biology:Ctenophora|Ctenophora]] ([[Biology:Ctenophora|comb jellies]])
              }}
              |2=[[Biology:Placozoa|Placozoa]]
              |3=[[Biology:Sponge|Porifera]] (sponges): [[Biology:Calcareous sponge|Calcarea]]
             }}
          |2=[[Biology:Sponge|Porifera]]: [[Biology:Hexactinellid|Hexactinellid]]a & [[Biology:Demosponge|Demospongiae]]
         }}
      |2=[[Biology:Choanoflagellate|Choanoflagellata]]
      |3=[[Biology:Mesomycetozoea|Mesomycetozoea]]
     }}
}}
</div>A family tree of the animals<ref>{{cite journal |last1=Gaidos |first1=Eric J. |last2=Dubuc |first2=Timothy |last3=Dunford |first3=Mike |last4=McAndrew |first4=Patricia |last5=Padilla-Gamiño |first5=Jacqueline |last6=Studer |first6=Benjamin |last7=Weersing |first7=Kim |last8=Stanley |first8=Steven |date=December 2007 |title=The Precambrian emergence of animal life: a geobiological perspective |journal=Geobiology |volume=5 |issue=4 |pages=351–373 |doi=10.1111/j.1472-4669.2007.00125.x |display-authors=3 }}</ref></div>
Animals are multicellular eukaryotes,<ref group=note>[[Biology:Myxozoa|Myxozoa]] were thought to be an exception, but are now thought to be heavily modified members of the [[Biology:Cnidaria|Cnidaria]]. {{cite journal |last1=Jímenez-Guri |first1=Eva |last2=Philippe |first2=Hervé |last3=Okamura |first3=Beth |last4=Holland |first4=Peter W. H. |date=July 6, 2007 |title=''Buddenbrockia'' Is a Cnidarian Worm |journal=Science |volume=317 |issue=5834 |pages=116–118 |bibcode=2007Sci...317..116J |doi=10.1126/science.1142024 |pmid=17615357 }}</ref> and are distinguished from plants, algae, and fungi by lacking [[Biology:Cell wall|cell wall]]s.<ref>{{cite web |url=http://micro.magnet.fsu.edu/cells/animalcell.html |title=Animal Cell Structure |last=Davidson |first=Michael W. |date=May 26, 2005 |website=Molecular Expressions |publisher=[[Organization:Florida State University|Florida State University]] |location=Tallahassee, FL |accessdate=2008-09-03}}</ref> All animals are motile,<ref>{{cite web |url=http://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm |title=Concepts of Biology |last=Saupe |first=Stephen G. |date=January 3, 2004 |website=Concepts of Biology (BIOL116) |publisher=College of Saint Benedict and Saint John's University |location=St. Joseph, MN |type=Lecture |accessdate=2008-09-03}}</ref> if only at certain life stages. All animals except sponges have bodies differentiated into separate [[Biology:Tissue|tissue]]s, including [[Biology:Muscle|muscle]]s, which move parts of the animal by contracting, and [[Biology:Nervous system|nerve tissue]], which transmits and processes signals.<ref>{{harvnb|Hinde|2001|pp=28–57}}</ref> In November 2019, researchers reported the discovery of ''[[Biology:Caveasphaera|Caveasphaera]]'', a [[Biology:Multicellular organism|multicellular organism]] found in 609-million-year-old rocks, that is not easily defined as an animal or non-animal, which may be related to one of the earliest instances of animal evolution.<ref name="EA-20191127">{{cite news |author=Chinese Academy of Sciences |title=Researchers say animal-like embryos preceded animal appearance |url=https://www.eurekalert.org/pub_releases/2019-11/caos-rsa112719.php |date=27 November 2019 |work=EurekAlert! |accessdate=28 November 2019 |author-link=Chinese Academy of Sciences }}</ref><ref name="NYT-20191127">{{cite news |last=Zimmer |first=Carl |title=Is This the First Fossil of an Embryo? - Mysterious 609-million-year-old balls of cells may be the oldest animal embryos — or something else entirely. |url=https://www.nytimes.com/2019/11/27/science/fossil-embryo-paleontology-caveaspharea.html |date=27 November 2019 |work={{wipe|The New York Times}} |accessdate=28 November 2019 }}</ref> Fossil studies of ''Caveaspaera'' have suggested that animal-like embryonic development arose much earlier than the oldest clearly defined animal fossils.<ref name="EA-20191127" /> and may be consistent with studies suggesting that animal evolution may have begun about 750 million years ago.<ref name="NYT-20191127" /><ref name="BE-20161205">{{cite journal |author=Cunningham, John A. |display-authors=et al. |title=The origin of animals: Can molecular clocks and the fossil record be reconciled? |date=5 December 2016 |journal=[[Biology:BioEssays|BioEssays]] |volume=39 |issue=1 |pages=e201600120 |doi=10.1002/bies.201600120 |pmid=27918074 |doi-access=free }}</ref>

Nonetheless, the earliest widely accepted animal fossils are the rather modern-looking [[Biology:Cnidaria|cnidaria]]ns (the group that includes [[Biology:Jellyfish|jellyfish]], [[Biology:Sea anemone|sea anemone]]s and ''[[Biology:Hydra (genus)|Hydra]]''), possibly from around {{ma|580|Ma}}, although fossils from the [[Earth:Doushantuo Formation|Doushantuo Formation]] can only be dated approximately. Their presence implies that the cnidarian and [[Biology:Bilateria|bilateria]]n lineages had already diverged.<ref>{{cite journal |author1=Jun-Yuan Chen |last2=Oliveri |first2=Paola |author3=Feng Gao |last4=Dornbos |first4=Stephen Q. |author5=Chia-Wei Li |last6=Bottjer |first6=David J. |last7=Davidson |first7=Eric H. |date=August 1, 2002 |title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China |url=https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |journal=Developmental Biology |volume=248 |issue=1 |pages=182–196 |doi=10.1006/dbio.2002.0714 |pmid=12142030 |accessdate=2015-02-04 |display-authors=3 |url-status=dead |archiveurl=https://web.archive.org/web/20130526203745/https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |archivedate=May 26, 2013 }}</ref>

The Ediacara biota, which flourished for the last 40 million years before the start of the [[Earth:Cambrian|Cambrian]],<ref name="Grazhdankin2004">{{cite journal |last=Grazhdankin |first=Dima |date=June 2004 |title=Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution |journal=Paleobiology |volume=30 |issue=2 |pages=203–221 |doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 }}</ref> were the first animals more than a very few centimetres long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate [[Biology:Kingdom|kingdom]], Vendozoa.<ref name="Seilacher1992">{{cite journal |last=Seilacher |first=Adolf |date=August 1992 |title=Vendobionta and Psammocorallia: lost constructions of Precambrian evolution |url=http://jgs.lyellcollection.org/content/149/4/607.abstract |journal=Journal of the Geological Society |volume=149 |issue=4 |pages=607–613 |doi=10.1144/gsjgs.149.4.0607 |accessdate=2015-02-04 |bibcode=1992JGSoc.149..607S }}</ref> Others, however, have been interpreted as early [[Biology:Mollusca|mollusc]]s (''[[Biology:Kimberella|Kimberella]]''<ref name="Martin2000">{{cite journal |last1=Martin |first1=Mark W. |last2=Grazhdankin |first2=Dmitriy V. |last3=Bowring |first3=Samuel A. |last4=Evans |first4=David A. D. |last5=Fedonkin |first5=Mikhail A. |last6=Kirschvink |first6=Joseph L. |date=May 5, 2000 |title=Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution |journal=Science |volume=288 |issue=5467 |pages=841–845 |doi=10.1126/science.288.5467.841 |bibcode=2000Sci...288..841M |pmid=10797002 |display-authors=3 }}</ref><ref name="FedonkinWaggoner1997KimberellaMollusc">{{cite journal |last1=Fedonkin |first1=Mikhail A. |last2=Waggoner |first2=Benjamin M. |date=August 28, 1997 |title=The late Precambrian fossil ''Kimberella'' is a mollusc-like bilaterian organism |journal=Nature |volume=388 |issue=6645 |pages=868–871 |bibcode=1997Natur.388..868F |doi=10.1038/42242 }}</ref>), [[Biology:Echinoderm|echinoderm]]s (''[[Biology:Arkarua|Arkarua]]''<ref>{{cite journal |last1=Mooi |first1=Rich |last2=David |first2=Bruno |date=December 1998 |title=Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms |url=http://icb.oxfordjournals.org/content/38/6/965.full.pdf |journal=American Zoologist |volume=38 |issue=6 |pages=965–974 |doi=10.1093/icb/38.6.965 |accessdate=2015-02-05 }}</ref>), and [[Biology:Arthropod|arthropod]]s (''[[Biology:Spriggina|Spriggina]]'',<ref>{{cite conference |url=https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |title=''Spriggina'' is a trilobitoid ecdysozoan |first=Mark A. S. |last=McMenamin |authorlink=Mark McMenamin |date=September 2003 |conference=Geoscience Horizons Seattle 2003 |conference-url=https://www.geosociety.org/meetings/2003/ |volume=35 |issue=6 |series=Abstracts with Programs |publisher=Geological Society of America |location=Boulder, CO |page=105 |oclc=249088612 |accessdate=2007-11-24 |archive-url=https://web.archive.org/web/20160412064305/https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |archive-date=2016-04-12 |url-status=dead }} Paper No. 40-2 presented at the Geological Society of America's 2003 Seattle Annual Meeting (November 2–5, 2003) on November 2, 2003, at the Washington State Convention Center.</ref> ''[[Biology:Parvancorina|Parvancorina]]''<ref>{{cite journal |author1=Jih-Pai Lin |last2=Gon |first2=Samuel M., III |last3=Gehling |first3=James G. |last4=Babcock |first4=Loren E. |author5=Yuan-Long Zhao |author6=Xing-Liang Zhang |author7=Shi-Xue Hu |author8=Jin-Liang Yuan |author9=Mei-Yi Yu |author10=Jin Peng |display-authors=3 |year=2006 |title=A ''Parvancorina''-like arthropod from the Cambrian of South China |journal=Historical Biology: An International Journal of Paleobiology |volume=18 |issue=1 |pages=33–45 |doi=10.1080/08912960500508689 }}</ref>). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that ''Kimberella'' was at least a [[Biology:Triploblasty|triploblastic]] bilaterian animal, in other words, an animal significantly more complex than the cnidarians.<ref name="Butterfield2006">{{cite journal |last=Butterfield |first=Nicholas J. |date=December 2006 |title=Hooking some stem-group 'worms': fossil lophotrochozoans in the Burgess Shale |journal=[[Biology:BioEssays|BioEssays]] |volume=28 |issue=12 |pages=1161–1166 |doi=10.1002/bies.20507 |pmid=17120226 }}</ref>

The [[Biology:Small shelly fauna|small shelly fauna]] are a very mixed collection of fossils found between the Late Ediacaran and Middle Cambrian periods. The earliest, ''Cloudina'', shows signs of successful defense against predation and may indicate the start of an [[Biology:Evolutionary arms race|evolutionary arms race]]. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates," ''[[Biology:Halkieriid|Halkieria]]'' and ''[[Biology:Microdictyon|Microdictyon]]'', were eventually identified when more complete specimens were found in Cambrian lagerstätten that preserved soft-bodied animals.<ref name="Bengtson2004">{{harvnb|Bengtson|2004|pp=67–78}}</ref>

[[File:Opabinia BW2.jpg|thumb|left|''Opabinia'' made the largest single contribution to modern interest in the Cambrian explosion.<ref>{{harvnb|Gould|1989|pp=124–136}}</ref>]]
In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of [[Earth:Precambrian|Precambrian]] animal fossils.<ref name="Bengtson2004" /> A re-analysis of fossils from the [[Earth:Burgess Shale|Burgess Shale]] lagerstätte increased interest in the issue when it revealed animals, such as ''[[Biology:Opabinia|Opabinia]]'', which did not fit into any known [[Biology:Phylum|phylum]]. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.<ref>{{harvnb|Gould|1989}}</ref> Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups<ref name="Budd2003">{{cite journal |last=Budd |first=Graham E. |date=February 2003 |title=The Cambrian Fossil Record and the Origin of the Phyla |url=http://icb.oxfordjournals.org/content/43/1/157.full.pdf+html |format=PDF |journal=Integrative and Comparative Biology |volume=43 |issue=1 |pages=157–165 |doi=10.1093/icb/43.1.157 |pmid=21680420 |accessdate=2015-02-06 |doi-access=free }}</ref>—for example that ''Opabinia'' was a member of the [[Biology:Lobopodia|lobopod]]s, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern [[Biology:Tardigrade|tardigrade]]s.<ref name="Budd1996">{{cite journal |last=Budd |first=Graham E. |date=March 1996 |title=The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group |journal=Lethaia |volume=29 |issue=1 |pages=1–14 |doi=10.1111/j.1502-3931.1996.tb01831.x }}</ref> Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.<ref name="Marshall2006Explaining">{{cite journal |last=Marshall |first=Charles R. |s2cid=85623607 |date=May 2006 |title=Explaining the Cambrian 'Explosion' of Animals |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=355–384 |bibcode=2006AREPS..34..355M |doi=10.1146/annurev.earth.33.031504.103001 }}</ref>
{{clear}}

===Deuterostomes and the first vertebrates===
[[File:Acanthodes BW.jpg|thumb|right|[[Biology:Acanthodii|Acanthodian]]s were among the earliest vertebrates with jaws.<ref>{{cite encyclopedia |last1=Janvier |first1=Philippe |title=Encyclopedia of Life Sciences |encyclopedia=Encyclopedia of Life Sciences |year=2001 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0001531|chapter=Vertebrata (Vertebrates) }}</ref> ]]
Most of the animals at the heart of the Cambrian explosion debate are [[Biology:Protostome|protostome]]s, one of the two main groups of complex animals. The other major group, the [[Biology:Deuterostome|deuterostome]]s, contains [[Biology:Invertebrate|invertebrate]]s such as [[Biology:Starfish|starfish]] and [[Biology:Sea urchin|sea urchin]]s (echinoderms), as well as [[Biology:Chordate|chordate]]s (see below). Many  echinoderms have hard [[Chemistry:Calcite|calcite]] "shells," which are fairly common from the Early Cambrian small shelly fauna onwards.<ref name="Bengtson2004" /> Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the Chengjiang fauna, a lagerstätte in {{wipe|China}}.<ref>{{cite journal |last=Conway Morris |first=Simon |date=August 2, 2003 |title=Once we were worms |url=http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm |journal=New Scientist |volume=179 |issue=2406 |page=34 |archiveurl=https://web.archive.org/web/20080725103609/http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm |archivedate=2008-07-25 |accessdate=2008-09-05 }}</ref> The chordates are another major deuterostome group: animals with a distinct dorsal nerve cord. Chordates include soft-bodied invertebrates such as [[Biology:Tunicate|tunicate]]s as well as vertebrates—animals with a backbone. While tunicate fossils predate the Cambrian explosion,<ref name="Jun-Yuan2003">{{cite journal|author1=Jun-Yuan Chen |author2=Di-Ying Huang |author3=Qing-Qing Peng |author4=Hui-Mei Chi |author5=Xiu-Qiang Wang |author6=Man Feng |date=July 8, 2003 |title=The first tunicate from the Early Cambrian of South China |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=100 |issue=14 |pages=8314–8318 |bibcode=2003PNAS..100.8314C |doi=10.1073/pnas.1431177100 |pmc=166226 |pmid=12835415 |display-authors=3 }}</ref> the Chengjiang fossils ''[[Biology:Haikouichthys|Haikouichthys]]'' and ''[[Biology:Myllokunmingia|Myllokunmingia]]'' appear to be true vertebrates,<ref name="Shu et al. 1999" /> and ''Haikouichthys'' had distinct vertebrae, which may have been slightly [[Earth:Biomineralization|mineralized]].<ref>{{cite journal |author1=D.-G. Shu |last2=Conway Morris |first2=Simon |author3=J. Han |author4=Z.-F. Zhang |author5=K. Yasui |last6=Janvier |first6=Philippe |author7=L. Chen |author8=X.-L. Zhang |author9=J.-N. Liu |author10=Y. Li |author11=H.-Q. Liu |date=January 30, 2003 |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |journal=Nature |volume=421 |issue=6922 |pages=526–529 |bibcode=2003Natur.421..526S |doi=10.1038/nature01264 |pmid=12556891 |display-authors=3 }}</ref> Vertebrates with [[Medicine:Jaw|jaw]]s, such as the [[Biology:Acanthodii|acanthodian]]s, first appeared in the Late [[Earth:Ordovician|Ordovician]].<ref>{{harvnb|Sansom|Smith|Smith|2001|pp=156–171}}</ref>

== Colonization of land ==
Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity; [[Biology:Respiration (physiology)|respiration]] and [[Physics:Gas exchange|gas exchange]] systems have to change; reproductive systems cannot depend on water to carry [[Biology:Egg|egg]]s and [[Biology:Sperm|sperm]] towards each other.<ref name="CowenHistLifeEd3P120To122">{{harvnb|Cowen|2000|pp=120–122}}</ref><ref name="Selden2001TerrestrializationOfAnimals" /><ref name="Garwood">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y|doi-access=free }}</ref> Although the earliest good evidence of land plants and animals dates back to the Ordovician period ({{ma|488|444|Ma}}), and a number of microorganism lineages made it onto land much earlier,<ref name="Battistuzzi2004">{{cite journal |last1=Battistuzzi |first1=Fabia U. |last2=Feijao |first2=Andreia |last3=Hedges |first3=S. Blair |date=November 9, 2004 |title=A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land |journal=BMC Evolutionary Biology |volume=4 |page=44 |doi=10.1186/1471-2148-4-44 |pmc=533871 |pmid=15535883}}</ref><ref>{{cite book|last1=Beraldi-Campesi|first1=Hugo|last2=Retallack|first2=Gregory J.|title=Biological Soil Crusts: An Organizing Principle in Drylands|publisher=Springer International Publishing|isbn=9783319302126|pages=37–54|year=2016|doi=10.1007/978-3-319-30214-0_3|language=en|chapter=Terrestrial Ecosystems in the Precambrian|quote=terrestrial ecosystems were indeed present, full of life, and functional since the Archean}}</ref> modern land [[Earth:Ecosystem|ecosystem]]s only appeared in the Late [[Earth:Devonian|Devonian]], about {{ma|385|359|Ma}}.<ref name="Shear2000EarlyTerrestrialEcosystems" /> In May 2017, evidence of the [[Biology:Earliest known life forms#Earliest life forms|earliest known life on land]] may have been found in 3.48-billion-year-old [[Earth:Geyserite|geyserite]] and other related mineral deposits (often found around [[Earth:Hot spring|hot spring]]s and [[Earth:Geyser|geyser]]s) uncovered in the [[Earth:Pilbara Craton|Pilbara Craton]] of Western Australia.<ref name="PO-20170509">{{cite news |author=Staff |title=Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks |url=https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html |date=9 May 2017 |work=Phys.org |accessdate=13 May 2017 }}</ref><ref name="NC-20170509">{{cite journal |last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits |date=9 May 2017 |journal=Nature Communications |volume=8 |pages=15263 |doi=10.1038/ncomms15263 |pmid=28486437 |pmc=5436104 |bibcode=2017NatCo...815263D }}</ref> In July 2018, scientists reported that the earliest life on land may have been [[Biology:Bacteria|bacteria]] living on land 3.22 billion years ago.<ref name="NG-20180723">{{cite journal |author=Homann, Martin |display-authors=et al. |title=Microbial life and biogeochemical cycling on land 3,220 million years ago |date=23 July 2018 |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=11 |issue=9 |pages=665–671 |doi=10.1038/s41561-018-0190-9 |bibcode=2018NatGe..11..665H }}</ref> In May 2019, scientists reported the discovery of a [[Biology:Fossil|fossil]]ized [[Biology:Fungus|fungus]], named ''[[Biology:Ourasphaira giraldae|Ourasphaira giraldae]]'', in the [[Place:Northern Canada|Canadian Arctic]], that may have grown on land a billion years ago, well before [[Biology:Plant|plant]]s were living on land.<ref name="NYT-20190522">{{cite news |last=Zimmer |first=Carl |title=How Did Life Arrive on Land? A Billion-Year-Old Fungus May Hold Clues - A cache of microscopic fossils from the Arctic hints that fungi reached land long before plants. |url=https://www.nytimes.com/2019/05/22/science/fungi-fossils-plants.html |date=22 May 2019 |work={{wipe|The New York Times}} |accessdate=23 May 2019 }}</ref><ref name="NAT-20190522">{{cite journal |last1=Loron |first1=Corentin C. |last2=François |first2=Camille |last3=Rainbird |first3=Robert H. |last4=Turner |first4=Elizabeth C. |last5=Borensztajn |first5=Stephan |last6=Javaux |first6=Emmanuelle J. |title=Early fungi from the Proterozoic era in Arctic Canada |journal=Nature |volume=570 |issue=7760 |pages=232–235 |publisher=[[Company:Springer Science+Business Media|Springer Science and Business Media LLC]] |date=22 May 2019 |issn=0028-0836 |doi=10.1038/s41586-019-1217-0 |pmid=31118507 |bibcode=2019Natur.570..232L }}</ref><ref name="Ars Technica 2019">{{cite web |last=Timmer |first=John |title=Billion-year-old fossils may be early fungus |website=[[Ars Technica]] |date=22 May 2019 |url=https://arstechnica.com/science/2019/05/billion-year-old-fossils-may-be-early-fungus/ |access-date=23 May 2019}}</ref>

=== Evolution of terrestrial antioxidants ===
Oxygen is a potent [[Chemistry:Oxidizing agent|oxidant]] whose accumulation in terrestrial atmosphere resulted from the development of [[Earth:Photosynthesis|photosynthesis]] over 3 Ga, in cyanobacteria (blue-green algae), which were the most primitive oxygenic photosynthetic organisms. Brown algae accumulate inorganic mineral [[Biology:Antioxidant|antioxidant]]s such as [[Chemistry:Rubidium|rubidium]], [[Chemistry:Vanadium|vanadium]], [[Chemistry:Zinc|zinc]], iron, [[Chemistry:Copper|copper]], [[Chemistry:Molybdenum|molybdenum]], [[Chemistry:Selenium|selenium]] and [[Chemistry:Iodine|iodine]] which is concentrated more than 30,000 times the concentration of this element in seawater. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. Most marine mineral antioxidants act in the cells as essential [[Biology:Trace element|trace element]]s in [[Chemistry:Redox|redox]] and antioxidant [[Biology:Metalloprotein|metalloenzymes]].

When plants and animals began to enter rivers and land about 500 Ma, environmental deficiency of these marine mineral antioxidants was a challenge to the evolution of terrestrial life.<ref>{{cite journal |last=Venturi |first=Sebastiano |date=September 2011 |title=Evolutionary Significance of Iodine |journal=Current Chemical Biology |volume=5 |issue=3 |pages=155–162 |doi=10.2174/187231311796765012 }}</ref><ref>{{cite journal |last=Crockford |first=Susan J. |date=August 2009 |title=Evolutionary roots of iodine and thyroid hormones in cell-cell signaling |journal=Integrative and Comparative Biology |volume=49 |issue=2 |pages=155–166 |doi=10.1093/icb/icp053 |pmid=21669854 |doi-access=free }}</ref> Terrestrial plants slowly optimized the production of “new” endogenous antioxidants such as [[Chemistry:Ascorbic acid|ascorbic acid]], [[Chemistry:Polyphenol|polyphenol]]s, [[Chemistry:Flavonoid|flavonoid]]s, [[Chemistry:Tocopherol|tocopherol]]s, etc. A few of these appeared more recently, in last 200–50 Ma, in [[Biology:Fruit|fruit]]s and [[Biology:Flower|flower]]s of [[Biology:Angiosperm|angiosperm]] plants.

In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the Late Jurassic period. Plants employ antioxidants to defend their structures against [[Biology:Reactive oxygen species|reactive oxygen species]] produced during photosynthesis. Animals are exposed to the same oxidants, and they have evolved endogenous enzymatic antioxidant systems.<ref>{{cite journal |last1=Venturi |first1=Sebastiano |last2=Donati |first2=Francesco M. |last3=Venturi |first3=Alessandro |last4=Venturi |first4=Mattia |date=August 2000 |title=Environmental Iodine Deficiency: A Challenge to the Evolution of Terrestrial Life? |journal=Thyroid |volume=10 |issue=8 |pages=727–729 |doi=10.1089/10507250050137851 |pmid=11014322 }}</ref> [[Chemistry:Iodine|Iodine]] is the most primitive and abundant electron-rich essential element in the diet of marine and terrestrial organisms, and as iodide acts as an [[Chemistry:Electron donor|electron donor]] and has this ancestral antioxidant function in all iodide-concentrating cells from primitive marine algae to more recent terrestrial vertebrates.<ref>{{cite journal |last1=Küpper |first1=Frithjof C. |last2=Carpenter |first2=Lucy J. |last3=McFiggans |first3=Gordon B. |last4=Palmer |first4=Carl J. |last5=Waite |first5=Tim J. |last6=Boneberg |first6=Eva-Maria |last7=Woitsch |first7=Sonja |last8=Weiller |first8=Markus |last9=Abela |first9=Rafael |date=May 13, 2008 |title=Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=105 |issue=19 |pages=6954–6958 |bibcode=2008PNAS..105.6954K |doi=10.1073/pnas.0709959105 |pmc=2383960 |pmid=18458346 |display-authors=3 }}</ref>

=== Evolution of soil ===
Before the colonization of land, [[Chemistry:Soil|soil]], a combination of mineral particles and decomposed [[Chemistry:Organic matter|organic matter]], did not exist. Land surfaces would have been either bare rock or unstable sand produced by [[Earth:Weathering|weathering]]. Water and any nutrients in it would have drained away very quickly.<ref name="Shear2000EarlyTerrestrialEcosystems" /> In the [[Earth:Sub-Cambrian peneplain|Sub-Cambrian peneplain]] in Sweden for example maximum depth of kaolinitization by [[Earth:Neoproterozoic|Neoproterozoic]] [[Earth:Weathering|weathering]] is about 5 m, in contrast nearby kaolin deposits developed in the [[Earth:Mesozoic|Mesozoic]] are [[Earth:Sub-Mesozoic hilly peneplains|much thicker]].<ref name=Lidmar-Bergstrometal2013>{{cite journal |last1=Lidmar-Bergström |first1=Karna |last2=Bonow |first2=Johan M. |last3=Japsen |first3=Peter |date=2013 |title=Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence |journal=[[Earth:Global and Planetary Change|Global and Planetary Change]] |volume=100 |issue= |pages=153–171 |doi= 10.1016/j.gloplacha.2012.10.015|bibcode=2013GPC...100..153L }}</ref> It has been argued that in the late Neoproterozoic  [[Earth:Sheet erosion|sheet wash]] was a dominant process of erosion of surface material due to the [[Biology:Evolutionary history of plants|lack of plants]] on land.<ref name=Karna1993>{{cite journal |last1=Lidmar-Bergström |first1=Karna |author-link1=Karna Lidmar-Bergström |date=1993 |title=Denudation surfaces and tectonics in the southernmost part of the Baltic Shield |url= |journal=Precambrian Research |volume=64 |issue= 1–4|pages=337–345 |doi=10.1016/0301-9268(93)90086-H |access-date= |bibcode=1993PreR...64..337L }}</ref>
[[File:Lichen.jpg|thumb|right|Lichens growing on concrete]]
Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts, and only in areas that are unsuitable for [[Biology:Vascular plant|vascular plant]]s. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to [[Earth:Intertidal zone|intertidal zone]]s and then to land.<ref name="Shear2000EarlyTerrestrialEcosystems">Shear 2000, "The Early Development of Terrestrial Ecosystems," pp. [https://books.google.com/?id=ZJe_Dmdbm-QC&pg=PA169 169–184]</ref> [[Biology:Lichen|Lichen]]s, which are [[Earth:Symbiosis|symbiotic]] combinations of a fungus (almost always an [[Biology:Ascomycota|ascomycete]]) and one or more photosynthesizers (green algae or cyanobacteria),<ref name="Hawksworth2001Lichens">{{cite encyclopedia |last=Hawksworth |first=David L. |encyclopedia=Encyclopedia of Life Sciences |year=2002 |publisher=John Wiley & Sons |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0000368|chapter=Lichens |title=Encyclopedia of Life Sciences }}</ref> are also important colonizers of lifeless environments,<ref name="Shear2000EarlyTerrestrialEcosystems" /> and their ability to break down rocks contributes to [[Earth:Pedogenesis|soil formation]] in situations where plants cannot survive.<ref name="Hawksworth2001Lichens" /> The earliest known ascomycete fossils date from {{ma|423|419|Ma}} in the [[Earth:Silurian|Silurian]].<ref name="Shear2000EarlyTerrestrialEcosystems" />

Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose [[Biology:Feces|feces]] are a major source of the organic components.<ref name="Shear2000EarlyTerrestrialEcosystems" /> Burrows have been found in Ordovician sediments, and are attributed to [[Biology:Annelid|annelid]]s ("worms") or arthropods.<ref name="Shear2000EarlyTerrestrialEcosystems" /><ref>{{cite journal |last1=Retallack |first1=Gregory J. |last2=Feakes |first2=Carolyn R. |date=January 2, 1987 |title=Trace Fossil Evidence for Late Ordovician Animals on Land |journal=Science |volume=235 |issue=4784 |pages=61–63 |bibcode=1987Sci...235...61R |doi=10.1126/science.235.4784.61 |pmid=17769314 }}</ref>

=== Plants and the Late Devonian wood crisis ===
{{main|Biology:Evolutionary history of plants}}
[[File:Cooksonia pertoni.png|thumb|150px|right|Reconstruction of ''[[Biology:Cooksonia|Cooksonia]]'', a vascular plant from the Silurian]]
[[File:Gilboa.jpg|thumb|150px|right|Fossilized trees from the Middle Devonian Gilboa Fossil Forest]]
In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land required plants to become internally more complex and specialized: photosynthesis was most efficient at the top; roots were required in order to extract water from the ground; the parts in between became supports and transport systems for water and nutrients.<ref name="CowenHistLifeEd3P120To122" /><ref name="KenrickCrane1997EvolutionOfLandPlants" />

Spores of land plants, possibly rather like [[Biology:Marchantiophyta|liverworts]], have been found in Middle Ordovician rocks dated to about {{ma|476|Ma|Ordovician}}. In [[Earth:Wenlock (Silurian)|Middle Silurian]] rocks {{ma|430|Ma|Silurian}}, there are fossils of actual plants including [[Biology:Lycopodiopsida|clubmoss]]es such as ''[[Biology:Baragwanathia|Baragwanathia]]''; most were under {{convert|10|cm|in}} high, and some appear closely related to [[Biology:Vascular plant|vascular plant]]s, the group that includes [[Earth:Tree|tree]]s.<ref name="KenrickCrane1997EvolutionOfLandPlants">{{cite journal |last1=Kenrick |first1=Paul |last2=Crane |first2=Peter R. |date=September 4, 1997 |title=The origin and early evolution of plants on land |url=http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%207/land%20plants.pdf |journal=Nature |volume=389 |issue=6646 |pages=33–39 |bibcode=1997Natur.389...33K |doi=10.1038/37918 |accessdate=2015-02-10 }}</ref>

By the Late Devonian {{ma|370|Ma|Devonian}}, trees such as ''[[Biology:Archaeopteris|Archaeopteris]]'' were so abundant that they changed river systems from mostly [[Earth:Braided river|braided]] to mostly [[Earth:Meander|meander]]ing, because their roots bound the soil firmly.<ref>Scheckler 2001, "Afforestation—the First Forests," pp. 67–70</ref> In fact, they caused the "Late Devonian wood crisis"<ref>The phrase "Late Devonian wood crisis" is used at {{cite web |url=http://palaeos.com/vertebrates/tetrapoda/acanthostega.html |title=Tetrapoda: ''Acanthostega'' |website=Palaeos |accessdate=2015-02-10}}</ref> because:
* They removed more carbon dioxide from the atmosphere, reducing the [[Earth:Greenhouse effect|greenhouse effect]] and thus causing an [[Earth:Ice age|ice age]] in the [[Earth:Carboniferous|Carboniferous]] period.<ref name="AlgeoScheckler1998errestrialMarineTeleconnectionsInDevonian">{{cite journal |last1=Algeo |first1=Thomas J. |last2=Scheckler |first2=Stephen E. |date=January 29, 1998 |title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events |journal=Philosophical Transactions of the Royal Society B |volume=353 |issue=1365 |pages=113–130 |doi=10.1098/rstb.1998.0195 |pmc=1692181 }}</ref> In later ecosystems the carbon dioxide "locked up" in wood is returned to the atmosphere by decomposition of dead wood. However, the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian.<ref>{{cite journal |last1=Taylor |first1=Thomas N. |last2=Osborn |first2=Jeffrey M. |date=February 1996 |title=The importance of fungi in shaping the paleoecosystem |journal=Review of Palaeobotany and Palynology |volume=90 |issue=3–4 |pages=249–262 |doi=10.1016/0034-6667(95)00086-0 }}</ref>
* The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused [[Biology:Algal bloom|algal bloom]]s whose high consumption of oxygen caused [[Earth:Anoxic event|anoxic event]]s in deeper waters, increasing the extinction rate among deep-water animals.<ref name="AlgeoScheckler1998errestrialMarineTeleconnectionsInDevonian" />

=== Land invertebrates ===
Animals had to change their feeding and [[Biology:Excretion|excretory]] systems, and most land animals developed internal fertilization of their eggs.<ref name="Garwood" /> The difference in [[Physics:Refractive index|refractive index]] between water and air required changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in air encouraged the development of [[Physics:Hearing|hearing]].<ref name="Selden2001TerrestrializationOfAnimals" />
[[File:AnimalsRelativeNumbers.png|thumb|300px|right|The relative number of species contributed to the total by each phylum of animals. Nematoda is the phylum with the most individual organisms while arthropod has the most species.]]
The oldest known air-breathing animal is ''[[Biology:Pneumodesmus|Pneumodesmus]]'', an [[Biology:Archipolypoda|archipolypoda]]n [[Biology:Millipede|millipede]] from the Middle Silurian, about {{ma|428|Ma}}.<ref name=Wilson2004>{{cite journal |last1=Wilson |first1=Heather M. |last2=Anderson |first2=Lyall I. |date=January 2004 |title=Morphology and taxonomy of Paleozoic millipedes (Diplopoda: Chilognatha: Archipolypoda) from Scotland |url=http://jpaleontol.geoscienceworld.org/content/78/1/169.abstract |journal=Journal of Paleontology |volume=78 |issue=1 |pages=169–184 |doi=10.1666/0022-3360(2004)078<0169:MATOPM>2.0.CO;2  }}</ref><ref name="SeldenRead2008">{{cite journal |last1=Selden |first1=Paul |last2=Read |first2=Helen J. |year=2008 |title=The Oldest Land Animals: Silurian Millipedes from Scotland |url=http://www.paulselden.net/uploads/7/5/3/2/7532217/seldenread2008.pdf |journal=Bulletin of the British Myriapod & Isopod Group |volume=23 |pages=36–37 |accessdate=2015-07-21 }}</ref> Its air-breathing, terrestrial nature is evidenced by the presence of [[Biology:Spiracle (arthropods)|spiracle]]s, the openings to [[Biology:Trachea#Invertebrates|tracheal systems]].<ref name=Shear2010>{{cite journal |last1=Shear |first1=William A. |last2=Edgecombe |first2=Gregory D. |date=March–May 2010 |title=The geological record and phylogeny of the Myriapoda |journal=Arthropod Structure & Development |volume=39 |issue=2–3 |pages=174–190 |doi=10.1016/j.asd.2009.11.002 |pmid=19944188 }}</ref> However, some earlier [[Biology:Trace fossil|trace fossil]]s from the Cambrian-Ordovician boundary about {{ma|490|Ma|Cambrian}} are interpreted as the tracks of large [[Biology:Amphibian|amphibious]] arthropods on coastal [[Earth:Dune|sand dune]]s, and may have been made by [[Biology:Euthycarcinoidea|euthycarcinoid]]s,<ref>{{cite journal |last1=MacNaughton |first1=Robert B. |last2=Cole |first2=Jennifer M. |last3=Dalrymple |first3=Robert W. |last4=Braddy |first4=Simon J. |last5=Briggs |first5=Derek E. G. |last6=Lukie |first6=Terrence D. |date=May 2002 |title=First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada |journal=Geology |volume=30 |issue=5 |pages=391–394 |bibcode=2002Geo....30..391M |doi=10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2 |display-authors=3 }}</ref> which are thought to be evolutionary "aunts" of myriapods.<ref>{{cite journal |last1=Vaccari |first1=N. Emilio |last2=Edgecombe |first2=Gregory D. |last3=Escudero |first3=C. |date=July 29, 2004 |title=Cambrian origins and affinities of an enigmatic fossil group of arthropods |journal=Nature |volume=430 |issue=6999 |pages=554–557 |bibcode=2004Natur.430..554V |doi=10.1038/nature02705 |pmid=15282604 }}</ref> Other trace fossils from the Late Ordovician a little over {{ma|445|Ma|Ordovician}} probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and [[Earth:Alluvial plain|alluvial plain]]s shortly before the Silurian-Devonian boundary, about {{ma|415|Ma|Silurian}}, including signs that some arthropods ate plants.<ref>{{cite journal |last1=Buatois |first1=Luis A. |last2=Mangano |first2=M. Gabriela |last3=Genise |first3=Jorge F. |last4=Taylor |first4=Thomas N. |date=June 1998 |title=The Ichnologic Record of the Continental Invertebrate Invasion: Evolutionary Trends in Environmental Expansion, Ecospace Utilization, and Behavioral Complexity |url=http://palaios.sepmonline.org/content/13/3/217.abstract |archive-url=https://archive.today/20130415204251/http://palaios.sepmonline.org/content/13/3/217.abstract |url-status=dead |archive-date=2013-04-15 |journal=PALAIOS |volume=13 |issue=3 |pages=217–240 |doi=10.2307/3515447 |jstor=3515447 |accessdate=2015-02-11 |bibcode=1998Palai..13..217B }}</ref> Arthropods were well pre-adapted to colonise land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.<ref name="Garwood" /><ref name="CowenHistLifeEd3P126">{{harvnb|Cowen|2000|p=126}}</ref>

The [[Biology:Fossil#Fossil record|fossil record]] of other major invertebrate groups on land is poor: none at all for non-[[Earth:Parasitism|parasitic]] [[Biology:Flatworm|flatworm]]s, [[Biology:Nematode|nematode]]s or [[Biology:Nemertea|nemertea]]ns; some parasitic nematodes have been fossilized in [[Earth:Amber|amber]]; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of [[Biology:Gastropoda|gastropod]]s on land date from the Late Carboniferous, and this group may have had to wait until [[Biology:Plant litter|leaf litter]] became abundant enough to provide the moist conditions they need.<ref name="Selden2001TerrestrializationOfAnimals">Selden 2001, "Terrestrialization of Animals," pp. 71–74</ref>

The earliest confirmed fossils of flying [[Biology:Insect|insect]]s date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of [[Biology:Ecological niche|ecological niche]]s for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment.<ref>{{harvnb|Grimaldi|Engel|2005|pp=155–160}}</ref> About 99% of modern insect species fly or are descendants of flying species.<ref>{{harvnb|Grimaldi|Engel|2005|p=12}}</ref>

=== Early land vertebrates ===
{{main|Biology:Tetrapod|Biology:Evolution of tetrapods}}
[[File:Acanthostega BW.jpg|thumb|left|''Acanthostega'' changed views about the early evolution of tetrapods.<ref name="Clack2005GettingLegUp" />]]
<div style="border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
| label1="Fish"
| 1={{clade
     |1=[[Biology:Osteolepiformes|Osteolepiformes]]
     |2={{clade
         | 1=[[Biology:Elpistostegalia|Panderichthyida]]e
         | 2={{clade
              |1=[[Biology:Obruchevichthys|Obruchevichthidae]]
              | 2={{clade
                   |1=''[[Biology:Acanthostega|Acanthostega]]''
                   | 2={{clade
                        |1=''[[Biology:Ichthyostega|Ichthyostega]]''
                        | 2={{clade
                             |1=''[[Biology:Tulerpeton|Tulerpeton]]''
                             |2={{clade
                                 |1=Early [[Biology:Labyrinthodontia|labyrinthodont]]s
                                 |2={{clade
                                    |1=[[Biology:Anthracosauria|Anthracosauria]]
                                    |2=[[Biology:Amniote|Amniote]]s
                                    }}
                                }}
                            }}
                       }}
                  }}
             }}
        }}
    }}
}}
</div>Family tree of [[Biology:Tetrapod|tetrapod]]s<ref name="AhlbergMilner1994OriginOfTetrapods" /></div>
[[Biology:Tetrapod|Tetrapod]]s, vertebrates with four limbs, evolved from other [[Biology:Rhipidistia|rhipidistia]]n fish over a relatively short timespan during the Late Devonian ({{ma|370|360|Ma|Devonian}}).<ref>{{cite journal |last1=Gordon |first1=Malcolm S. |last2=Graham |first2=Jeffrey B. |last3=Wang |first3=Tobias |date=September–October 2004 |title=Introduction to the Special Collection: Revisiting the Vertebrate Invasion of the Land |journal=Physiological and Biochemical Zoology |volume=77 |issue=5 |pages=697–699 |doi=10.1086/425182 }}</ref> The early groups are grouped together as [[Biology:Labyrinthodontia|Labyrinthodontia]]. They retained aquatic, fry-like [[Biology:Tadpole|tadpole]]s, a system still seen in [[Biology:Lissamphibia|modern amphibians]].

Iodine and T4/T3  stimulate the amphibian metamorphosis and the evolution of nervous systems transforming the aquatic, vegetarian tadpole into  a “more evoluted” terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting.<ref>{{Cite journal|author=Venturi, Sebastiano|title=Evolutionary Significance of Iodine|journal=Current Chemical Biology|volume=5 |pages=155–162|year=2011|doi=10.2174/187231311796765012|issue=3}}</ref>
The new hormonal action of T3 was made possible by the formation of T3-receptors in the cells of vertebrates. Firstly, about 600-500 million years ago, in primitive Chordata appeared the alpha T3-receptors with a metamorphosing action and then, about 250-150 million years ago, in the Birds and Mammalia appeared the beta T3-receptors with metabolic and thermogenetic actions.<ref name=Venturi>{{cite book | last = Venturi | first = S |author2=Bégin ME | year = 2010 | chapter = Thyroid Hormone, Iodine and Human Brain Evolution | title= Environmental Influences on Human Brain Evolution |editor= Cunnane S |editor2=Stewart K | publisher = {{wipe|John Wiley & Sons}} | pages = [https://books.google.com/books?id=gfkRnv20GtsC&pg=PA105#v=onepage&q&f=false 105–124] | isbn = 978-0-470-45268-4}}</ref>

From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of ''[[Biology:Acanthostega|Acanthostega]]'' from about {{ma|363|Ma|Devonian}} showed that this Late Devonian [[Biology:Transitional fossil|transitional]] animal had [[Biology:Leg|leg]]s and both [[Medicine:Lung|lung]]s and [[Biology:Gill|gill]]s, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that ''Acanthostega'', which was about {{convert|1|m|ft}} long, was a wholly aquatic predator that hunted in shallow water<!-- , using its limbs to hold on to vegetation while it lay in ambush -->. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the [[Biology:Shoulder girdle|shoulder girdle]] and it has a distinct neck.<ref name="Clack2005GettingLegUp">{{cite journal |last=Clack |first=Jennifer A. |date=December 2005 |title=Getting a Leg Up on Land |journal=Scientific American |volume=293 |issue=6 |pages=100–107 |doi=10.1038/scientificamerican1205-100 |pmid=16323697|bibcode=2005SciAm.293f.100C }}</ref>

The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less [[Biology:Oxygen saturation|dissolved oxygen]] than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.<ref name="Clack2005GettingLegUp" />

Later discoveries revealed earlier transitional forms between ''Acanthostega'' and completely fish-like animals.<ref>{{cite journal |last1=Daeschler |first1=Edward B. |last2=Shubin |first2=Neil H. |last3=Jenkins |first3=Farish A., Jr. |authorlink3=Farish Jenkins |date=April 6, 2006 |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |url=http://courses.washington.edu/bio354/Paper%203.pdf |journal=Nature |volume=440 |pages=757–763 |issue=7085 |bibcode=2006Natur.440..757D |doi=10.1038/nature04639 |pmid=16598249 |accessdate=2015-02-12 }}</ref> Unfortunately, there is then a gap ([[Biology:Romer's gap|Romer's gap]]) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land. Some of these look like early relatives of modern amphibians, most of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the [[Biology:Amniote|amniote]]s, whose waterproof skin and egg membranes enable them to live and breed far from water.<ref name="AhlbergMilner1994OriginOfTetrapods">{{cite journal |last1=Ahlberg |first1=Per E. |authorlink1=Per E. Ahlberg |last2=Milner |first2=Andrew R. |date=April 7, 1994 |title=The origin and early diversification of tetrapods |journal=Nature |volume=368 |issue=6471 |pages=507–514 |bibcode=1994Natur.368..507A |doi=10.1038/368507a0 }}</ref>

{{clear}}

== Dinosaurs, birds and mammals ==
{{main|Biology:Evolution of dinosaurs|Biology:Origin of birds|Biology:Evolution of mammals}}

<div style="border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Biology:Amniote|Amniote]]s
 |1={{clade
    |label1=[[Biology:Synapsid|Synapsid]]s
    |1={{clade
        |1=Early synapsids (extinct)
        |label2=[[Biology:Pelycosaur|Pelycosaur]]s
        |2={{clade
            |1=Extinct pelycosaurs
            |label2=Therapsids
            |2={{clade
                |1=Extinct therapsids
                |label2=[[Biology:Mammaliaformes|Mammaliaformes]]
                |2={{clade
                    |label1=&nbsp;&nbsp;&nbsp;
                    |1=Extinct mammaliaforms
                    |label2=&nbsp;&nbsp;&nbsp;
                    |2=[[Biology:Mammal|Mammal]]s
                   }}
               }}
           }}
       }}
    |label2=[[Biology:Sauropsida|Sauropsid]]s
    |2={{clade
        |1={{clade
            |1=[[Biology:Anapsid|Anapsid]]s; whether [[Biology:Turtle|turtle]]s belong here is debated<ref>{{cite journal 
|last1=deBraga |first1=Michael |last2=Rieppel |first2=Olivier |date=July 1997 |title=Reptile phylogeny and the interrelationships of turtles |journal=Zoological Journal of the Linnean Society |volume=120 |issue=3 |pages=281–354 |doi=10.1111/j.1096-3642.1997.tb01280.x |doi-access=free }}</ref>
            |label2=&nbsp;&nbsp;&nbsp;
            |2={{clade
               |1=[[Biology:Captorhinidae|Captorhinidae]] and [[Biology:Protorothyrididae|Protorothyrididae]] (extinct)
               |label2=[[Biology:Diapsid|Diapsid]]s
               |2={{clade
                   |1=[[Biology:Araeoscelidia|Araeoscelidia]] (extinct)
                   |label2=&nbsp;&nbsp;&nbsp;
                   |2={{clade
                       |label1=&nbsp;&nbsp;&nbsp;
                       |1=[[Biology:Squamata|Squamata]] ([[Biology:Lizard|lizard]]s and [[Biology:Snake|snake]]s)
                       |label2=[[Biology:Archosaur|Archosaur]]s
                       |2={{clade
                           |1=Extinct archosaurs
                           |2=[[Biology:Crocodilia|Crocodilia]]ns
                           |label3=&nbsp;&nbsp;&nbsp;
                           |3={{clade
                               |1=[[Biology:Pterosaur|Pterosaur]]s (extinct)
                               |label2=[[Biology:Dinosaur|Dinosaur]]s
                               |2={{clade
                                   |label11=[[Biology:Saurischia|Saurischia]]ns
                                   |1={{clade
                                       |label1=[[Biology:Theropoda|Theropod]]s
                                       |1={{clade 
                                           |label1=&nbsp;&nbsp;&nbsp;
                                           |1=Extinct<br />theropods
                                           |label2=&nbsp;&nbsp;&nbsp;
                                           |2=[[Biology:Bird|Bird]]s
                                          }}
                                       |2=[[Biology:Sauropoda|Sauropod]]s<br />(extinct)
                                      }}
                                   |label2=&nbsp;&nbsp;&nbsp;
                                   |2=[[Biology:Ornithischia|Ornithischia]]ns (extinct)
                                  }}
                              }}
                          }}
                      }}
                  }}
               }}
           }}
       }}
   }}
}}
</div>Possible family tree of [[Biology:Dinosaur|dinosaur]]s, [[Biology:Bird|bird]]s and [[Biology:Mammal|mammal]]s<ref name="BentonDonoghue2007DateTreeOfLife" /><ref name="Benton1999PhylogenyOfMajorTetrapodGroups" /></div>
Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period ({{ma|330|Permian|Ma}}). The earliest fossils of the two surviving amniote groups, [[Biology:Synapsid|synapsid]]s and sauropsids, date from around {{ma|313|Ma|Carboniferous}}.<ref name="BentonDonoghue2007DateTreeOfLife">{{cite journal |last1=Benton |first1=Michael J. |last2=Donoghue |first2=Philip C. J. |date=January 2007 |title=Paleontological Evidence to Date the Tree of Life |url=http://mbe.oxfordjournals.org/content/24/1/26.full |journal=Molecular Biology and Evolution |volume=24 |issue=1 |pages=26–53 |doi=10.1093/molbev/msl150 |pmid=17047029 |accessdate=2015-02-12 |doi-access=free }}</ref><ref name="Benton1999PhylogenyOfMajorTetrapodGroups">{{cite journal |last=Benton |first=Michael J. |date=May 1990 |title=Phylogeny of the Major Tetrapod Groups: Morphological Data and Divergence Dates |journal=Journal of Molecular Evolution |volume=30 |issue=5 |pages=409–424 |doi=10.1007/BF02101113 |pmid=2111854 |bibcode=1990JMolE..30..409B }}</ref> The synapsid [[Biology:Pelycosaur|pelycosaur]]s and their descendants the [[Biology:Therapsid|therapsid]]s are the most common land vertebrates in the best-known Permian ({{ma|Permian|Triassic|Ma}}) fossil beds. However, at the time these were all in temperate zones at middle [[Earth:Latitude|latitude]]s, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians.<ref>{{cite journal |last1=Sidor |first1=Christian A. |authorlink1=Christian Sidor |last2=O'Keefe |first2=F. Robin |last3=Damiani |first3=Ross |last4=Steyer |first4=J. Sébastien |last5=Smith |first5=Roger M. H. |last6=Larsson |first6=Hans C. E. |last7=Sereno |first7=Paul C. |authorlink7=Paul Sereno |last8=Ide |first8=Oumarou |last9=Maga |first9=Abdoulaye |date=April 14, 2005 |title=Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea |journal=Nature |volume=434 |issue=7035 |pages=886–889 |bibcode=2005Natur.434..886S|doi=10.1038/nature03393 |pmid=15829962 |display-authors=3 }}</ref>

The [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]] wiped out almost all land vertebrates,<ref>{{cite journal |last1=Smith |first1=Roger |last2=Botha |first2=Jennifer |date=September–October 2005 |title=The recovery of terrestrial vertebrate diversity in the South African Karoo Basin after the end-Permian extinction  |journal=Comptes Rendus Palevol |volume=4 |issue=6–7 |pages=623–636 |doi=10.1016/j.crpv.2005.07.005 }}</ref> as well as the great majority of other life.<ref>{{harvnb|Benton|2005}}</ref> During the slow recovery from this catastrophe, estimated to have taken 30 million years,<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |last1=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |date=April 7, 2008 |title=Recovery from the most profound mass extinction of all time |journal=Proceedings of the Royal Society B |volume=275 |pages=759–765 |issue=1636 |doi=10.1098/rspb.2007.1370 |pmc=2596898 |pmid=18198148 }}</ref> a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of [[Biology:Archosauriformes|archosauriformes]] ("ruling lizard forms") have been found in Late Permian rocks,<ref>{{harvnb|Gauthier|Cannatella|de Queiroz|Kluge|1989|p=[https://repository.si.edu/bitstream/handle/10088/4689/VZ_1989GauthieretalHierLife.pdf 345]}}</ref> but, by the [[Earth:Middle Triassic|Middle Triassic]], archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods ({{ma|Jurassic|Paleogene|Ma}}).<ref>{{cite journal |last=Benton |first=Michael J. |date=March 1983 |title=Dinosaur Success in the Triassic: A Noncompetitive Ecological Model |url=http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf |journal=[[Biology:The Quarterly Review of Biology|The Quarterly Review of Biology]] |volume=58 |issue=1 |pages=29–55 |jstor=2828101 |accessdate=2008-09-08 |doi=10.1086/413056 |archive-url=https://web.archive.org/web/20080911075351/http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf |archive-date=2008-09-11 |url-status=dead }}</ref>

During the Late Jurassic, birds evolved from small, predatory theropod dinosaurs.<ref name="padian2004">{{harvnb|Padian|2004|pp=210–231}}</ref> The first birds inherited teeth and long, bony tails from their dinosaur ancestors,<ref name="padian2004" /> but some had developed horny, toothless beaks by the very Late Jurassic<ref>{{cite journal |author1=Lian-hai Hou |author2=Zhonghe Zhou |last3=Martin |first3=Larry D. |authorlink3=Larry Martin |last4=Feduccia |first4=Alan |authorlink4=Alan Feduccia |date=October 19, 2002 |title=A beaked bird from the Jurassic of China |journal=Nature |volume=377 |pages=616–618 |issue=6550 |bibcode=1995Natur.377..616H |doi=10.1038/377616a0 }}</ref> and short [[Biology:Pygostyle|pygostyle]] tails by the [[Earth:Early Cretaceous|Early Cretaceous]].<ref>{{cite journal |last1=Clarke |first1=Julia A. |author2=Zhonghe Zhou |author3=Fucheng Zhang |date=March 2006 |title=Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui''|journal=Journal of Anatomy |volume=208 |issue=3 |pages=287–308 |doi=10.1111/j.1469-7580.2006.00534.x |pmc=2100246 |pmid=16533313 }}</ref>

<!-- Nice pics but causing layout problems ******** {| align="right" border="0" cellspacing="0" cellpadding="3"
|- valign="top"
|[[File:Yanoconodon BW.jpg| thumb| 150px | The Early Cretaceous [[Yanoconodon]] was only about {{convert|13|cm|in}} long, but was longer than the average mammal of its time.<ref name="NatureLCLC">{{cite journal 
| author=Luo, Z., Chen, P., Li, G. and Chen, M. | date=March 2007 
| title=A new eutriconodont mammal and evolutionary development in early mammals 
| journal=Nature | volume=446 | pages=288–293 | doi=10.1038/nature05627 
| pmid=17361176 
| issue=7133
|bibcode = 2007Natur.446..288L }}</ref> ]]
|[[File:House sparrow04.jpg|thumb|right|Birds are the last surviving dinosaurs.<ref name="padian2004" /> ]]
|} **** -->
While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the [[Biology:Mammaliaformes|mammaliaform]] successors of the therapsids evolved into small, mainly nocturnal [[Biology:Insectivore|insectivore]]s. This ecological role may have promoted the [[Biology:Evolution of mammals|evolution of mammals]], for example nocturnal life may have accelerated the development of [[Biology:Endotherm|endotherm]]y ("warm-bloodedness") and hair or fur.<ref name="RubenJones2000FurAndFeathers">{{cite journal |last1=Ruben |first1=John |last2=Jones |first2=Terry D. |date=August 2000 |title=Selective Factors Associated with the Origin of Fur and Feathers |url=http://icb.oxfordjournals.org/content/40/4/585.full |journal=American Zoologist |volume=40 |issue=4 |pages=585–596 |doi=10.1093/icb/40.4.585 |accessdate=2015-02-16 |doi-access=free }}</ref> By {{ma|195|Ma|Jurassic}} in the Early Jurassic there were animals that were very like today's mammals in a number of respects.<ref>{{cite journal |author1=Zhe-Xi Luo |last2=Crompton |first2=Alfred W. |author3=Ai-Lin Sun |date=May 25, 2001 |title=A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics |journal=Science |volume=292 |issue=5521 |pages=1535–1540 |bibcode=2001Sci...292.1535L |doi=10.1126/science.1058476 |pmid=11375489 }}</ref> Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic.<ref name="Cifelli2001">{{cite journal |last=Cifelli |first=Richard L. |date=November 2001 |title=Early mammalian radiations |url=http://jpaleontol.geoscienceworld.org/content/75/6/1214.extract |journal=Journal of Paleontology |doi=10.1666/0022-3360(2001)075<1214:EMR>2.0.CO;2 |volume=75 |issue=6 |pages=1214–1226 |accessdate=2015-02-16  }}</ref> However, fossil teeth discovered in {{wipe|Madagascar}} indicate that the split between the lineage leading to [[Biology:Monotreme|monotreme]]s and the one leading to other living mammals had occurred by {{ma|167|Ma|Jurassic}}.<ref>{{cite journal |last1=Flynn |first1=John |last2=Parrish |first2=J. Michael |last3=Rakotosamimanana |first3=Berthe |last4=Simpson |first4=William F. |last5=Wyss |first5=André R. |authorlink5=Andre Wyss |date=September 2, 1999 |title=A Middle Jurassic mammal from Madagascar |journal=Nature |volume=401 |issue=6748 |pages=57–60 |bibcode=1999Natur.401...57F |doi=10.1038/43420 |display-authors=3 }}</ref> After dominating land vertebrate niches for about 150 Ma, the non-avian dinosaurs perished in the Cretaceous–Paleogene extinction event ({{ma|Paleogene|Ma}}) along with many other groups of organisms.<ref name="MacLeod">{{cite journal |last1=MacLeod |first1=Norman |last2=Rawson |first2=Peter F. |last3=Forey |first3=Peter L. |last4=Banner |first4=Frederick T. |last5=Boudagher-Fadel |first5=Marcelle K. |last6=Bown |first6=Paul R. |last7=Burnett |first7=Jackie A. |last8=Chambers |first8=Paul |last9=Culver |first9=Stephen |last10=Evans |first10=Susan E. |last11=Jeffery |first11=Charlotte |last12=Kaminski |first12=Michael A. |last13=Lord |first13=Allan R. |last14=Milner |first14=Angela C. |last15=Milner |first15=Andrew R. |last16=Morris |first16=Noel |last17=Owen |first17=Ellis |last18=Rosen |first18=Brian R. |last19=Smith |first19=Andrew B. |last20=Taylor |first20=Paul D. |last21=Urquhart |first21=Elspeth |last22=Young |first22=Jeremy R. |date=April 1997 |title=The Cretaceous–Tertiary biotic transition |url=http://jgs.geoscienceworld.org/content/154/2/265.abstract |journal=Journal of the Geological Society |volume=154 |issue=2 |pages=265–292 |doi=10.1144/gsjgs.154.2.0265 |accessdate=2015-02-16 |display-authors=3 |bibcode=1997JGSoc.154..265M }}</ref> Mammals throughout the time of the dinosaurs had been restricted to a narrow range of [[Biology:Taxon|taxa]], sizes and shapes, but increased rapidly in size and diversity after the extinction,<ref>{{cite journal |last=Alroy |first=John |authorlink=John Alroy |date=March 1999 |title=The Fossil Record of North American Mammals: Evidence for a Paleocene Evolutionary Radiation |journal=Systematic Biology |volume=48 |issue=1 |pages=107–118 |doi=10.1080/106351599260472 |pmid=12078635 |doi-access=free }}</ref><ref>{{cite journal |last1=Archibald |first1=J. David |last2=Deutschman |first2=Douglas H. |date=June 2001 |title=Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders |url=http://www.bio.sdsu.edu/faculty/archibald.html/ArchDeut01JME8p107.pdf |journal=Journal of Mammalian Evolution |volume=8 |issue=2 |pages=107–124 |doi=10.1023/A:1011317930838 |accessdate=2015-02-16 |url-status=dead |archiveurl=https://web.archive.org/web/20150217234632/http://www.bio.sdsu.edu/faculty/archibald.html/ArchDeut01JME8p107.pdf |archivedate=2015-02-17 }}</ref> with [[Biology:Bat|bat]]s taking to the air within 13&nbsp;million years,<ref>{{cite journal |last1=Simmons |first1=Nancy B. |last2=Seymour |first2=Kevin L. |last3=Habersetzer |first3=Jörg |last4=Gunnell |first4=Gregg F. |date=February 14, 2008 |title=Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation |journal=Nature |volume=451 |pages=818–821 |issue=7180 |bibcode=2008Natur.451..818S |doi=10.1038/nature06549 |pmid=18270539 |hdl=2027.42/62816 |hdl-access=free }}</ref> and [[Biology:Cetacea|cetacea]]ns to the sea within 15&nbsp;million years.<ref>{{harvnb|Thewissen|Madar|Hussain|1996}}</ref>
{{clear}}

== Flowering plants ==
{{main|Biology:Flowering plant|Biology:Gymnosperm}}
{| align="right" border="0" cellspacing="0" cellpadding="3"
|- valign="top"
|<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=[[Biology:Gymnosperm|Gymnosperm]]s
  |1={{clade
      |1={{clade     
          |1={{clade     
              |1={{clade     
                  |1=[[Biology:Gnetum|Gnetales]]<br />(gymnosperm)
                  |2=''[[Biology:Welwitschia|Welwitschia]]''<br />(gymnosperm)
                 }}
              |2=''[[Biology:Ephedra (plant)|Ephedra]]''<br />(gymnosperm)
             }}
          |2=[[Biology:Bennettitales|Bennettitales]]
          |3=[[Biology:Angiosperm|Angiosperm]]s<br />(flowering plants)
         }}
     }}
}}
</div>One possible family tree of [[Biology:Flowering plant|flowering plant]]s<ref name="CraneFriisPedersen2000OriginAndDiversificationOfAngiosperms" /></div>
|<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=[[Biology:Gymnosperm|Gymnosperm]]s
  |1={{clade
      |1=[[Biology:Angiosperm|Angiosperm]]s<br />(flowering plants)
      |2={{clade
           |1={{clade
                |1={{clade
                    |1=[[Biology:Cycad|Cycad]]s<br />(gymnosperm)
                    |2=[[Biology:Bennettitales|Bennettitales]]
                  }}
               |2=''[[Biology:Ginkgo|Ginkgo]]''
             }}
          |2={{clade
              |1=[[Biology:Gnetales|Gnetales]]<br />(gymnosperm)
              |2=[[Biology:Pinophyta|Conifer]]s<br />(gymnosperm)
             }}
         }}
     }}
}}
</div>Another possible family tree<ref name="Crepet2000" /></div>
|}
The first flowering plants appeared around 130 Ma.<ref>{{cite encyclopedia |encyclopedia=[[Encyclopædia Britannica Online]] |title=evolution: plant timeline |url=http://www.britannica.com/EBchecked/media/24/Significant-events-in-plant-evolution |accessdate=2015-02-16 |year=1996 |publisher=Encyclopædia Britannica, Inc. |oclc=42796406}}</ref> The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from {{ma|130|90|Ma|Cretaceous}},<ref name="CraneFriisPedersen2000OriginAndDiversificationOfAngiosperms">Crane, Friis and Pedersen 2000, "The Origin and Early Diversification of Angiosperms," pp. [https://books.google.com/?id=ZJe_Dmdbm-QC&pg=PA233 233–250]</ref><ref name="Crepet2000">{{cite journal |last=Crepet |first=William L. |date=November 21, 2000 |title=Progress in understanding angiosperm history, success, and relationships: Darwin's abominably 'perplexing phenomenon' |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=24 |pages=12939–12941 |bibcode=2000PNAS...9712939C |doi=10.1073/pnas.97.24.12939 |pmc=34068 |pmid=11087846 }}</ref> and that their rise was associated with that of [[Biology:Pollination|pollinating]] insects.<ref name="Crepet2000" /> Among modern flowering plants ''[[Biology:Magnoliaceae|Magnolia]]'' are thought to be close to the common ancestor of the group.<ref name="CraneFriisPedersen2000OriginAndDiversificationOfAngiosperms" /> However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.<ref name="CraneFriisPedersen2000OriginAndDiversificationOfAngiosperms" /><ref name="Crepet2000" />
{{clear}}

== Social insects ==
[[File:Termite cathedral mounds in a bushfire blackened tropical savanna.jpg|thumb|right|upright|These termite mounds have survived a bush fire.]]
{{main|Biology:Eusociality}}
The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the [[Biology:Gene-centered view of evolution|selfish gene]]. In fact, there are very few [[Biology:Eusociality|eusocial]] insect species: only 15 out of approximately 2,600 living families of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some eusocial lineages have diversified into several families. Nevertheless, social insects have been spectacularly successful; for example although [[Biology:Ant|ant]]s and [[Biology:Termite|termite]]s account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success.<ref name="WilsonHölldobler2005EusocialityOriginConsequences">{{cite journal |last1=Wilson |first1=Edward O. |last2=Hölldobler |first2=Bert |date=September 20, 2005 |title=Eusociality: Origin and consequences |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=102 |issue=38 |pages=13367–13371 |bibcode=2005PNAS..10213367W |doi=10.1073/pnas.0505858102 |pmc=1224642 |pmid=16157878 }}</ref>

The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual [[Biology:Haplodiploidy|haplodiploid]] method of [[Biology:Sex-determination system|sex determination]], which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed.<ref>{{cite journal |last1=Hughes |first1=William O. H. |last2=Oldroyd |first2=Benjamin P. |last3=Beekman |first3=Madeleine |last4=Ratnieks |first4=Francis L. W. |date=May 30, 2008 |title=Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality |journal=Science |volume=320 |issue=5880 |pages=1213–1216 |bibcode=2008Sci...320.1213H |doi=10.1126/science.1156108 |pmid=18511689 }}</ref> However, [[Biography:E. O. Wilson|E. O. Wilson]] and Bert Hölldobler argue that this explanation is faulty: for example, it is based on [[Philosophy:Kin selection|kin selection]], but there is no evidence of [[Social:Nepotism|nepotism]] in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative foraging. In support of this explanation they cite the appearance of eusociality in [[Biology:Blesmol|bathyergid]] mole rats,<ref name="WilsonHölldobler2005EusocialityOriginConsequences" /> which are not haplodiploid.<ref>{{cite journal |last=Lovegrove |first=Barry G. |date=January 1991 |title=The evolution of eusociality in molerats (Bathyergidae): a question of risks, numbers, and costs |journal=Behavioral Ecology and Sociobiology |volume=28 |issue=1 |pages=37–45 |doi=10.1007/BF00172137 }}</ref>

The earliest fossils of insects have been found in Early Devonian rocks from about {{ma|400|Ma|Devonian}}, which preserve only a few varieties of flightless insect. The [[Earth:Mazon Creek fossil beds|Mazon Creek lagerstätten]] from the Late Carboniferous, about {{ma|300|Ma|Carboniferous}}, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as [[Earth:Herbivore|herbivore]]s, [[Biology:Detritivore|detritivore]]s and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle [[Earth:Cenozoic|Cenozoic]].<ref>{{harvnb|Labandeira|Eble|1999}}</ref>

== Humans ==
{{Human timeline}}
{{main|Biology:Human evolution}}
The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by Robert Chambers in 1844 and taken up by [[Biography:Charles Darwin|Charles Darwin]] in 1871.<ref>{{cite web |url=http://darwin200.christs.cam.ac.uk/pages/index.php?page_id=c8 |title=Darwin & the Descent of Man |last=Montgomery |first=Stephen |year=2009 |website=Charles Darwin & Evolution |publisher=Christ's College |location=Cambridge |accessdate=2015-02-17 |url-status=dead |archiveurl=https://web.archive.org/web/20150225222347/http://darwin200.christs.cam.ac.uk/pages/index.php?page_id=c8 |archivedate=2015-02-25 }}</ref> Modern humans evolved from a lineage of upright-walking [[Biology:Ape|ape]]s that has been traced back over {{ma|6|Ma}} to ''[[Biology:Sahelanthropus|Sahelanthropus]]''.<ref>{{cite journal |last1=Brunet |first1=Michel |last2=Guy |first2=Franck |last3=Pilbeam |first3=David |authorlink3=David Pilbeam |last4=Mackaye |first4=Hassane Taisso |last5=Likius |first5=Andossa |last6=Ahounta |first6=Djimdoumalbaye |last7=Beauvilain |first7=Alain |last8=Blondel |first8=Cécile |date=July 11, 2002 |title=A new hominid from the Upper Miocene of Chad, Central Africa |journal=Nature |volume=418 |issue=6894 |pages=145–151 |doi=10.1038/nature00879 |pmid=12110880 |bibcode=2002Natur.418..145B |display-authors=3 }}</ref> The first known [[Engineering:Stone tool|stone tool]]s were made about {{ma|2.5|Ma}}, apparently by ''[[Biology:Australopithecus garhi|Australopithecus garhi]]'', and were found near animal bones that bear scratches made by these tools.<ref>{{cite journal |last1=de Heinzelin |first1=Jean |authorlink1=Jean de Heinzelin de Braucourt |last2=Clark |first2=J. Desmond |last3=White |first3=Tim |authorlink3=Tim D. White |last4=Hart |first4=William |last5=Renne |first5=Paul |authorlink5=Paul Renne |last6=WoldeGabriel |first6=Giday |authorlink6=Giday WoldeGabriel |last7=Beyene |first7=Yonas |last8=Vrba |first8=Elisabeth |authorlink8=Elisabeth Vrba |date=April 23, 1999 |title=Environment and Behavior of 2.5-Million-Year-Old Bouri Hominids |journal=Science |volume=284 |issue=5414 |pages=625–629 |doi=10.1126/science.284.5414.625 |pmid=10213682 |display-authors=3 |bibcode=1999Sci...284..625D }}</ref> The earliest [[Biology:Homininae|hominine]]s had [[Biology:Common chimpanzee|chimpanzee]]-sized brains, but there has been a fourfold increase in the last 3&nbsp;Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence.<ref>{{cite journal |last1=De Miguel |first1=Carmen |last2=Henneberg |first2=Maciej |year=2001 |title=Variation in hominid brain size: How much is due to method? |journal=HOMO: Journal of Comparative Human Biology |volume=52 |issue=1 |pages=3–58 |doi=10.1078/0018-442X-00019 |pmid=11515396 }}</ref> There is a long-running debate about whether modern humans evolved [[Biology:Multiregional origin of modern humans|all over the world]] simultaneously from existing advanced hominines or are descendants of a [[Biology:Recent African origin of modern humans|single small population in Africa]], which then migrated all over the world less than 200,000&nbsp;years ago and replaced previous hominine species.<ref>{{harvnb|Leakey|1994|pp=87–89}}</ref> There is also debate about whether anatomically modern humans had an intellectual, cultural and technological [[Philosophy:Behavioral modernity|"Great Leap Forward"]] under 100,000&nbsp;years ago and, if so, whether this was due to neurological changes that are not visible in fossils.<ref>{{cite journal |last=Mellars |first=Paul |authorlink=Paul Mellars |date=June 20, 2006 |title=Why did modern human populations disperse from Africa ''ca.'' 60,000 years ago? A new model |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |pages=9381–9386 |issue=25 |bibcode=2006PNAS..103.9381M |doi=10.1073/pnas.0510792103 |pmid=16772383 |pmc=1480416 }}</ref>
{{clear}}

== Mass extinctions ==
{{main|Biology:Extinction event}}
{{annotated image/Extinction|caption=Apparent extinction intensity, i.e. the fraction of [[Biology:Genus|genera]] going extinct at any given time, as reconstructed from the [[Biology:Fossil#Fossil record|fossil record]]. (Graph not meant to include the recent, ongoing [[Earth:Holocene extinction|Holocene extinction]] event).}}
Life on Earth has suffered occasional mass extinctions at least since {{ma|542|Ma}}. Although they were disasters at the time, mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.<ref name="Van Valkenburgh 1999 463–493"/><ref>{{harvnb|Benton|2005a|loc=Chapter 6: "Tetrapods of the Triassic"}}</ref>
{{Phanerozoic biodiversity | float=right}}
The fossil record appears to show that the gaps between mass extinctions are becoming longer and the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:<ref name="MacLeod2001Extinction">{{cite web |url=http://www.firstscience.com/SITE/ARTICLES/macleod.asp |title=Extinction! |last=MacLeod |first=Norman |date=January 6, 2001 |website=FirstScience.com |accessdate=2015-02-20}}</ref>
* The oceans may have become more hospitable to life over the last 500&nbsp;Ma and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of [[Earth:Eutrophication|eutrophication]] and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.<ref>{{cite journal |last=Martin |first=Ronald E. |date=June 1995 |title=Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans |journal=[[Earth:Global and Planetary Change|Global and Planetary Change]] |volume=11 |issue=1–2 |pages=1–23 |doi=10.1016/0921-8181(94)00011-2 |bibcode=1995GPC....11....1M }}</ref><ref>{{cite journal |last=Martin |first=Ronald E. |date=June 1996 |title=Secular Increase in Nutrient Levels through the Phanerozoic: Implications for Productivity, Biomass, and Diversity of the Marine Biosphere |journal=PALAIOS |volume=11 |issue=3 |pages=209–219 |doi=10.2307/3515230 |jstor=3515230 |bibcode=1996Palai..11..209M }}</ref>
* Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera, which were often defined solely to accommodate these finds—the story of ''[[Biology:Anomalocaris|Anomalocaris]]'' is an example of this. The risk of this mistake is higher for older fossils because these are often both unlike parts of any living organism and poorly conserved. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.<ref name="MacLeod2001Extinction" />

[[Biology:Biodiversity|Biodiversity]] in the fossil record, which is "...the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"<ref name="RohdeMuller2005" /> shows a different trend: a fairly swift rise from {{ma|542|400|Ma}}; a slight decline from {{ma|400|200|Ma}}, in which the devastating Permian–Triassic extinction event is an important factor; and a swift rise from {{ma|200|Ma}} to the present.<ref name="RohdeMuller2005">{{cite journal |last1=Rohde |first1=Robert A. |last2=Muller |first2=Richard A. |date=March 10, 2005 |title=Cycles in fossil diversity |url=http://muller.lbl.gov/papers/Rohde-Muller-Nature.pdf |journal=Nature |volume=434 |issue=7030 |pages=208–210 |bibcode=2005Natur.434..208R |doi=10.1038/nature03339 |pmid=15758998 |accessdate=2008-09-22 }}</ref>
{{clear}}

== See also ==
{{div col|colwidth=22em}}
* History of evolutionary thought
* ''On the Origin of Species''
* [[Biology:Speciation|Speciation]]
* [[Biology:Taxonomy of commonly fossilised invertebrates|Taxonomy of commonly fossilised invertebrates]]
* ''[[Treatise on Invertebrate Paleontology]]''
* [[Biology:Viral evolution|Viral evolution]]
{{div col end}}

== Footnotes ==
{{Reflist|group=note}}

== References ==
{{Reflist|30em}}

== Bibliography ==
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* {{cite book |last1=Krumbein |first1=Wolfgang E. |last2=Brehm |first2=Ulrike |last3=Gerdes |first3=Gisela |last4=Gorbushina |first4=Anna A. |last5=Levit |first5=George |last6=Palinska |first6=Katarzyna A. |year=2003 |chapter=Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites Geophysiology, Global Mechanism, Parahistology |chapterurl=http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf |editor1-last=Krumbein |editor1-first=Wolfgang E. |editor2-last=Paterson |editor2-first=David M. |editor3-last=Zavarzin |editor3-first=Georgii A. |title=Fossil and Recent Biofilms: A Natural History of Life on Earth |location=Dordrecht, the Netherlands |publisher=[[Company:Springer Science+Business Media|Kluwer Academic Publishers]] |isbn=978-1-4020-1597-7 |lccn=2003061870 |oclc=52901566 |archiveurl=https://web.archive.org/web/20070106201614/http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf |display-authors=3 |archive-date=2007-01-06 }}
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* {{cite book |last=O'Leary |first=Margaret R. |year=2008 |title=Anaxagoras and the Origin of Panspermia Theory |location=Bloomington, IN |publisher=iUniverse |isbn=978-0-595-49596-2 |oclc=757322661 }}
* {{cite book |last=Padian |first=Kevin |year=2004 |chapter=Basal Avialae |editor1-last=Weishampel |editor1-first=David B. |editor1-link=David B. Weishampel |editor2-last=Dodson |editor2-first=Peter |editor2-link=Peter Dodson |editor3-last=Osmólska |editor3-first=Halszka |editor3-link=Halszka Osmólska |title=The Dinosauria |edition=2nd |location=Berkeley |publisher=University of California Press |isbn=978-0-520-24209-8 |lccn=2004049804 |oclc=55000644 |title-link=The Dinosauria }}
* {{cite book |last1=Sansom |first1=Ivan J. |last2=Smith |first2=Moya M. |last3=Smith |first3=M. Paul |authorlink3=M. Paul Smith |year=2001 |chapter=The Ordovician radiation of vertebrates |editor-last=Ahlberg |editor-first=Per Erik |title=Major Events in Early Vertebrate Evolution: Palaeontology, phylogeny, genetics and development |series=Systematics Association special volume series |volume=61 |location=London; New York |publisher=Taylor & Francis |isbn=978-0-415-23370-5 |lccn=00062919 |oclc=51667292 }}
* {{cite book |last1=Thewissen |first1=J. G. M. |last2=Madar |first2=S. I. |last3=Hussain |first3=S. T. |year=1996 |title=Ambulocetus natans, an Eocene cetacean (Mammalia) from Pakistan |series=Courier Forschungsinstitut Senckenberg |volume=191 |location=Frankfurt |publisher=[[Organization:Senckenberg Nature Research Society|Senckenbergische Naturforschende Gesellschaft]] |isbn=978-3-929907-32-2 |lccn=97151576 |oclc=36463214 }}
{{Refend}}

== Further reading ==
* {{cite book |last=Dawkins |first=Richard |year=1989 |title=The Selfish Gene |edition=New |location=Oxford; New York |publisher=Oxford University Press |isbn=978-0-19-286092-7 |lccn=89016077 |oclc=20012195 |title-link=The Selfish Gene }}
* {{cite book |last=Dawkins |first=Richard |year=2004 |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Life |location=Boston |publisher=Houghton Mifflin Company |isbn=978-0-618-00583-3 |lccn=2004059864 |oclc=56617123 |title-link=The Ancestor's Tale }}
* {{cite book |editor1-last=Ruse |editor1-first=Michael |editor1-link=Michael Ruse |editor2-last=Travis |editor2-first=Joseph |editor2-link=Joseph Travis |year=2009 |title=Evolution: The First Four Billion Years |others=Foreword by Edward O. Wilson |location=Cambridge, MA |publisher=Belknap Press of Harvard University Press |isbn=978-0-674-03175-3 |lccn=2008030270 |oclc=225874308 |url=https://archive.org/details/evolutionfirstfo00mich }}
* {{cite book |last1=Smith |first1=John Maynard |last2=Szathmáry |first2=Eörs |year=1997 |origyear=Originally published 1995; Oxford: W. H. Freeman/Spektrum |title=The Major Transitions in Evolution |location=Oxford; New York |publisher=Oxford University Press |isbn=978-0-19-850294-4 |lccn=94026965 |oclc=715217397 |title-link=The Major Transitions in Evolution }}

== External links ==

===General information===
* {{cite web |url=http://www.fossilmuseum.net/Evolution.htm |title=Evolution |website=The Virtual Fossil Museum |accessdate=2015-02-22}} General information on evolution compiled by Roger Perkins
* {{cite web |url=http://evolution.berkeley.edu/ |title=Understanding Evolution: your one-stop resource for information on evolution |publisher=[[Organization:University of California, Berkeley|University of California, Berkeley]] |accessdate=2015-02-22}}
* {{cite web |url=http://nationalacademies.org/evolution/ |title=Evolution Resources |publisher=National Academies |location=Washington, D.C. |accessdate=2015-02-23}}
* {{cite web |url=http://tellapallet.com/tree_of_life.htm |title=Tree of Life |accessdate=2015-02-23 |url-status=dead |archiveurl=https://web.archive.org/web/20150210112109/http://www.tellapallet.com/tree_of_life.htm |archivedate=2015-02-10 }} Tree of life diagram by Neal Olander
* {{cite magazine |url=https://www.newscientist.com/topic/evolution |title=Evolution |magazine=New Scientist |accessdate=2015-02-23}}
* {{cite web |url=http://anthro.palomar.edu/synthetic/ |title=Modern Theories of Evolution: An Introduction to the Concepts and Theories That Led to Our Current Understanding of Evolution |publisher=Palomar College |accessdate=2015-02-23}} Tutorial created by Dennis O'Neil

===History of evolutionary thought===
* {{cite web |url=http://darwin-online.org.uk |title=The Complete Work of Charles Darwin Online |editor-last=van Wyhe |editor-first=John |accessdate=2015-02-23}}
* {{cite web |url=http://www.rationalrevolution.net/articles/understanding_evolution.htm |title=Understanding Evolution: History, Theory, Evidence, and Implications |last=Price |first=R. G. |website=rationalrevolution.net |accessdate=2015-02-23}}

{{Evolution}}
{{Nature}}
{{Earth}}

{{DEFAULTSORT:Evolutionary History Of Life}}
[[Category:Paleontology]]
[[Category:Evolutionary biology]]
[[Category:Earth sciences]]
[[Category:Geochronology]]

{{Sourceattribution|Evolutionary history of life|1}}