View source for Earth:History of Earth

{{Short description|Development of planet Earth from its formation to the present day}}
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[[File:Geologic Clock with events and periods.svg|upright=2.2|thumb|Earth's history with time-spans of the [[Earth:Geologic time scale|eons]] to scale]]
The '''history of Earth''' concerns the development of [[Astronomy:Planet|planet]] [[Astronomy:Earth|Earth]] from its formation to the present day.{{r|Stanley2005|TimeScale}} Nearly all branches of [[Natural science|natural science]] have contributed to understanding of the main events of Earth's past, characterized by constant [[Earth:Geology|geological]] change and biological [[Biology:Evolution|evolution]].

The [[Earth:Geologic time scale|geological time scale]] (GTS), as defined by international convention,<ref>"International Stratigraphic Chart". International Commission on Stratigraphy</ref> depicts the large spans of time from the beginning of the Earth to the present, and its divisions chronicle some definitive events of Earth history. (In the graphic, [[Year#Abbreviations yr and ya|Ma]] means "million years ago".) Earth formed around 4.54 billion years ago, approximately one-third the [[Astronomy:Age of the universe|age of the universe]], by [[Physics:Accretion (astrophysics)|accretion]] from the [[Astronomy:Formation and evolution of the Solar System#Formation of the planets|solar nebula]].<ref name="USGS1997" /><ref>{{cite journal |last=Dalrymple |first=G. Brent |title=The age of the Earth in the twentieth century: a problem (mostly) solved |journal=Special Publications, Geological Society of London |date=2001 |volume=190 |issue=1 |pages=205–221 |doi=10.1144/GSL.SP.2001.190.01.14 |bibcode=2001GSLSP.190..205D |s2cid=130092094}}</ref><ref>{{cite journal |last1=Manhesa |first1=Gérard |last2=Allègre |first2=Claude J. |last3=Dupréa |first3=Bernard |last4=Hamelin |first4=Bruno |name-list-style=amp |title=Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics |journal=[[Earth:Earth and Planetary Science Letters|Earth and Planetary Science Letters]] |date=1980 |volume=47 |issue= 3 |pages=370–382 |doi=10.1016/0012-821X(80)90024-2 |bibcode=1980E&PSL..47..370M}}</ref> Volcanic [[Physics:Outgassing|outgassing]] probably created the primordial [[Astronomy:Atmosphere|atmosphere]] and then the ocean, but the early atmosphere contained almost no [[Chemistry:Oxygen|oxygen]]. Much of the Earth was molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage ([[Earth:Early Earth|Early Earth]]), a giant impact collision with a planet-sized body named [[Astronomy:Theia (planet)|Theia]] is thought to have formed the Moon. Over time, the Earth cooled, causing the formation of a solid [[Earth:Crust (geology)|crust]], and allowing liquid water on the surface. In June 2023, scientists reported evidence that the planet Earth may have formed in just three million years, much faster than the 10−100 million years thought earlier.<ref name="WP-202130616" /><ref name="NAT-20230614" />

The [[Earth:Hadean|Hadean]] eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0 billion years ago. The following [[Earth:Archean|Archean]] and [[Earth:Proterozoic|Proterozoic]] eons produced the [[Biology:Abiogenesis|beginnings of life]] on Earth and its earliest [[Biology:Evolution|evolution]]. The succeeding eon is the [[Earth:Phanerozoic|Phanerozoic]], divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land; the [[Earth:Mesozoic|Mesozoic]], which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the [[Earth:Cenozoic|Cenozoic]], which saw the rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale.

The [[Biology:Earliest known life forms|earliest undisputed evidence of life on Earth]] dates at least from 3.5 billion years ago,<ref name="Origin1">{{cite journal |last1=Schopf |first1=J. William |last2=Kudryavtsev |first2=Anatoliy B. |last3=Czaja |first3=Andrew D. |last4=Tripathi |first4=Abhishek B. |date=5 October 2007 |title=Evidence of Archean life: Stromatolites and microfossils |journal=Precambrian Research |location=Amsterdam |publisher=Elsevier |volume=158 |pages=141–155 |issue=3–4 |doi=10.1016/j.precamres.2007.04.009 |issn=0301-9268|bibcode = 2007PreR..158..141S }}</ref><ref name="Origin2">{{cite journal |last=Schopf |first=J. William |date=29 June 2006 |title=Fossil evidence of Archaean life |journal=[[Organization:Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society B]] |location=London |publisher=Royal Society |volume=361 |issue=1470 |pages=869–885 |doi=10.1098/rstb.2006.1834 |issn=0962-8436 |pmid=16754604 |pmc=1578735}}</ref><ref>{{cite book |last1=Raven |first1=Peter H. |last2=Johnson |first2=George B. |year=2002 |title=Biology |url=https://archive.org/details/biologyrave00rave|url-access=registration |edition=6th |location=Boston, MA |publisher=McGraw Hill |isbn=978-0-07-112261-0 |lccn=2001030052 |oclc=45806501 |page=[https://archive.org/details/biologyrave00rave/page/68 68]}}</ref> during the [[Earth:Eoarchean|Eoarchean]] Era, after a geological crust started to solidify following the earlier molten [[Earth:Hadean|Hadean]] [[Earth:Geologic time scale|Eon]]. There are [[Biology:Microbial mat|microbial mat]] [[Biology:Fossil|fossil]]s such as stromatolites found in 3.48 billion-year-old [[Earth:Sandstone|sandstone]] discovered in Western Australia.<ref name="AP-20131113">{{cite news |last=Borenstein |first=Seth |date=13 November 2013 |title=Oldest fossil found: Meet your microbial mom |url=http://apnews.excite.com/article/20131113/DAA1VSC01.html |work=[[Company:Excite (web portal)|Excite]] |location=Yonkers, NY |publisher=Mindspark Interactive Network |agency=Associated Press |access-date=2015-06-02}}</ref><ref name="TG-20131113-JP">{{cite news |last=Pearlman |first=Jonathan |date=13 November 2013 |title=Oldest signs of life on Earth found |url=https://www.telegraph.co.uk/news/science/science-news/10445788/Oldest-signs-of-life-on-Earth-found.html |archive-url=https://ghostarchive.org/archive/20220111/https://www.telegraph.co.uk/news/science/science-news/10445788/Oldest-signs-of-life-on-Earth-found.html |archive-date=2022-01-11 |url-access=subscription |url-status=live |newspaper=The Daily Telegraph |location=London |access-date=2014-12-15}}{{cbignore}}</ref><ref name="AST-20131108">{{cite journal |last1=Noffke |first1=Nora |author1-link=Nora Noffke |last2=Christian |first2=Daniel |last3=Wacey |first3=David |last4=Hazen |first4=Robert M. |author-link4=Robert Hazen |date=16 November 2013 |title=Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ''ca.'' 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia |journal=Astrobiology |location=New Rochelle, NY |publisher=Mary Ann Liebert, Inc. |volume=13 |issue=12 |pages=1103–1124 |bibcode=2013AsBio..13.1103N |doi=10.1089/ast.2013.1030 |issn=1531-1074 |pmc=3870916 |pmid=24205812}}</ref> Other early physical evidence of a [[Earth:Biogenic substance|biogenic substance]] is [[Chemistry:Graphite|graphite]] in 3.7 billion-year-old metasedimentary rocks discovered in southwestern [[Place:Greenland|Greenland]]<ref name="NG-20131208">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |location=London |publisher=[[Company:Nature Publishing Group|Nature Publishing Group]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}}</ref> as well as "remains of biotic life" 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 (web portal)|Excite]] |location=Yonkers, NY |publisher=Mindspark Interactive Network |agency=Associated Press |url-status=dead |archive-url=https://web.archive.org/web/20151023200248/http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |archive-date=23 October 2015 |access-date=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. |location=Washington, DC |publisher=National Academy of Sciences |doi=10.1073/pnas.1517557112 |issn=1091-6490 |access-date=2015-10-20 |pages=14518–14521 |pmid=26483481 |pmc=4664351 |volume=112|issue=47 |bibcode = 2015PNAS..11214518B |doi-access=free}} Early edition, published online before print.</ref> According to one of the researchers, "If life arose relatively quickly on Earth … then it could be common in the [[Astronomy:Universe|universe]]."<ref name="AP-20151019" />

Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life remained mostly small and microscopic until about [[Biology:Timeline of human evolution#Unicellular life|580 million years ago]], when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 538.8 million years ago. This sudden diversification of life forms produced most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated that 99 percent of all species that ever lived on Earth, over five billion,<ref name="Book-Biology">{{cite book |editor1-last=Kunin |editor1-first=W.E. |editor2-last=Gaston |editor2-first=Kevin |title=The Biology of Rarity: Causes and consequences of rare–common differences 
|url={{Google books|plainurl=yes|id=4LHnCAAAQBAJ|page=110}} |year=1996 |publisher=Springer |isbn=978-0-412-63380-5 |access-date=26 May 2015}}</ref> have gone [[Biology:Extinction|extinct]].<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|year=2000 |publisher=Yale University Press |isbn=978-0-300-08469-6|page=preface x}}</ref><ref name="NYT-20141108-MJN">{{cite news |last=Novacek |first=Michael J. |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |date=8 November 2014 |work={{wipe|New York Times}} |access-date=2014-12-25 }}</ref> Estimates on the number of Earth's current [[Biology:Species|species]] range from 10 million to 14 million,<ref name="MillerSpoolman2012">{{cite book |author1-first=G. |author1-last=Miller |author2-first=Scott |author2-last=Spoolman |title=Environmental Science – Biodiversity Is a Crucial Part of the Earth's Natural Capital |url={{Google books|plainurl=yes|id=NYEJAAAAQBAJ|page=62}} |date=2012 |publisher=Cengage Learning |isbn=978-1-133-70787-5 |page=62 |access-date=2014-12-27}}</ref> of which about 1.2 million are documented, but over 86 percent have not been described.<ref name="PLoS-20110823">{{cite journal |last1=Mora |first1=C. |last2=Tittensor |first2=D.P. |last3=Adl |first3=S. |last4=Simpson |first4=A.G. |last5=Worm |first5=B. |title=How many species are there on Earth and in the ocean? |date=23 August 2011 |journal=[[Biology:PLOS Biology|PLOS Biology]] |doi=10.1371/journal.pbio.1001127 |pmid=21886479 |pmc=3160336 |volume=9 |issue=8 |pages=e1001127 |doi-access=free }}</ref> 

The Earth's crust has constantly changed since its formation, as has life since its first appearance. Species continue to [[Biology:Evolution|evolve]], taking on new forms, splitting into daughter species, or going extinct in the face of ever-changing physical environments. The process of [[Earth:Plate tectonics|plate tectonics]] continues to shape the Earth's continents and oceans and the life they harbor.

== Eons ==
In [[Physics:Geochronology|geochronology]], time is generally measured in [[Astronomy:Myr|mya]] (million years ago), each unit representing the period of approximately 1,000,000 years in the past. The history of Earth is divided into four great [[Earth:Eon (geology)|eons]], starting 4,540 mya with the formation of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided into [[Earth:Era (geology)|eras]], which in turn are divided into periods, which are further divided into [[Earth:Epoch (geology)|epochs]].
{|class="wikitable"
! Eon
! Time (mya)
! Description
|-
| [[Earth:Hadean|Hadean]]
| 4,540–4,000
| The Earth is formed out of debris around the solar [[Astronomy:Protoplanetary disk|protoplanetary disk]]. There is no life. Temperatures are extremely hot, with frequent volcanic activity and hellish-looking environments (hence the eon's name, which comes from [[Religion:Hades|Hades]]). The atmosphere is nebular. Possible early oceans or bodies of liquid water. The Moon is formed around this time probably due to a [[Astronomy:Giant-impact hypothesis|protoplanet's collision into Earth]].
|-
| [[Earth:Archean|Archean]]
| 4,000–2,500
| [[Biology:Prokaryote|Prokaryote]] life, the first form of life, emerges at the very beginning of this eon, in a process known as [[Biology:Abiogenesis|abiogenesis]]. The continents of [[Earth:Ur (continent)|Ur]], [[Earth:Vaalbara|Vaalbara]] and [[Earth:Kenorland|Kenorland]] may have existed around this time. The atmosphere is composed of volcanic and greenhouse gases.
|-
| [[Earth:Proterozoic|Proterozoic]]
| 2,500–538.8
| The name of this eon means "early life". [[Biology:Eukaryote|Eukaryote]]s, a more complex form of life, emerge, including some forms of [[Biology:Multicellular organism|multicellular organism]]s. [[Biology:Bacteria|Bacteria]] begin producing oxygen, shaping the third and current of Earth's atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time. The early and late phases of this eon may have undergone "[[Earth:Snowball Earth|Snowball Earth]]" periods, in which all of the planet suffered below-zero temperatures. The early continents of [[Earth:Columbia (supercontinent)|Columbia]], [[Earth:Rodinia|Rodinia]] and [[Earth:Pannotia|Pannotia]], in that order, may have existed in this eon.
|-
| [[Earth:Phanerozoic|Phanerozoic]]
| 538.8–present
| Complex life, including [[Biology:Vertebrate|vertebrate]]s, begin to dominate the Earth's ocean in a process known as the [[Earth:Cambrian explosion|Cambrian explosion]]. [[Earth:Pangaea|Pangaea]] forms and later dissolves into [[Earth:Laurasia|Laurasia]] and [[Earth:Gondwana|Gondwana]], which in turn dissolve into the current continents. Gradually, life expands to land and familiar forms of plants, animals and fungi begin appearing, including annelids, insects and reptiles, hence the eon's name, which means "visible life". Several mass extinctions occur, among which birds, the descendants of non-avian dinosaurs, and more recently mammals emerge. Modern animals—[[Biology:Human evolution|including humans]]—evolve at the most recent phases of this eon.
|-
|}

==Geologic time scale==
{{Main|Earth:Geologic time scale}}
The history of the Earth can be organized chronologically according to the [[Earth:Geologic time scale|geologic time scale]], which is split into intervals based on [[Earth:Stratigraphy|stratigraphic]] analysis.{{r|TimeScale|shortTimeScale}}
{{Timeline Geological Timescale}}

==Solar System formation==
{{Main|Astronomy:Formation and evolution of the Solar System}}
[[File:Protoplanetary-disk.jpg|thumb|right|350px|An artist's rendering of a protoplanetary disk]]
The standard model for the formation of the [[Astronomy:Solar System|Solar System]] (including the [[Astronomy:Earth|Earth]]) is the [[Astronomy:Nebular hypothesis|solar nebula hypothesis]].<ref>{{Cite book |last=Encrenaz |first=T.|title=The solar system |date=2004 |publisher=Springer |location=Berlin |isbn=978-3-540-00241-3 |page=89 |edition=3rd}}</ref> In this model, the Solar System formed from a large, rotating cloud of interstellar dust and gas called the solar nebula. It was composed of [[Chemistry:Hydrogen|hydrogen]] and [[Chemistry:Helium|helium]] created [[Astronomy:Recombination (cosmology)|shortly after]] the [[Astronomy:Big Bang|Big Bang]] 13.8&nbsp;Ga (billion years ago) and heavier [[Chemistry:Chemical element|elements]] ejected by supernovae. About 4.5&nbsp;Ga, the nebula began a contraction that may have been triggered by the [[Physics:Shock wave|shock wave]] from a nearby [[Astronomy:Supernova|supernova]].{{r|Matson}} A shock wave would have also made the nebula rotate. As the cloud began to accelerate, its [[Physics:Angular momentum|angular momentum]], gravity, and [[Physics:Inertia|inertia]] flattened it into a [[Astronomy:Protoplanetary disk|protoplanetary disk]] perpendicular to its axis of rotation. Small [[Astronomy:Perturbation|perturbations]] due to collisions and the angular momentum of other large debris created the means by which kilometer-sized [[Astronomy:Protoplanet|protoplanet]]s began to form, orbiting the nebular center.<ref name=Goldreich1973>{{cite journal |author=P. Goldreich |author2=W.R. Ward |title=The Formation of Planetesimals |journal=Astrophysical Journal |date=1973 |volume=183 |pages=1051–1062 |bibcode=1973ApJ...183.1051G |doi=10.1086/152291|doi-access= }}</ref>

The center of the nebula, not having much angular momentum, collapsed rapidly, the compression heating it until [[Physics:Nuclear fusion|nuclear fusion]] of hydrogen into helium began. After more contraction, a [[Astronomy:T Tauri star|T Tauri star]] ignited and evolved into the [[Astronomy:Sun|Sun]]. Meanwhile, in the outer part of the nebula gravity caused [[Physics:Matter|matter]] to condense around density perturbations and dust particles, and the rest of the protoplanetary disk began separating into rings. In a process known as runaway [[Physics:Accretion (astrophysics)|accretion]], successively larger fragments of dust and debris clumped together to form planets.<ref name=Goldreich1973/> Earth formed in this manner about 4.54&nbsp;billion years ago (with an [[Measurement uncertainty|uncertainty]] of 1%){{r|age_earth1c|age_of_earth_faq|USGS1997}} and was largely completed within 10–20&nbsp;million&nbsp;years.{{r|Yin}} In June 2023, scientists reported evidence that the planet Earth may have formed in just three million years, much faster than the 10−100 million years thought earlier.<ref name="WP-202130616">{{cite news |last=Patel |first=Kasha |title=Scientists have a controversial theory for how — and how fast — Earth formed |url=https://www.washingtonpost.com/climate-environment/2023/06/16/earth-formation-pebble-accretion-theory/ |date=16 June 2023 |newspaper={{wipe|The Washington Post}} |url-status=live |archiveurl=https://archive.today/20230617115127/https://www.washingtonpost.com/climate-environment/2023/06/16/earth-formation-pebble-accretion-theory/ |archivedate=17 June 2023  |accessdate=17 June 2023 }}</ref><ref name="NAT-20230614">{{cite journal |author=Onyett, Isaac J. |display-authors=et al.|title=Silicon isotope constraints on terrestrial planet accretion |date=14 June 2023 |journal=Nature |volume=619 |issue=7970 |pages=539–544 |doi=10.1038/s41586-023-06135-z |pmid=37316662 |s2cid=259161680 |pmc=10356600 |bibcode=2023Natur.619..539O }}</ref> Nonetheless, the [[Astronomy:Solar wind|solar wind]] of the newly formed T Tauri star cleared out most of the material in the disk that had not already condensed into larger bodies. The same process is expected to produce accretion disks around virtually all newly forming stars in the universe, some of which yield planets.{{r|Kokubo2002}}

The proto-Earth grew by accretion until its interior was hot enough to melt the heavy, siderophile [[Chemistry:Metal|metal]]s. Having higher [[Physics:Density|densities]] than the silicates, these metals sank. This so-called ''[[Astronomy:Iron catastrophe|iron catastrophe]]'' resulted in the separation of a [[Chemistry:Primitive mantle|primitive mantle]] and a (metallic) core only 10&nbsp;million years after the Earth began to form, producing the layered [[Earth:Structure of the Earth|structure of Earth]] and setting up the formation of [[Earth:Earth's magnetic field|Earth's magnetic field]].<ref>{{cite book |first=Charles |last=Frankel |date=1996 |title=Volcanoes of the Solar System |publisher={{wipe|Cambridge University Press}} |pages=7–8 |isbn=978-0-521-47770-3}}</ref> J.A. Jacobs <ref>{{cite journal |first=J.A. |last=Jacobs |title=The Earth's inner core |journal=Nature |volume=172 |issue=4372 |pages=297–298 |date=1953 |doi=10.1038/172297a0 |bibcode=1953Natur.172..297J |s2cid=4222938}}</ref> was the first to suggest that [[Earth:Earth's inner core|Earth's inner core]]—a solid center distinct from the liquid [[Earth:Earth's outer core|outer core]]—is [[Earth:Earth's inner core#Growth|freezing]] and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years<ref>{{Cite journal |last1=van Hunen |first1=J. |last2=van den Berg |first2=A.P. |title=Plate tectonics on the early Earth: Limitations imposed by strength and buoyancy of subducted lithosphere |journal=Lithos |volume=103 |issue=1–2 |pages=217–235 |date=2007 |doi=10.1016/j.lithos.2007.09.016 |bibcode=2008Litho.103..217V}}</ref>).

== Hadean and Archean Eons ==
{{Main|Earth:Hadean|Earth:Archean}}
[[File:Earth formation.jpg|thumb|right|Artist's conception of Hadean Eon Earth, when it was much hotter and inhospitable to all forms of life.]]
The first [[Earth:Eon (geology)|eon]] in Earth's history, the ''Hadean'', begins with the Earth's formation and is followed by the ''Archean'' eon at 3.8&nbsp;Ga.{{r|TimeScale|page1=145}} The oldest rocks found on Earth date to about 4.0&nbsp;Ga, and the oldest detrital [[Chemistry:Zircon|zircon]] crystals in rocks to about 4.4&nbsp;Ga,{{r|nature1|Lindsey|Cavosie}} soon after the formation of the Earth's [[Earth:Crust (geology)|crust]] and the Earth itself. The giant impact hypothesis for the Moon's formation states that shortly after formation of an initial crust, the proto-Earth was impacted by a smaller protoplanet, which ejected part of the [[Earth:Mantle (geology)|mantle]] and crust into space and created the Moon.{{r|belbruno|Carsten|moonwalk}}

From crater counts on other celestial bodies, it is inferred that a period of intense meteorite impacts, called the ''[[Astronomy:Late Heavy Bombardment|Late Heavy Bombardment]]'', began about 4.1&nbsp;Ga, and concluded around 3.8&nbsp;Ga, at the end of the Hadean.{{r|space.com-bombardment}} In addition, volcanism was severe due to the large heat flow and [[Geothermal gradient|geothermal gradient]].{{r|Green}} Nevertheless, detrital zircon crystals dated to 4.4&nbsp;Ga show evidence of having undergone contact with liquid water, suggesting that the Earth already had oceans or seas at that time.{{r|nature1}}

By the beginning of the Archean, the Earth had cooled significantly. Present life forms could not have survived at Earth's surface, because the Archean atmosphere lacked [[Chemistry:Oxygen|oxygen]] hence had no [[Physics:Ozone layer|ozone layer]] to block ultraviolet light. Nevertheless, it is believed that primordial life began to evolve by the early Archean, with candidate [[Biology:Fossil|fossil]]s dated to around 3.5&nbsp;Ga.{{r|Taylor-2006}} Some scientists even speculate that life could have begun during the early Hadean, as far back as 4.4&nbsp;Ga, surviving the possible Late Heavy Bombardment period in hydrothermal vents below the Earth's surface.{{r|reuters1}}

=== Formation of the Moon ===
{{Main|Astronomy:Moon|Origin of the Astronomy:Moon|Astronomy:Giant-impact hypothesis}}
[[File:Artist's concept of collision at HD 172555.jpg|thumb|right|Artist's impression of the enormous collision that probably formed the Moon]]
Earth's only [[Astronomy:Natural satellite|natural satellite]], the Moon, is larger relative to its planet than any other satellite in the Solar System.{{#tag:ref|[[Astronomy:Pluto|Pluto]]'s satellite [[Astronomy:Charon (moon)|Charon]] is relatively larger,<ref>{{cite web |url=http://www.planetary.org/explore/topics/pluto/ |title=Space Topics: Pluto and Charon |publisher=The Planetary Society |access-date=6 April 2010 |url-status=dead |archive-url=https://web.archive.org/web/20120218223842/http://www.planetary.org/explore/topics/pluto/ |archive-date=18 February 2012}}</ref> but Pluto is defined as a [[Astronomy:Dwarf planet|dwarf planet]].<ref>{{cite web |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto |archive-url=https://web.archive.org/web/20021216191657/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Pluto |url-status=dead |archive-date=16 December 2002 |title=Pluto: Overview |work=Solar System Exploration |publisher=[[Organization:National Aeronautics and Space Administration|National Aeronautics and Space Administration]] |access-date=19 April 2012}}</ref>|group=nb}} During the [[Astronomy:Apollo program|Apollo program]], rocks from the Moon's surface were brought to Earth. [[Physics:Radiometric dating|Radiometric dating]] of these rocks shows that the Moon is 4.53&nbsp;±&nbsp;0.01&nbsp;billion years old,{{r|Kleine}} formed at least 30&nbsp;million years after the Solar System.{{r|Halliday}} New evidence suggests the Moon formed even later, 4.48&nbsp;±&nbsp;0.02&nbsp;Ga, or 70–110&nbsp;million years after the start of the Solar System.{{r|halliday-2008}}

Theories for the formation of the Moon must explain its late formation as well as the following facts. First, the Moon has a low density (3.3 times that of water, compared to 5.5 for the Earth{{r|earth_fact_sheet}}) and a small metallic core. Second, the Earth and Moon have the same oxygen [[Physics:Isotopic signature|isotopic signature]] (relative abundance of the oxygen isotopes). Of the theories proposed to account for these phenomena, one is widely accepted: The ''giant impact hypothesis'' proposes that the Moon originated after a body the size of [[Astronomy:Mars|Mars]] (sometimes named [[Astronomy:Theia (planet)|Theia]]{{r|Halliday}}) struck the proto-Earth a glancing blow.{{r|Stanley2005|StarChild|Canup|page1=256}}

The collision released about 100 million times more energy than the more recent [[Earth:Chicxulub crater|Chicxulub impact]] that is believed to have caused the extinction of the non-avian dinosaurs. It was enough to vaporize some of the Earth's outer layers and melt both bodies.{{r|StarChild|Stanley2005|page2=256}} A portion of the mantle material was [[Astronomy:Ejecta|ejected]] into orbit around the Earth. The giant impact hypothesis predicts that the Moon was depleted of metallic material,{{r|Liu}} explaining its abnormal composition.{{r|Newsom}} The ejecta in orbit around the Earth could have condensed into a single body within a couple of weeks. Under the influence of its own gravity, the ejected material became a more spherical body: the Moon.{{r|Taylor}}

[[File:Hadean.png|thumb|right|Artist's impression of a Hadean landscape with the relatively newly formed Moon still looming closely over Earth and both bodies sustaining strong volcanism.]]

=== First continents ===
[[File:North america terrain 2003 map.jpg|thumb|Geologic map of North America, color-coded by age. From most recent to oldest, age is indicated by yellow, green, blue, and red. The reds and pinks indicate rock from the Archean. |alt=Map with color and texture]]
[[Physics:Mantle convection|Mantle convection]], the process that drives plate tectonics, is a result of heat flow from the Earth's interior to the Earth's surface.{{r|DaviesMantle|page1=2}} It involves the creation of rigid tectonic plates at mid-oceanic ridges. These plates are destroyed by [[Earth:Subduction|subduction]] into the mantle at subduction zones. During the early Archean (about 3.0&nbsp;Ga) the mantle was much hotter than today, probably around {{convert|1600|C}},{{r|Cattermole|page1=82}} so convection in the mantle was faster. Although a process similar to present-day plate tectonics did occur, this would have gone faster too. It is likely that during the Hadean and Archean, subduction zones were more common, and therefore tectonic plates were smaller.{{r|Stanley2005|page1=258}}<ref>{{Cite book|last=Davies|first=Geoffrey F.|title=Mantle convection for geologists|date=2011|publisher=Cambridge University Press|location=Cambridge|isbn=978-0-521-19800-4}}</ref>

The initial crust, formed when the Earth's surface first solidified, totally disappeared from a combination of this fast Hadean plate tectonics and the intense impacts of the Late Heavy Bombardment. However, it is thought that it was [[Earth:Basalt|basalt]]ic in composition, like today's [[Earth:Oceanic crust|oceanic crust]], because little crustal differentiation had yet taken place.{{r|Stanley2005|page1=258}} The first larger pieces of [[Earth:Continental crust|continental crust]], which is a product of differentiation of lighter elements during [[Earth:Partial melting|partial melting]] in the lower crust, appeared at the end of the Hadean, about 4.0&nbsp;Ga. What is left of these first small continents are called [[Earth:Craton|craton]]s. These pieces of late Hadean and early Archean crust form the cores around which today's continents grew.<ref>{{cite conference |bibcode=2004AGUSM.T41C..01B |title=What is a craton? |last1=Bleeker |first1=W. |first2=B.W. |last2=Davis |date=May 2004 |publisher=American Geophysical Union |conference=Spring meeting |id=T41C-01}}</ref>

The [[Physics:Oldest dated rocks|oldest rocks]] on Earth are found in the North American craton of {{wipe|Canada}}. They are tonalites from about 4.0&nbsp;Ga. They show traces of [[Earth:Metamorphism|metamorphism]] by high temperature, but also sedimentary grains that have been rounded by erosion during transport by water, showing that rivers and seas existed then.{{r|Lunine}} Cratons consist primarily of two alternating types of terranes. The first are so-called [[Earth:Greenstone belt|greenstone belt]]s, consisting of low-grade metamorphosed sedimentary rocks. These "greenstones" are similar to the sediments today found in [[Earth:Oceanic trench|oceanic trench]]es, above subduction zones. For this reason, greenstones are sometimes seen as evidence for subduction during the Archean. The second type is a complex of [[Earth:Felsic|felsic]] magmatic rocks. These rocks are mostly tonalite, trondhjemite or [[Earth:Granodiorite|granodiorite]], types of rock similar in composition to [[Chemistry:Granite|granite]] (hence such terranes are called TTG-terranes). TTG-complexes are seen as the [[Earth:Relict (geology)|relicts]] of the first continental crust, formed by partial melting in basalt.{{r|Condie|page1=Chapter 5}}

=== Oceans and atmosphere ===

Earth is often described as having had three atmospheres. The first atmosphere, captured from the solar nebula, was composed of light (atmophile) elements from the solar nebula, mostly hydrogen and helium. A combination of the solar wind and Earth's heat would have driven off this atmosphere, as a result of which the atmosphere is now depleted of these elements compared to cosmic abundances.<ref name=Kasting93/> After the impact which created the Moon, the molten Earth released volatile gases; and later more gases were released by [[Earth:Volcano|volcano]]es, completing a second atmosphere rich in [[Chemistry:Greenhouse gas|greenhouse gas]]es but poor in oxygen. {{r|Stanley2005|page1=256}} Finally, the third atmosphere, rich in oxygen, emerged when bacteria [[Earth:Great Oxygenation Event|began to produce oxygen]] about 2.8&nbsp;Ga.{{r|Gale|page1=83–84, 116–117}}

[[File:NASA-EarlyEarth-PaleOrangeDot-20190802.jpg|thumb|250px|''The pale orange dot'', an artist's impression of the [[Earth:Early Earth|early Earth]] which might have appeared orange through its hazy methane rich ''prebiotic second atmosphere''.<ref name="Trail Elsila Müller Lyons p. ">{{cite journal |last1=Trail |first1=Dustin |last2=Elsila |first2=Jamie |last3=Mller |first3=Ulrich |last4=Lyons |first4=Timothy |last5=Rogers |first5=Karyn |title=Rethinking the Search for the Origins of Life |journal=Eos |publisher=American Geophysical Union (AGU) |volume=103 |date=2022-02-04 |issn=2324-9250 |doi=10.1029/2022eo220065 |page= |s2cid=246620824|doi-access=free }}</ref><ref name="Astrobiology 2017">{{cite web |title=NASA Astrobiology |website=Astrobiology |date=2017-06-05 |url=https://astrobiology.nasa.gov/news/how-hot-were-the-oceans-when-life-first-evolved/ |access-date=2022-09-13}}</ref> Earth's atmosphere at this stage was somewhat comparable to today's atmosphere of Titan.<ref name="Trainer Pavlov DeWitt Jimenez pp. 18035–18042">{{cite journal |last1=Trainer |first1=Melissa G. |last2=Pavlov |first2=Alexander A. |last3=DeWitt |first3=H. Langley |last4=Jimenez |first4=Jose L. |last5=McKay |first5=Christopher P. |last6=Toon |first6=Owen B. |last7=Tolbert |first7=Margaret A. |title=Organic haze on Titan and the early Earth |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=48 |date=2006-11-28 |issn=0027-8424 |doi=10.1073/pnas.0608561103 |pages=18035–18042 |pmid=17101962 |pmc=1838702 |doi-access=free}}</ref>]]

In early models for the formation of the atmosphere and ocean, the second atmosphere was formed by outgassing of [[Earth:Volatile (astrogeology)|volatiles]] from the Earth's interior. Now it is considered likely that many of the volatiles were delivered during accretion by a process known as ''impact degassing'' in which incoming bodies vaporize on impact. The ocean and atmosphere would, therefore, have started to form even as the Earth formed.{{r|Kasting03}} The new atmosphere probably contained [[Physics:Water vapor|water vapor]], carbon dioxide, nitrogen, and smaller amounts of other gases.{{r|Kasting-2006}}

Planetesimals at a distance of 1&nbsp;[[Astronomy:Astronomical unit|astronomical unit]] (AU), the distance of the Earth from the Sun, probably did not contribute any water to the Earth because the solar nebula was too hot for ice to form and the hydration of rocks by water vapor would have taken too long.{{r|Kasting03}}<ref name=Selsis>{{cite book |last=Selsis |first=Franck |chapter=Chapter 11. The Prebiotic Atmosphere of the Earth |title=Astrobiology: Future perspectives |series=Astrophysics and space science library |volume=305 |pages=267–286 |date=2005 |doi=10.1007/1-4020-2305-7_11 |isbn=978-1-4020-2304-0}}</ref> The water must have been supplied by meteorites from the outer asteroid belt and some large planetary embryos from beyond 2.5&nbsp;AU.{{r|Kasting03|Morbidelli}} Comets may also have contributed. Though most comets are today in orbits farther away from the Sun than [[Astronomy:Neptune|Neptune]], computer simulations show that they were originally far more common in the inner parts of the Solar System.{{r|Lunine|page1=130–132}}

As the Earth cooled, [[Earth:Cloud|cloud]]s formed. Rain created the oceans. Recent evidence suggests the oceans may have begun forming as early as 4.4&nbsp;Ga.{{r|nature1}} By the start of the Archean eon, they already covered much of the Earth. This early formation has been difficult to explain because of a problem known as the [[Faint young Sun paradox|faint young Sun paradox]]. Stars are known to get brighter as they age, and the Sun has become 30% brighter since its formation 4.5 billion years ago.<ref>{{Cite web|url=http://faculty.wcas.northwestern.edu/~infocom/The%20Website/evolution.html|title=The Sun's Evolution}}</ref> Many models indicate that the early Earth should have been covered in ice.{{r|Sagan|Kasting03}} A likely solution is that there was enough carbon dioxide and methane to produce a [[Earth:Greenhouse effect|greenhouse effect]]. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. It is hypothesized that there also existed an organic haze created from the products of methane photolysis that caused an [[Earth:Anti-greenhouse effect|anti-greenhouse effect]] as well.<ref>{{Cite book |last=Kump |first=Lee R. |url=https://www.worldcat.org/oclc/268789401 |title=The earth system |date=2010 |publisher=Prentice Hall |others=James F. Kasting, Robert G. Crane |isbn=978-0-321-59779-3 |edition=3rd |location=San Francisco |oclc=268789401}}</ref> Another greenhouse gas, [[Chemistry:Ammonia|ammonia]], would have been ejected by volcanos but quickly destroyed by ultraviolet radiation.{{r|Gale|page1=83}}

=== Origin of life ===
{{Life timeline}}
{{Main|Biology:Abiogenesis}}
One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose. There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism.{{r|Szathmary|Luisi}}

The first step in the emergence of life may have been chemical reactions that produced many of the simpler [[Chemistry:Organic chemistry|organic]] compounds, including nucleobases and amino acids, that are the building blocks of life. An [[Biology:Miller–Urey experiment|experiment in 1953]] by Stanley Miller and [[Biography:Harold Urey|Harold Urey]] showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of [[Astronomy:Lightning|lightning]].{{r|Lazcano}} Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules.{{r|NYTimes}} [[Computer simulation]]s show that [[Biology:Abiogenesis#Observed extraterrestrial organic molecules|extraterrestrial organic molecules]] could have formed in the protoplanetary disk before the formation of the Earth.{{r|Space-20120329}}

Additional complexity could have been reached from at least three possible starting points: self-replication, an organism's ability to produce offspring that are similar to itself; [[Biology:Metabolism|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.{{r|Pereto}}

==== Replication first: RNA world ====
{{Main|Biology: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 a kind of RNA molecule called a [[Biology:Ribozyme|ribozyme]] can [[Chemistry:Catalyst|catalyze]] both its own replication and the construction of proteins led to the hypothesis that earlier life-forms were based entirely on RNA.{{r|Joyce}} They could have formed an [[Biology:RNA world|RNA world]] in which there were individuals but no [[Biology:Species|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 genomes from those that their parents started with.{{r|Hoenigsberg}} 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.{{r|Forterre}} Ribozymes remain as the main components of [[Biology:Ribosome|ribosome]]s, the "protein factories" of modern cells.{{r|Cech}}

Although short, self-replicating RNA molecules have been artificially produced in laboratories,{{r|Johnston}} doubts have been raised about whether natural non-biological synthesis of RNA is possible.{{r|Levy|Larralde|Lindahl}} 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 GNA, which would have been replaced later by RNA.{{r|Orgel|Nelson}} Other pre-RNA replicators have been posited, including [[Earth:Crystal|crystal]]s{{r|Dawkins-Watchmaker|page1=150}} and even quantum systems.{{r|Daviesb}}

In 2003 it was proposed that porous metal sulfide precipitates would assist RNA synthesis at about {{convert|100|°C|°F}} and at ocean-bottom pressures near [[Earth:Hydrothermal vent|hydrothermal vent]]s. In this hypothesis, the proto-cells would be confined in the pores of the metal substrate until the later development of lipid membranes.{{r|Martin}}

==== Metabolism first: iron–sulfur world ====
[[File:DNA replication split.svg|thumb|upright|The replicator in virtually all known life is deoxyribonucleic acid. DNA is far more complex than the original replicator and its replication systems are highly elaborate.]]
{{Main|Biology:Iron–sulfur world hypothesis}}
Another long-standing hypothesis is that the first life was composed of protein molecules. Amino acids, the building blocks of [[Biology:Proteins|proteins]], are easily synthesized in plausible prebiotic conditions, as are small peptides ([[Chemistry:Polymers|polymers]] of amino acids) that make good catalysts.{{r|Kauffman|page1=295–297}} A series of experiments starting in 1997 showed that amino acids and peptides could form in the presence of [[Chemistry:Carbon monoxide|carbon monoxide]] and [[Chemistry:Hydrogen sulfide|hydrogen sulfide]] with [[Chemistry:Iron(II) sulfide|iron sulfide]] and [[Chemistry:Nickel sulfide|nickel sulfide]] as catalysts. Most of the steps in their assembly required temperatures of about {{convert|100|C}} and moderate pressures, although one stage required {{convert|250|C}} and a pressure equivalent to that found under {{convert|7|km|mi|sp=us}} of rock. Hence, self-sustaining synthesis of proteins could have occurred near hydrothermal vents.{{r|Wachtershauser}}

A difficulty with the metabolism-first scenario is finding a way for organisms to evolve. Without the ability to replicate as individuals, aggregates of molecules would have "compositional genomes" (counts of molecular species in the aggregate) as the target of natural selection. However, a recent model shows that such a system is unable to evolve in response to natural selection.{{r|Vasas}}

==== Membranes first: Lipid world ====
It has been suggested that double-walled "bubbles" of [[Chemistry:Lipid|lipid]]s like those that form the external membranes of cells may have been an essential first step.{{r|Trevors01}} 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. 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.{{r|Segre}}

==== The clay theory ====
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 crystalline pattern, are subject to an analog of [[Biology:Natural selection|natural selection]] (as the clay "species" that grows fastest in a particular environment rapidly becomes dominant), and can catalyze the formation of RNA molecules.{{r|Cairns-Smith}} Although this idea has not become the scientific consensus, it still has active supporters.{{r|Ferris|Dawkins-Watchmaker|page1=150–158}}
[[File:Liposome cross section.png|thumb|upright|Cross-section through a liposome]]
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. Bubbles can then grow by absorbing additional lipids and dividing. The formation of the earliest [[Biology:Cell|cells]] may have been aided by similar processes.{{r|Hanczyc}}

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of [[Biology:Nucleotide|nucleotide]]s, lipids and amino acids.{{r|Hartman}}

==== Last universal common ancestor ====
{{Main|Biology:Last universal common ancestor}}
It is believed that of this multiplicity of protocells, only one [[Biology:Lineage (evolution)|line]] survived. Current [[Biology:Phylogenetics|phylogenetic]] evidence suggests that the last universal ancestor (LUA) lived during the early [[Earth:Archean|Archean]] eon, perhaps 3.5&nbsp;Ga or earlier.<ref name=":0">{{Cite web |last=TimeTree |date=2023 |title=TimeTree: the timescale of life |url=http://www.timetree.org/ |access-date=6 February 2023 |website=TimeTree: the timescale of life |publisher=Temple University}}</ref><ref>{{Cite journal |last=Hedges |first=SB |date=2009 |year=2009 |editor-last=Hedges |editor-first=SB |editor2-last=Kumar |editor2-first=S |title=Life |url=http://www.timetree.org/public/data/pdf/Hedges2009Chap05.pdf |journal=The Timetree of Life |publisher=Oxford University Press |pages=89–98|doi=10.1093/oso/9780199535033.003.0005 |isbn=978-0-19-953503-3 }}</ref> This LUA cell is the ancestor of all life on Earth today. It was probably a [[Biology:Prokaryote|prokaryote]], possessing a cell membrane and probably ribosomes, but lacking a [[Biology:Cell nucleus|nucleus]] or membrane-bound [[Biology:Organelle|organelle]]s such as [[Biology:Mitochondrion|mitochondria]] or [[Biology:Chloroplast|chloroplast]]s. Like modern cells, it used DNA as its genetic code, RNA for information transfer and protein synthesis, and enzymes to [[Biology:Enzyme catalysis|catalyze reactions]]. Some scientists believe that instead of a single organism being the last universal common ancestor, there were populations of organisms exchanging genes by lateral gene transfer.{{r|Penny-LUCA}}

[[File:Archean.png|thumb|upright=1.3|Artist's impression of Earth during the later Archean, the largely cooled [[Earth:Earth's crust|planetary crust]] and water-rich barren [[Planetary surface|surface]], marked by volcanoes and continents, features already round microbialites. The Moon, still orbiting Earth much closer than today and still dominating Earth's sky, produced strong tides.<ref name="Lunar and Planetary Institute">{{cite web |title=Earth-Moon Dynamics |website=Lunar and Planetary Institute |url=https://www.lpi.usra.edu/exploration/training/illustrations/earthMoon/ |access-date=September 2, 2022}}</ref>]]

== Proterozoic Eon ==
{{Main|Earth:Proterozoic}}
The Proterozoic eon lasted from 2.5&nbsp;Ga to 538.8&nbsp;Ma (million years) ago.<ref name="StratChart 2022">{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |publisher=International Stratigraphic Commission |date=February 2022 |access-date=22 April 2022}}</ref> In this time span, cratons grew into continents with modern sizes. The change to an oxygen-rich atmosphere was a crucial development. Life developed from prokaryotes into [[Biology:Eukaryote|eukaryote]]s and multicellular forms. The Proterozoic saw a couple of severe ice ages called [[Earth:Snowball Earth|Snowball Earth]]s. After the last Snowball Earth about 600&nbsp;Ma, the evolution of life on Earth accelerated. About 580&nbsp;Ma, the [[Earth:Ediacaran biota|Ediacaran biota]] formed the prelude for the Cambrian Explosion.{{citation needed|date=January 2016}}

=== Oxygen revolution ===
{{Main|Earth:Great Oxidation Event}}
[[File:Lake Thetis-Stromatolites-LaRuth.jpg|thumb|right|[[Earth:Lithification|Lithified]] stromatolites on the shores of Lake Thetis, Western Australia. Archean stromatolites are the first direct fossil traces of life on Earth.]]
[[File:Banded Iron Formation Barberton.jpg|thumb|A [[Earth:Banded iron formation|banded iron formation]] from the 3.15&nbsp;Ga Moodies Group, Barberton Greenstone Belt, {{wipe|South Africa}}. Red layers represent the times when oxygen was available; gray layers were formed in anoxic circumstances.]]
The earliest cells absorbed energy and food from the surrounding environment. They used fermentation, the breakdown of more complex compounds into less complex compounds with less energy, and used the energy so liberated to grow and reproduce. Fermentation can only occur in an ''anaerobic'' (oxygen-free) environment. The evolution of [[Earth:Photosynthesis|photosynthesis]] made it possible for cells to derive energy from the Sun.{{r|CondieSystem|page1=377}}

Most of the life that covers the surface of the Earth depends directly or indirectly on photosynthesis. The most common form, oxygenic photosynthesis, turns carbon dioxide, water, and sunlight into food. It captures the energy of sunlight in energy-rich molecules such as ATP, which then provide the energy to make sugars. To supply the electrons in the circuit, hydrogen is stripped from water, leaving oxygen as a waste product.{{r|Leslie}} Some organisms, including [[Biology:Purple bacteria|purple bacteria]] and [[Biology:Green sulfur bacteria|green sulfur bacteria]], use an [[Biology:Anoxygenic photosynthesis|anoxygenic form of photosynthesis]] that uses alternatives to hydrogen stripped from water as electron donors; examples are hydrogen sulfide, sulfur and iron. Such [[Earth:Extremophile|extremophile]] organisms are restricted to otherwise inhospitable environments such as hot springs and hydrothermal vents.{{r|CondieSystem|Nisbet|page1=379–382}}

The simpler anoxygenic form arose about 3.8&nbsp;Ga, not long after the appearance of life. The timing of oxygenic photosynthesis is more controversial; it had certainly appeared by about 2.4&nbsp;Ga, but some researchers put it back as far as 3.2&nbsp;Ga.{{r|Leslie}} The latter "probably increased global productivity by at least two or three orders of magnitude".{{r|De-Marais-photosynthesis|Olson-2006}} Among the oldest remnants of oxygen-producing lifeforms are fossil [[Earth:Stromatolite|stromatolite]]s.{{r|De-Marais-photosynthesis|Olson-2006|Holland-2006}}

At first, the released oxygen was bound up with [[Earth:Limestone|limestone]], [[Chemistry:Iron|iron]], and other minerals. The oxidized iron appears as red layers in geological strata called banded iron formations that formed in abundance during the [[Earth:Siderian|Siderian]] period (between 2500&nbsp;Ma and 2300&nbsp;Ma).{{r|TimeScale|page1=133}} When most of the exposed readily reacting minerals were oxidized, oxygen finally began to accumulate in the atmosphere. Though each cell only produced a minute amount of oxygen, the combined metabolism of many cells over a vast time transformed Earth's atmosphere to its current state. This was Earth's third atmosphere.{{r|ForteyDtL|page1=50–51|Gale|page2=83–84, 116–117}}

Some oxygen was stimulated by solar ultraviolet radiation to form [[Chemistry:Ozone|ozone]], which collected in a layer near the upper part of the atmosphere. The ozone layer absorbed, and still absorbs, a significant amount of the ultraviolet radiation that once had passed through the atmosphere. It allowed cells to colonize the surface of the ocean and eventually the land: without the ozone layer, ultraviolet radiation bombarding land and sea would have caused unsustainable levels of mutation in exposed cells.{{r|cosmic-evolution-bio1|Lunine|page2=219–220}}

[[File:Oxygenation-atm.svg|thumb|300px|Graph showing range of estimated [[Physics:Partial pressure|partial pressure]] of atmospheric oxygen through geologic time {{r|Holland-2006}}]]

Photosynthesis had another major impact. Oxygen was toxic; much life on Earth probably died out as its levels rose in what is known as the ''oxygen catastrophe''. Resistant forms survived and thrived, and some developed the ability to use oxygen to increase their metabolism and obtain more energy from the same food.{{r|cosmic-evolution-bio1}}

=== Snowball Earth ===
{{Main|Earth:Snowball Earth}}
[[File:Snowball Huronian.jpg|thumb|250px|Artist's rendition of an oxinated fully-frozen Snowball Earth with no remaining liquid surface water.]]

The [[Astronomy:Stellar evolution|natural evolution of the Sun]] made it progressively more [[Astronomy:Luminosity|luminous]] during the Archean and Proterozoic eons; the Sun's luminosity increases 6% every billion years.{{r|Lunine|page1=165}} As a result, the Earth began to receive more heat from the Sun in the Proterozoic eon. However, the Earth did not get warmer. Instead, the geological record suggests it cooled dramatically during the early Proterozoic. Glacial deposits found in South Africa date back to 2.2&nbsp;Ga, at which time, based on [[Earth:Paleomagnetism|paleomagnetic]] evidence, they must have been located near the equator. Thus, this glaciation, known as the [[Earth:Huronian glaciation|Huronian glaciation]], may have been global. Some scientists suggest this was so severe that the Earth was frozen over from the poles to the equator, a hypothesis called Snowball Earth.{{r|Snowball}}

The Huronian ice age might have been caused by the [[Earth:Great Oxygenation Event|increased oxygen concentration]] in the atmosphere, which caused the decrease of methane (CH<sub>4</sub>) in the atmosphere. Methane is a strong greenhouse gas, but with oxygen it reacts to form CO<sub>2</sub>, a less effective greenhouse gas.{{r|Lunine|page1=172}} When free oxygen became available in the atmosphere, the concentration of methane could have decreased dramatically, enough to counter the effect of the increasing heat flow from the Sun.{{r|SnowballCause}}

However, the term Snowball Earth is more commonly used to describe later extreme ice ages during the [[Earth:Cryogenian|Cryogenian]] period. There were four periods, each lasting about 10 million years, between 750 and 580 million years ago, when the Earth is thought to have been covered with ice apart from the highest mountains, and average temperatures were about {{convert|-50|C}}.<ref>{{cite encyclopedia |encyclopedia=Oxford Dictionary of Geology & Earth Sciences |editor-first=Michael |editor-last=Allaby |title=Snowball Earth |page=539 |edition=4th |year=2013 |publisher=[[Organization:Oxford University Press|Oxford University Press]] |isbn=978-0-19-965306-5}}</ref> The snowball may have been partly due to the location of the supercontinent [[Earth:Rodinia|Rodinia]] straddling the [[Astronomy:Equator|Equator]]. Carbon dioxide combines with rain to weather rocks to form carbonic acid, which is then washed out to sea, thus extracting the greenhouse gas from the atmosphere. When the continents are near the poles, the advance of ice covers the rocks, slowing the reduction in carbon dioxide, but in the Cryogenian the weathering of Rodinia was able to continue unchecked until the ice advanced to the tropics. The process may have finally been reversed by the emission of carbon dioxide from volcanoes or the destabilization of methane gas hydrates. According to the alternative Slushball Earth theory, even at the height of the ice ages there was still open water at the Equator.<ref>{{cite book |first=Marcia |last=Bjornerud |title=Reading the Rocks: The Autobiography of the Earth |publisher=Westview Press |year=2005 |pages=131–138 |isbn=978-0-8133-4249-8}}</ref><ref>{{cite encyclopedia |url=https://www.britannica.com/science/Slushball-Earth-hypothesis |encyclopedia=Encyclopædia Britannica |title=Slushball Earth hypothesis}}</ref>

=== Emergence of eukaryotes ===
[[File:Plagiomnium affine laminazellen.jpeg|thumb|Chloroplasts in the cells of a moss]]
Modern [[Biology:Taxonomy|taxonomy]] classifies life into three domains. The time of their origin is uncertain. The [[Biology:Bacteria|Bacteria]] domain probably first split off from the other forms of life (sometimes called [[Biology:Neomura|Neomura]]), but this supposition is controversial. Soon after this, by 2 Ga,{{r|SciAm-eukaryote}} the Neomura split into the [[Biology:Archaea|Archaea]] and the Eukaryota. Eukaryotic cells (Eukaryota) are larger and more complex than prokaryotic cells (Bacteria and Archaea), and the origin of that complexity is only now becoming known.<ref>{{Cite book |last=Cooper |first=Geoffrey M. |date=2000 |chapter=The Origin and Evolution of Cells |url=https://www.ncbi.nlm.nih.gov/books/NBK9841/ |title=The Cell: A Molecular Approach |publisher=Sinauer Associates |edition=2nd |language=en}}</ref> The earliest fossils possessing features typical of [[Biology:Fungus|fungi]] date to the [[Earth:Paleoproterozoic|Paleoproterozoic]] era, some 2.4 Ga ago; these multicellular benthic organisms had filamentous structures capable of [[Biology:Anastomosis|anastomosis]].<ref>{{Cite journal |last1=Bengtson |first1=Stefan |last2=Rasmussen |first2=Birger |last3=Ivarsson |first3=Magnus |last4=Muhling |first4=Janet |last5=Broman |first5=Curt |last6=Marone |first6=Federica |last7=Stampanoni |first7=Marco |last8=Bekker |first8=Andrey |date=2017-04-24 |title=Fungus-like mycelial fossils in 2.4-billion-year-old vesicular basalt. |url=https://escholarship.org/uc/item/4883d4qh |journal=Nature Ecology & Evolution |volume=1 |issue=6 |pages=141 |doi=10.1038/s41559-017-0141 |pmid=28812648 |hdl=20.500.11937/67718 |s2cid=25586788 |issn=2397-334X |hdl-access=free }}</ref>

Around this time, the first [[Biology:Proto-mitochondrion|proto-mitochondrion]] was formed. A bacterial cell related to today's ''[[Biology:Rickettsia|Rickettsia]]'',{{r|Andersson}} which had evolved to [[Biology:Cellular respiration|metabolize oxygen]], entered a larger prokaryotic cell, which lacked that capability. Perhaps the large cell attempted to digest the smaller one but failed (possibly due to the evolution of prey defenses). The smaller cell may have tried to [[Biology:Parasite|parasitize]] the larger one. In any case, the smaller cell survived inside the larger cell. Using oxygen, it metabolized the larger cell's waste products and derived more energy. Part of this excess energy was returned to the host. The smaller cell replicated inside the larger one. Soon, a stable [[Earth:Symbiosis|symbiosis]] developed between the large cell and the smaller cells inside it. Over time, the host cell acquired some genes from the smaller cells, and the two kinds became dependent on each other: the larger cell could not survive without the energy produced by the smaller ones, and these, in turn, could not survive without the raw materials provided by the larger cell. The whole cell is now considered a single organism, and the smaller cells are classified as [[Biology:Organelle|organelle]]s called mitochondria.{{r|BerkeleyEvolution}}

A similar event occurred with photosynthetic [[Biology:Cyanobacteria|cyanobacteria]]{{r|Bergland}} entering large heterotrophic cells and becoming chloroplasts.{{r|ForteyDtL|Dawkins-Ancestors|page1=60–61|page2=536–539}} Probably as a result of these changes, a line of cells capable of photosynthesis split off from the other eukaryotes more than 1&nbsp;billion years ago. There were probably several such inclusion events. Besides the well-established endosymbiotic theory of the cellular origin of mitochondria and chloroplasts, there are theories that cells led to peroxisomes, spirochetes led to cilia and flagella, and that perhaps a [[Biology:DNA virus|DNA virus]] led to the cell nucleus,{{r|takemura|bell}} though none of them are widely accepted.{{r|peroxisome}}

Archaeans, bacteria, and eukaryotes continued to diversify and to become more complex and better adapted to their environments. Each domain repeatedly split into multiple lineages. Around 1.1 Ga, the [[Biology:Plant|plant]], [[Biology:Animal|animal]], and [[Biology:Fungi|fungi]] lines had split, though they still existed as solitary cells. Some of these lived in colonies, and gradually a division of labor began to take place; for instance, cells on the periphery might have started to assume different roles from those in the interior. Although the division between a colony with specialized cells and a multicellular organism is not always clear, around 1&nbsp;billion years ago{{r|Cosmic-evolution-bio2}}, the first multicellular plants emerged, probably [[Biology:Green algae|green algae]].{{r|bhattacharya}} Possibly by around 900 Ma{{r|Dawkins-Ancestors|page1=488}} true multicellularity had also evolved in animals.<ref>{{Cite journal |last1=Sperling |first1=Erik A |last2=Stockey |first2=Richard G |date=2018-10-01 |title=The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an "Explosion" |journal=Integrative and Comparative Biology |volume=58 |issue=4 |pages=605–622 |doi=10.1093/icb/icy088 |pmid=30295813 |issn=1540-7063|doi-access=free }}</ref>

At first, it probably resembled today's [[Biology:Sponge|sponge]]s, which have totipotent cells that allow a disrupted organism to reassemble itself.{{r|Dawkins-Ancestors|page1=483–487}} As the division of labor was completed in the different lineages of multicellular organisms, cells became more specialized and more dependent on each other.<ref>{{Cite journal |last1=Traxler |first1=Matthew F |last2=Rozen |first2=Daniel E |date=2022-06-01 |title=Ecological drivers of division of labour in Streptomyces |journal=Current Opinion in Microbiology |volume=67 |pages=102148 |doi=10.1016/j.mib.2022.102148 |pmid=35468363 |s2cid=248349511 |issn=1369-5274|doi-access=free }}</ref>

=== Supercontinents in the Proterozoic ===
{{Main|Supercontinent cycle}}
[[File:Positions of ancient continents, 550 million years ago.jpg|thumb|right|A reconstruction of Pannotia (550&nbsp;Ma).]]
Reconstructions of tectonic plate movement in the past 250&nbsp;million years (the Cenozoic and Mesozoic eras) can be made reliably using fitting of continental margins, ocean floor magnetic anomalies and [[Earth:Paleomagnetic pole|paleomagnetic pole]]s. No ocean crust dates back further than that, so earlier reconstructions are more difficult. Paleomagnetic poles are supplemented by geologic evidence such as [[Physics:Orogenic belt|orogenic belt]]s, which mark the edges of ancient plates, and past distributions of flora and fauna. The further back in time, the scarcer and harder to interpret the data get and the more uncertain the reconstructions.{{r|Kearey|page1=370}}

Throughout the history of the Earth, there have been times when continents collided and formed a supercontinent, which later broke up into new continents. About 1000 to 830&nbsp;Ma, most continental mass was united in the supercontinent Rodinia.{{r|Kearey|page1=370}}{{r|Torsvik2003}} Rodinia may have been preceded by Early-Middle Proterozoic continents called Nuna and Columbia.{{r|Kearey|page1=374}}<ref name="Zhao1">{{Cite journal |doi=10.1016/S0012-8252(02)00073-9 |first1=Guochun |last1=Zhao |last2=Cawood |first2=Peter A. |last3=Wilde |first3=Simon A. |last4=Sun |first4=M. |date=2002|title=Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia supercontinent |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=59 |issue=1–4 |pages=125–162 |bibcode=2002ESRv...59..125Z }}</ref><ref name="Zhao2">{{Cite journal |doi=10.1016/j.earscirev.2004.02.003 |first1=Guochun |last1=Zhao |last2=Sun |first2=M. |last3=Wilde |first3=Simon A. |last4=Li |first4=S.Z. |date=2004|title=A Paleo-Mesoproterozoic supercontinent: assembly, growth and breakup |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=67 |issue=1–2 |pages=91–123|bibcode = 2004ESRv...67...91Z |url=http://www.gt-crust.ru/jour/article/view/518}}</ref>

After the break-up of Rodinia about 800&nbsp;Ma, the continents may have formed another short-lived supercontinent around 550&nbsp;Ma. The hypothetical supercontinent is sometimes referred to as [[Earth:Pannotia|Pannotia or Vendia]].{{r|McElhinny|page1=321–322}} The evidence for it is a phase of [[Earth:Continental collision|continental collision]] known as the Pan-African orogeny, which joined the continental masses of current-day Africa, South America, Antarctica and Australia. The existence of Pannotia depends on the timing of the rifting between [[Earth:Gondwana|Gondwana]] (which included most of the landmass now in the Southern Hemisphere, as well as the [[Place:Arabian Peninsula|Arabian Peninsula]] and the [[Earth:Indian subcontinent|Indian subcontinent]]) and [[Earth:Laurentia|Laurentia]] (roughly equivalent to current-day North America).{{r|Kearey|page1=374}} It is at least certain that by the end of the Proterozoic eon, most of the continental mass lay united in a position around the south pole.{{r|Dalziel}}

=== Late Proterozoic climate and life ===
[[File:Spriggina Floundensi 4.png|thumb|A 580&nbsp;million year old fossil of ''[[Biology:Spriggina|Spriggina floundensi]]'', an animal from the Ediacaran period. Such life forms could have been ancestors to the many new forms that originated in the Cambrian Explosion.]]
The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about 716.5 and 635&nbsp;Ma, in the Cryogenian period.<ref>{{Cite news |url=https://www.sciencedaily.com/releases/2010/03/100304142228.htm |title=Snowball Earth: New Evidence Hints at Global Glaciation 716.5 Million Years Ago |newspaper=Science Daily |date=Mar 4, 2010 |access-date=April 18, 2012}}</ref> The intensity and mechanism of both glaciations are still under investigation and harder to explain than the early Proterozoic Snowball Earth.<ref name="Science Daily Snowball Earth challenged">{{cite web|title='Snowball Earth' Hypothesis Challenged|url=https://www.sciencedaily.com/releases/2011/10/111012083450.htm|access-date=29 September 2012}}</ref>
Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia.<ref name=Hoffman /> Because Rodinia was centered on the equator, rates of chemical weathering increased and carbon dioxide (CO<sub>2</sub>) was taken from the atmosphere. Because CO<sub>2</sub> is an important greenhouse gas, climates cooled globally.<ref>{{Cite web |title=Carbon Cycle and the Earth's Climate |url=http://www.columbia.edu/~vjd1/carbon.htm |access-date=2022-07-15 |website=www.columbia.edu}}</ref>

In the same way, during the Snowball Earths most of the continental surface was covered with [[Earth:Permafrost|permafrost]], which decreased chemical weathering again, leading to the end of the glaciations. An alternative hypothesis is that enough carbon dioxide escaped through volcanic outgassing that the resulting greenhouse effect raised global temperatures.{{r|Hoffman}} Increased volcanic activity resulted from the break-up of Rodinia at about the same time.<ref>{{Cite web |title=Rodinia - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/rodinia |access-date=2022-07-15 |website=www.sciencedirect.com}}</ref>

The Cryogenian period was followed by the [[Earth:Ediacaran|Ediacaran]] period, which was characterized by a rapid development of new multicellular lifeforms.<ref>{{Cite news |url=https://www.sciencedaily.com/releases/2008/01/080103144451.htm |title=Two Explosive Evolutionary Events Shaped Early History Of Multicellular Life |newspaper=Science Daily |date=Jan 3, 2008 |access-date=April 18, 2012}}</ref> Whether there is a connection between the end of the severe ice ages and the increase in diversity of life is not clear, but it does not seem coincidental. The new forms of life, called Ediacara biota, were larger and more diverse than ever. Though the taxonomy of most Ediacaran life forms is unclear, some were ancestors of groups of modern life.{{r|Xiao}} Important developments were the origin of muscular and neural cells. None of the Ediacaran fossils had hard body parts like skeletons. These first appear after the boundary between the Proterozoic and [[Earth:Phanerozoic|Phanerozoic]] eons or Ediacaran and Cambrian periods.<ref>{{Citation |last1=McArthur |first1=J.M. |title=Strontium Isotope Stratigraphy |date=2020 |url=https://www.sciencedirect.com/topics/earth-and-planetary-sciences/ediacaran |work=Geologic Time Scale 2020 |pages=211–238 |publisher=Elsevier |language=en |doi=10.1016/B978-0-12-824360-2.00007-3 |access-date=2022-07-15 |last2=Howarth |first2=R.J. |last3=Shields |first3=G.A. |last4=Zhou |first4=Y. |isbn=9780128243602|s2cid=263783179 }}</ref>

== Phanerozoic Eon ==
{{Main|Earth:Phanerozoic}}
The Phanerozoic is the current eon on Earth, which started approximately 538.8 million years ago. It consists of three eras: The [[Earth:Paleozoic|Paleozoic]], [[Earth:Mesozoic|Mesozoic]], and [[Earth:Cenozoic|Cenozoic]],<ref name="StratChart 2022"/> and is the time when multi-cellular life greatly diversified into almost all the organisms known today.<ref>{{cite book |title=The Dynamic Earth System |last=Patwardhan |first=A.M. |date=2010 |publisher=PHI Learning Private Limited |location=New Delhi |isbn=978-81-203-4052-7 |page=146 |url={{Google books|plainurl=yes|id=EOgZJZaA-Q0C}}}}</ref>

The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 538.8 to 251.9&nbsp;Ma.<ref name="StratChart 2022"/> During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Two major extinctions occurred. The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic slowly moved together again, forming the supercontinent [[Earth:Pangaea|Pangaea]] in the late Paleozoic.<ref>{{Cite web |title=Pangea {{!}} Definition, Map, History, & Facts {{!}} Britannica |url=https://www.britannica.com/place/Pangea |access-date=2022-07-15 |website=www.britannica.com |language=en}}</ref>

The Mesozoic ("middle life") era lasted from 251.9&nbsp;Ma to 66&nbsp;Ma.<ref name="StratChart 2022"/> It is subdivided into the [[Earth:Triassic|Triassic]], [[Earth:Jurassic|Jurassic]], and [[Earth:Cretaceous|Cretaceous]] periods. The era began with the [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]], the most severe extinction event in the fossil record; 95% of the species on Earth died out.{{r|bbc-permian-triassic}} It ended with the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] that wiped out the [[Biology:Dinosaur|dinosaur]]s.<ref>{{Cite web |title=K–T extinction {{!}} Overview & Facts {{!}} Britannica |url=https://www.britannica.com/science/K-T-extinction |access-date=2022-07-15 |website=www.britannica.com |language=en}}</ref>

The Cenozoic ("new life") era began at {{period start|Cenozoic}}&nbsp;Ma, and is subdivided into the [[Earth:Paleogene|Paleogene]], [[Earth:Neogene|Neogene]], and Quaternary periods. These three periods are further split into seven subdivisions, with the Paleogene composed of The [[Earth:Paleocene|Paleocene]], [[Earth:Eocene|Eocene]], and [[Earth:Oligocene|Oligocene]], the Neogene divided into the Miocene, Pliocene, and the Quaternary composed of the [[Earth:Pleistocene|Pleistocene]], and Holocene.<ref>{{cite web |url=http://www.ucmp.berkeley.edu/cenozoic/cenozoic.php |title=The Cenozoic Era |publisher=University of California Museum of Paleontology |date=June 2011 |access-date=2016-01-10}}</ref> Mammals, birds, amphibians, crocodilians, turtles, and lepidosaurs survived the Cretaceous–Paleogene extinction event that killed off the non-avian dinosaurs and many other forms of life, and this is the era during which they diversified into their modern forms.<ref>{{Cite web |title=GEOL 104 The Cretaceous-Paleogene Extinction: All Good Things... |url=https://www.geol.umd.edu/~tholtz/G104/lectures/104extinct.html |access-date=2022-07-15 |website=www.geol.umd.edu}}</ref>

=== Tectonics, paleogeography and climate ===
[[File:Pangaea continents.svg|thumb|right|Pangaea was a supercontinent that existed from about 300 to 180&nbsp;Ma. The outlines of the modern continents and other landmasses are indicated on this map.]]
At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia, [[Earth:Baltica|Baltica]], [[Earth:Siberia (continent)|Siberia]] and Gondwana.<ref>{{cite web |url=http://evolution.berkeley.edu/glossary_entry/glossary.php?word=Pannotia |title=Pannotia |work=UCMP Glossary |access-date=2006-03-12}}</ref> During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods. This causes the [[Sea level|sea level]] to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level.{{Citation needed|date=March 2016}}

Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short [[Earth:Ice age|ice age]] during which glaciers covered the south pole, where the huge continent Gondwana was situated. Traces of glaciation from this period are only found on former Gondwana. During the Late Ordovician ice age, a few mass extinctions took place, in which many [[Biology:Brachiopod|brachiopod]]s, trilobites, [[Biology:Bryozoa|Bryozoa]] and [[Biology:Coral|coral]]s disappeared. These marine species could probably not contend with the decreasing temperature of the sea water.{{r|bbc-ordovician}}

The continents Laurentia and Baltica collided between 450 and 400&nbsp;Ma, during the Caledonian Orogeny, to form Laurussia (also known as Euramerica).<ref>{{cite web|last=Murphy |first=Dennis C. |url=http://www.devoniantimes.org/who/pages/euramerica.html|title=The paleocontinent Euramerica|work=Devonian Times |date=May 20, 2006 |access-date=April 18, 2012}}</ref> Traces of the mountain belt this collision caused can be found in [[Place:Scandinavia|Scandinavia]], Scotland, and the northern Appalachians. In the [[Earth:Devonian|Devonian]] period (416–359&nbsp;Ma){{r|shortTimeScale}} Gondwana and Siberia began to move towards Laurussia. The collision of Siberia with Laurussia caused the Uralian Orogeny, the collision of Gondwana with Laurussia is called the Variscan or Hercynian Orogeny in Europe or the Alleghenian Orogeny in North America. The latter phase took place during the [[Earth:Carboniferous|Carboniferous]] period (359–299&nbsp;Ma){{r|shortTimeScale}} and resulted in the formation of the last supercontinent, Pangaea.{{r|Condie}}

By 180&nbsp;Ma, Pangaea broke up into [[Earth:Laurasia|Laurasia]] and Gondwana.{{Citation needed|date=March 2016}}

=== Cambrian explosion ===
{{Main|Earth:Cambrian explosion}}
[[File:Elrathia kingii growth series.jpg|thumb|Trilobites first appeared during the Cambrian period and were among the most widespread and diverse groups of Paleozoic organisms.]]
The rate of the evolution of life as recorded by fossils accelerated in the [[Earth:Cambrian|Cambrian]] period (542–488&nbsp;Ma).{{r|shortTimeScale}} The sudden emergence of many new species, [[Biology:Phylum|phyla]], and forms in this period is called the Cambrian Explosion. It was a form of adaptive radiation, where vacant [[Biology:Ecological niche|niches]] left by the extinct [[Earth:Ediacaran biota|Ediacaran biota]] were filled up by the emergence of new phyla.<ref name=Wille2008>{{cite journal|doi=10.1038/nature07072|date=June 2008|author=Wille, M|author2=Nägler, T.F.|author3=Lehmann, B|author4=Schröder, S|author5=Kramers, J.D|title=Hydrogen sulphide release to surface waters at the Precambrian/Cambrian boundary|volume=453|issue=7196|pages=767–9|pmid=18509331|journal=Nature|bibcode = 2008Natur.453..767W |s2cid=4425120}}</ref>The biological fomenting in the Cambrian Explosion was unprecedented before and since that time.{{r|Lunine|page1=229}} Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. The development of hard body parts such as shells, [[Biology:Skeleton|skeleton]]s or [[Biology:Exoskeleton|exoskeleton]]s in animals like [[Biology:Mollusc|mollusc]]s, [[Biology:Echinoderm|echinoderm]]s, [[Biology:Crinoid|crinoid]]s and [[Biology:Arthropod|arthropod]]s (a well-known group of arthropods from the lower Paleozoic are the [[Biology:Trilobite|trilobite]]s) made the preservation and [[Biology:Fossil|fossil]]ization of such life forms easier than those of their Proterozoic ancestors. For this reason, much more is known about life in and after the Cambrian than about that of older periods. Some of these Cambrian groups appear complex but are seemingly quite different from modern life; examples are ''[[Biology:Anomalocaris|Anomalocaris]]'' and ''[[Biology:Haikouichthys|Haikouichthys]]''. More recently, however, these seem to have found a place in modern classification.<ref>{{Cite web |title=Welcome to CK-12 Foundation {{!}} CK-12 Foundation |url=https://www.ck12.org/section/modern-classification-systems/ |access-date=2022-07-15 |website=www.ck12.org}}</ref>

During the Cambrian, the first [[Biology:Vertebrate|vertebrate]] animals, among them the first fishes, had appeared.{{r|Dawkins-Ancestors|page1=357}} A creature that could have been the ancestor of the fishes, or was probably closely related to it, was ''[[Biology:Pikaia|Pikaia]]''. It had a primitive notochord, a structure that could have developed into a [[Biology:Vertebral column|vertebral column]] later. The first fishes with [[Medicine:Jaw|jaw]]s ([[Biology:Gnathostomata|Gnathostomata]]) appeared during the next geological period, the [[Earth:Ordovician|Ordovician]]. The colonisation of new [[Biology:Ecological niche|niches]] resulted in massive body sizes. In this way, fishes with increasing sizes evolved during the early Paleozoic, such as the titanic placoderm ''[[Biology:Dunkleosteus|Dunkleosteus]]'', which could grow {{convert|7|m|ft|sp=us}} long.<ref>{{Cite web |title=placoderm {{!}} fossil fish {{!}} Britannica |url=https://www.britannica.com/animal/placoderm |access-date=2022-07-15 |website=www.britannica.com |language=en}}</ref>

The diversity of life forms did not increase greatly because of a series of mass extinctions that define widespread biostratigraphic units called ''biomeres''.{{r|Runkel}} After each extinction pulse, the [[Earth:Continental shelf|continental shelf]] regions were repopulated by similar life forms that may have been evolving slowly elsewhere.{{r|Palmer}} By the late Cambrian, the trilobites had reached their greatest diversity and dominated nearly all fossil assemblages.{{r|Hallam|page1=34}}

=== Colonization of land ===
[[File:Devonianscene-green.jpg|thumb|Artist's conception of Devonian flora]]
Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun's ultraviolet radiation, meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryote lineages had probably colonized the land as early as 3 Ga<ref>{{Cite journal |last1=Battistuzzi |first1=Fabia U |last2=Feijao |first2=Andreia |last3=Hedges |first3=S Blair |date=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 |issue=1 |pages=44 |doi=10.1186/1471-2148-4-44 |pmc=533871 |pmid=15535883 |doi-access=free }}</ref><ref>{{Cite journal |last1=Battistuzzi |first1=F. U. |last2=Hedges |first2=S. B. |date=2009-02-01 |title=A Major Clade of Prokaryotes with Ancient Adaptations to Life on Land |url=https://academic.oup.com/mbe/article-lookup/doi/10.1093/molbev/msn247 |journal=Molecular Biology and Evolution |language=en |volume=26 |issue=2 |pages=335–343 |doi=10.1093/molbev/msn247 |pmid=18988685 |issn=0737-4038}}</ref> even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600&nbsp;Ma and then broke apart a short 50&nbsp;million years later.{{r|liebermean}} Fish, the earliest vertebrates, evolved in the oceans around 530&nbsp;Ma.{{r|Dawkins-Ancestors|page1=354}} A major extinction event occurred near the end of the Cambrian period,{{r|bbc-cambrian}} which ended 488 Ma.{{r|landing}}

Several hundred million years ago, plants (probably resembling [[Biology:Algae|algae]]) and fungi started growing at the edges of the water, and then out of it.{{r|Fortey|page1=138–140}} The oldest fossils of land fungi and plants date to 480–460&nbsp;Ma, though molecular evidence suggests the fungi may have colonized the land as early as 1000&nbsp;Ma and the plants 700&nbsp;Ma.{{r|heckman}} Initially remaining close to the water's edge, mutations and variations resulted in further colonization of this new environment. The timing of the first animals to leave the oceans is not precisely known: the oldest clear evidence is of arthropods on land around 450&nbsp;Ma,{{r|johnson}} perhaps thriving and becoming better adapted due to the vast food source provided by the terrestrial plants. There is also unconfirmed evidence that arthropods may have appeared on land as early as 530 Ma.{{r|macnaughton}}

=== Evolution of tetrapods ===
{{further|Biology:Evolution of tetrapods}}
[[File:Tiktaalik BW.jpg|thumb|right|''[[Biology:Tiktaalik|Tiktaalik]]'', a fish with limb-like fins and a predecessor of tetrapods. Reconstruction from fossils about 375&nbsp;million years old.]]
At the end of the Ordovician period, 443&nbsp;Ma,{{r|shortTimeScale}} additional extinction events occurred, perhaps due to a concurrent ice age.{{r|bbc-ordovician}} Around 380 to 375&nbsp;Ma, the first [[Biology:Tetrapod|tetrapod]]s evolved from fish.{{r|clack-sa}} Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water.{{r|clack-sa}} They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the [[Biology:Amphibian|amphibian]]s. About 365&nbsp;Ma, another [[Earth:Late Devonian extinction|period of extinction]] occurred, perhaps as a result of [[Earth:Global cooling|global cooling]].<ref>{{Cite book|last=McGhee |first=George R. Jr. |date=1996 |title=The Late Devonian Mass Extinction: the Frasnian/Famennian Crisis |publisher=Columbia University Press |isbn=978-0-231-07504-6}}</ref> Plants evolved [[Biology:Seed|seed]]s, which dramatically accelerated their spread on land, around this time (by approximately 360&nbsp;Ma).{{r|willis|waikato}}

About 20&nbsp;million years later (340&nbsp;Ma{{r|Dawkins-Ancestors|page1=293–296}}), the amniotic egg evolved, which could be laid on land, giving a survival advantage to tetrapod embryos. This resulted in the divergence of [[Biology:Amniote|amniote]]s from amphibians. Another 30&nbsp;million years (310&nbsp;Ma{{r|Dawkins-Ancestors|page1=254–256}}) saw the divergence of the [[Biology:Synapsid|synapsid]]s (including mammals) from the sauropsids (including birds and reptiles). Other groups of organisms continued to evolve, and lines diverged—in fish, insects, bacteria, and so on—but less is known of the details.{{Citation needed|date=March 2016}}

[[File:Europasaurus holgeri Scene 2.jpg|thumb|[[Biology:Dinosaur|Dinosaur]]s were the dominant terrestrial vertebrates throughout most of the Mesozoic]]
After yet another, the most severe extinction of the period (251~250&nbsp;Ma), around 230&nbsp;Ma, dinosaurs split off from their reptilian ancestors.{{r|bbc-new_blood}} The [[Biology:Triassic–Jurassic extinction event|Triassic–Jurassic extinction event]] at 200&nbsp;Ma spared many of the dinosaurs,{{r|shortTimeScale|bbc-triassic}} and they soon became dominant among the vertebrates. Though some mammalian lines began to separate during this period, existing mammals were probably small animals resembling shrews.{{r|Dawkins-Ancestors|page1=169}}

The boundary between avian and non-avian dinosaurs is not clear, but ''[[Biology:Archaeopteryx|Archaeopteryx]]'', traditionally considered one of the first birds, lived around 150&nbsp;Ma.{{r|archaeopteryx}}

The earliest evidence for the [[Biology:Flowering plant|angiosperms]] evolving flowers is during the Cretaceous period, some 20&nbsp;million years later (132&nbsp;Ma).{{r|tol-angiosperms}}

===Extinctions===
The first of five great mass extinctions was the Ordovician-Silurian extinction. Its possible cause was the intense glaciation of Gondwana, which eventually led to a [[Earth:Snowball Earth|Snowball Earth]]. 60% of marine invertebrates became extinct and 25% of all families.{{Citation needed|date=March 2016}}

The second mass extinction was the [[Earth:Late Devonian extinction|Late Devonian extinction]], probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like {{CO2}}) or the [[Earth:Eutrophication|eutrophication]] of water. 70% of all species became extinct.<ref>{{Cite web |title=Devonian extinctions {{!}} Definition & Facts {{!}} Britannica |url=https://www.britannica.com/science/Devonian-extinctions |access-date=2022-07-15 |website=www.britannica.com |language=en}}</ref>

The third mass extinction was the Permian-Triassic, or the Great Dying, event was possibly caused by some combination of the Siberian Traps volcanic event, an asteroid impact, [[Chemistry:Methane clathrate|methane hydrate]] gasification, sea level fluctuations, and a major [[Earth:Anoxic event|anoxic event]]. Either the proposed [[Earth:Wilkes Land crater|Wilkes Land crater]]{{r|bbc-antarctic-crater}} in Antarctica or [[Earth:Bedout|Bedout structure]] off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic extinction. But it remains uncertain whether either these or other proposed Permian-Triassic boundary craters are either real impact craters or even contemporaneous with the Permian-Triassic extinction event. This was by far the deadliest extinction ever, with about 57% of all families and 83% of all [[Biology:Genus|genera]] killed.<ref name="Benton">{{cite book|author=Benton M J|title=When life nearly died: the greatest mass extinction of all time |publisher=Thames & Hudson |location=London |year=2005 |isbn=978-0-500-28573-2 }}</ref><ref name="BergstromDugatkin2012">{{cite book|author1=Carl T. Bergstrom|author2=Lee Alan Dugatkin|title=Evolution|url=https://books.google.com/books?id=SeaEZwEACAAJ|year=2012|publisher=Norton|isbn=978-0-393-92592-0|page=515}}</ref>

The fourth mass extinction was the Triassic-Jurassic extinction event in which almost all synapsids and archosaurs became extinct, probably due to new competition from dinosaurs.<ref>{{Cite web |title=end-Triassic extinction {{!}} Evidence & Facts {{!}} Britannica |url=https://www.britannica.com/science/end-Triassic-extinction |access-date=2022-07-15 |website=www.britannica.com |language=en}}</ref>

The fifth and most recent mass extinction was the Cretaceous-Paleogene extinction event. In 66&nbsp;Ma, a {{convert|10|km|adj=on|sp=us}} [[Astronomy:Asteroid|asteroid]] struck Earth just off the Yucatán Peninsula—somewhere in the southwestern tip of then Laurasia—where the [[Earth:Chicxulub crater|Chicxulub crater]] is today. This ejected vast quantities of particulate matter and vapor into the air that occluded sunlight, inhibiting photosynthesis. 75% of all life, including the non-avian dinosaurs, became extinct,{{r|cosmic-evolution-bio4}} marking the end of the Cretaceous period and Mesozoic era.{{Citation needed|date=March 2016}}

===Diversification of mammals===
{{hatnote|Further information: [[Biology:Evolution of mammals|Evolution of mammals]]}}
The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that filled the world by the late Triassic. The first mammals were very small, and were probably nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-Paleogene extinction event.<ref>{{cite web |first=Bob |last=Strauss |title=The First Mammals:The Early Mammals of the Triassic, Jurassic and Cretaceous Periods |website=about.com |url=http://dinosaurs.about.com/od/otherprehistoriclife/a/earlymammals.htm |access-date=12 May 2015}}</ref> By the early Paleocene the Earth recovered from the extinction, and mammalian diversity increased. Creatures like ''[[Biology:Ambulocetus|Ambulocetus]]'' took to the oceans to eventually evolve into whales,<ref>{{cite web |url=http://www.amnh.org/explore/news-blogs/news-posts/a-walking-whale-ambulocetus |title=A Walking Whale: Ambulocetus |publisher=American Museum of Natural History |date=2014-05-01 |access-date=2016-01-10}}</ref> whereas some creatures, like primates, took to the trees.<ref>{{cite web |url=http://anthro.palomar.edu/earlyprimates/early_2.htm |title=Early Primate Evolution: The First Primates |last=O'Neil |first=Dennis |publisher=Palomar College |date=2012 |access-date=2016-01-10 |url-status=dead |archive-url=https://web.archive.org/web/20151225115518/http://anthro.palomar.edu/earlyprimates/early_2.htm |archive-date=2015-12-25 }}</ref> This all changed during the mid to late Eocene when the circum-Antarctic current formed between Antarctica and Australia which disrupted weather patterns on a global scale. Grassless [[Earth:Savanna|savanna]] began to predominate much of the landscape, and mammals such as ''[[Biology:Andrewsarchus|Andrewsarchus]]'' rose up to become the largest known terrestrial predatory mammal ever,<ref>{{cite web|url=http://www.amnh.org/explore/news-blogs/on-exhibit-posts/andrewsarchus-superb-skull-of-a-gigantic-beast |title=Andrewsarchus, "Superb Skull of a Gigantic Beast," Now on View in Whales Exhibit |publisher=[[Biology:American Museum of Natural History|American Museum of Natural History]] |date=2014-05-01 |access-date=2016-01-10}}</ref> and [[Biology:Archaeoceti|early whales]] like ''[[Biology:Basilosaurus|Basilosaurus]]'' took control of the seas. {{Citation needed|date=March 2016}}

The evolution of [[Biology:Poaceae|grasses]] brought a remarkable change to the Earth's landscape, and the new open spaces created pushed mammals to get bigger and bigger. Grass started to expand in the Miocene, and the Miocene is where many modern- day mammals first appeared. Giant [[Biology:Ungulates|ungulates]] like ''[[Biology:Paraceratherium|Paraceratherium]]'' and ''[[Biology:Deinotherium|Deinotherium]]'' evolved to rule the grasslands. The evolution of grass also brought [[Biology:Primate|primate]]s down from the trees, and started [[Biology:Human evolution|human evolution]]. The first big cats evolved during this time as well.<ref>{{cite web|author=George Dvorsky |url=http://io9.com/the-worlds-first-big-cats-came-from-asia-not-africa-1463616060 |title=The world's first big cats came from Asia, not Africa |publisher=Io9.com |date=November 13, 2013 |access-date=2016-01-10}}</ref> The Tethys Sea was closed off by the collision of Africa and Europe.<ref>{{cite journal|url=http://www.clim-past.net/9/2687/2013/cp-9-2687-2013.pdf |title=The role of eastern Tethys seaway closure in the Middle Miocene Climatic Transition (c. 14 Ma) |first1=N. |last1=Hamon |first2=P. |last2=Sepulchre |first3=V. |last3=Lefebvre |first4=G. |last4=Ramstein |journal=Climate of the Past |date=2013 |volume=9 |issue=6 |pages=2687–2702 |access-date=2016-01-10 |doi=10.5194/cp-9-2687-2013|bibcode = 2013CliPa...9.2687H |s2cid=203104343 |doi-access=free }}</ref>

The formation of Panama was perhaps the most important geological event to occur in the last 60 million years. Atlantic and Pacific currents were closed off from each other, which caused the formation of the [[Earth:Gulf Stream|Gulf Stream]], which made Europe warmer. The land bridge allowed the isolated creatures of South America to migrate over to North America, and vice versa.<ref>{{cite web|url=http://earthobservatory.nasa.gov/IOTD/view.php?id=4073|title=Isthmus of Panama|author=N.A.S.A.|work=N.A.S.A|date=31 December 2003}}</ref> Various species migrated south, leading to the presence in South America of llamas, the [[Biology:Spectacled bear|spectacled bear]], [[Biology:Kinkajou|kinkajou]]s and [[Biology:Jaguar|jaguar]]s.{{citation needed|date=January 2016}}

Three million years ago saw the start of the Pleistocene epoch, which featured dramatic climatic changes due to the ice ages. The ice ages led to the evolution of modern man in Saharan Africa and expansion. The mega-fauna that dominated fed on grasslands that, by now, had taken over much of the subtropical world. The large amounts of water held in the ice allowed for various bodies of water to shrink and sometimes disappear such as the North Sea and the Bering Strait. It is believed by many that a huge migration took place along [[Earth:Beringia|Beringia]] which is why, today, there are camels (which evolved and became extinct in North America), horses (which evolved and became extinct in North America), and Native Americans. The ending of the last ice age coincided with the expansion of man, along with a massive die out of ice age mega-fauna. This extinction is nicknamed "[[Earth:Holocene extinction|the Sixth Extinction]]".

[[File:IceAgeEarth.jpg|thumb|upright=1.35|An artist's impression of ice age Earth at glacial maximum.]]

===Human evolution===
{{Human timeline}}
{{Main|Biology:Human evolution}}
A small African ape living around 6&nbsp;Ma was the last animal whose descendants would include both modern humans and their closest relatives, the [[Biology:Pan (genus)|chimpanzee]]s.<ref name=":0" />{{r|Dawkins-Ancestors|page1=100–101}} Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability to walk upright.{{r|Dawkins-Ancestors|page1=95–99}} [[Biology:Brain|Brain]] size increased rapidly, and by 2&nbsp;Ma, the first animals classified in the genus ''[[Biology:Homo|Homo]]'' had appeared.{{r|Fortey|page1=300}} Around the same time, the other branch split into the ancestors of the [[Biology:Common chimpanzee|common chimpanzee]] and the ancestors of the [[Biology:Bonobo|bonobo]] as evolution continued simultaneously in all life forms.{{r|Dawkins-Ancestors|page1=100–101}}

The ability to control [[Astronomy:Fire|fire]] probably began in ''[[Biology:Homo erectus|Homo erectus]]'' (or ''Homo ergaster''), probably at least 790,000&nbsp;years ago{{r|goren-inbar}} but perhaps as early as 1.5&nbsp;Ma.{{r|Dawkins-Ancestors|page1=67}} The use and discovery of controlled fire may even predate ''Homo erectus''. Fire was possibly used by the early Lower Paleolithic ([[History:Oldowan|Oldowan]]) hominid ''[[Biology:Homo habilis|Homo habilis]]'' or strong [[Biology:Australopithecine|australopithecine]]s such as ''[[Biology:Paranthropus|Paranthropus]].''{{r|McClellan}}
[[File:Human evolution chart-en.svg|thumb|left|A reconstruction of human history based on fossil data.<ref>{{Cite journal|last=Reed|first=David L.|display-authors=4|author2=Smith, Vincent S. |author3=Hammond, Shaless L. |author4=Rogers, Alan R. |author5= Clayton, Dale H. |title=Genetic Analysis of Lice Supports Direct Contact between Modern and Archaic Humans|journal=PLOS Biology|date=2004|volume=2|issue=11|pages=e340|doi=10.1371/journal.pbio.0020340|pmid=15502871|pmc=521174 |doi-access=free }}</ref>]]
It is more difficult to establish the [[Philosophy:Origin of language|origin of language]]; it is unclear whether ''Homo erectus'' could speak or if that capability had not begun until ''Homo sapiens''.{{r|Dawkins-Ancestors|page1=67}} As brain size increased, babies were born earlier, before their heads grew too large to pass through the [[Biology:Pelvis|pelvis]]. As a result, they exhibited more [[Biology:Neuroplasticity|plasticity]], and thus possessed an increased capacity to learn and required a longer period of dependence. Social skills became more complex, language became more sophisticated, and tools became more elaborate. This contributed to further cooperation and intellectual development.{{r|McNeill|page1=7}} Modern humans (''Homo sapiens'') are believed to have originated around 200,000&nbsp;years ago or earlier in Africa; the oldest fossils date back to around 160,000&nbsp;years ago.{{r|gibbons}}

The first humans to show signs of [[Philosophy:Spirituality|spirituality]] are the [[Biology:Neanderthal|Neanderthal]]s (usually classified as a separate species with no surviving descendants); they buried their dead, often with no sign of food or tools.{{r|hopfe|page1=17}} However, evidence of more sophisticated beliefs, such as the early Cro-Magnon cave paintings (probably with magical or religious significance){{r|hopfe|page1=17–19}} did not appear until 32,000&nbsp;years ago.{{r|Chauvet}} Cro-Magnons also left behind stone figurines such as Venus of Willendorf, probably also signifying religious belief.{{r|hopfe|page1=17–19}} By 11,000&nbsp;years ago, ''Homo sapiens'' had reached the southern tip of [[Earth:South America|South America]], the last of the uninhabited continents (except for Antarctica, which remained undiscovered until 1820 AD).{{r|oxford-atlas}} Tool use and communication continued to improve, and interpersonal relationships became more intricate.{{Citation needed|date=March 2016}}
{{Clear}}

====Human history====
{{Main|History:Human history|Social:Cradle of civilization}}
[[File:Da Vinci Vitruve Luc Viatour (cropped).jpg|thumb|''[[Vitruvian Man|Vitruvian Man]]'' by Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance.]]
Throughout more than 90% of its history, ''Homo sapiens'' lived in small bands as [[Social:Nomad|nomad]]ic [[Biology:Hunter-gatherer|hunter-gatherer]]s.{{r|McNeill|page1=8}} As language became more complex, the ability to remember and communicate information resulted, according to a theory proposed by [[Biography:Richard Dawkins|Richard Dawkins]], in a new replicator: the [[Philosophy:Meme|meme]].{{r|dawkins-sg}} Ideas could be exchanged quickly and passed down the generations. Cultural evolution quickly outpaced [[Biology:Evolution|biological evolution]], and [[History:History|history]] proper began. Between 8500 and 7000 [[History:Before Christ|BC]], humans in the [[Place:Fertile Crescent|Fertile Crescent]] in the [[Place:Middle East|Middle East]] began the systematic husbandry of plants and animals: [[Earth:Agriculture|agriculture]].{{r|Tudge}} This spread to neighboring regions, and developed independently elsewhere, until most ''Homo sapiens'' lived sedentary lives in permanent settlements as farmers. Not all societies abandoned nomadism, especially those in isolated areas of the globe poor in domesticable plant species, such as {{wipe|Australia}}.{{r|diamond}} However, among those civilizations that did adopt agriculture, the relative stability and increased productivity provided by farming allowed the population to expand.{{Citation needed|date=March 2016}}

Agriculture had a major impact; humans began to affect the environment as never before. Surplus food allowed a priestly or governing class to arise, followed by increasing [[Social:Division of labour|division of labor]]. This led to Earth's first civilization at Sumer in the Middle East, between 4000 and 3000 BC.{{r|McNeill|page1=15}} Additional civilizations quickly arose in ancient Egypt, at the Indus River valley and in China. The invention of [[Social:Writing|writing]] enabled complex societies to arise: record-keeping and libraries served as a storehouse of knowledge and increased the cultural transmission of information. Humans no longer had to spend all their time working for survival, enabling the first specialized occupations (e.g. craftsmen, merchants, priests, etc.). Curiosity and education drove the pursuit of knowledge and wisdom, and various disciplines, including [[Science|science]] (in a primitive form), arose. This in turn led to the emergence of increasingly larger and more complex civilizations, such as the first empires, which at times traded with one another, or fought for territory and resources.

By around 500 BC, there were advanced civilizations in the Middle East, Iran, India, China, and Greece, at times expanding, at times entering into decline.{{r|McNeill|page1=3}} In 221 BC, China became a single polity that would grow to spread its culture throughout [[Place:East Asia|East Asia]], and it has remained the most populous nation in the world. During this period, famous Hindu texts known as [[Religion:Vedas|vedas]] came in existence in Indus valley civilization. This civilization developed in [[Social:Warfare|warfare]], arts, [[Science|science]], mathematics and architecture.{{citation needed|date=April 2021}} The fundamentals of [[Social:Western culture|Western civilization]] were largely shaped in {{wipe|Ancient Greece}}, with the world's first [[Social:Democracy|democratic government]] and major advances in philosophy and [[Science|science]], and in [[History:Ancient Rome|Ancient Rome]] with advances in law, government, and engineering.<ref name="Daly2013">{{cite book|author=Jonathan Daly|title=The Rise of Western Power: A Comparative History of Western Civilization|url=https://books.google.com/books?id=9aZPAQAAQBAJ|date=19 December 2013|publisher=A&C Black|isbn=978-1-4411-1851-6|pages=7–9}}</ref> The Roman Empire was [[Religion:Religion in ancient Rome#History of Roman religion|Christianized by Emperor Constantine]] in the early 4th century and declined by the end of the 5th. Beginning with the 7th century, [[Religion:Christianization#Christianization of Europe (6th–9th centuries)|Christianization of Europe]] began, and since at least the 4th century [[Religion:Christianity|Christianity]] has played a prominent role in the [[History:History of Western civilization|shaping of Western civilization]].<ref>[https://www.britannica.com/EBchecked/topic/507284/Roman-Catholicism Roman Catholicism], "Roman Catholicism, Christian church that has been the decisive spiritual force in the history of Western civilization". [[Encyclopædia Britannica]]</ref><ref name="Caltron J.H Hayas">Caltron J.H Hayas, ''Christianity and Western Civilization'' (1953), Stanford University Press, p. 2: That certain distinctive features of our Western civilization—the civilization of western Europe and of America—have been shaped chiefly by Judaeo–Christianity, Catholic and Protestant.</ref><ref name="Orlandis">Jose Orlandis, 1993, "A Short History of the Catholic Church," 2nd edn. (Michael Adams, Trans.), Dublin:Four Courts Press, {{ISBN|1851821252}}, preface, see [https://books.google.com/books?id=KYdbpwAACAAJ], accessed 8 December 2014. p. (preface)</ref><ref name="How The Catholic Church Built Western Civilization">Thomas E. Woods and Antonio Canizares, 2012, "How the Catholic Church Built Western Civilization," Reprint edn., Washington, D.C.: Regnery History, {{ISBN|1596983280}}, see [https://books.google.com/books?id=jYvmAgAAQBAJ, accessed 8 December 2014. p. 1: "Western civilization owes far more to Catholic Church than most people—Catholic included—often realize. The Church in fact built Western civilization."]</ref><ref name="Perry2012">{{cite book |author=Marvin Perry |title=Western Civilization: A Brief History, Volume I: To 1789 |url=https://books.google.com/books?id=U2pnv0Aoh2EC&pg=PA33 |date=1 January 2012 |publisher=Cengage Learning |isbn=978-1-111-83720-4 |pages=33–}}</ref><ref>{{cite book |last=Spielvogel |first=Jackson J. |title=Western Civilization: A Brief History, Volume I: To 1715 |year=2016 |isbn=978-1-305-63347-6 |edition=Cengage Learning |page=156|publisher=Cengage Learning }}</ref><ref>{{cite book |last=Neill |first=Thomas Patrick |title=Readings in the History of Western Civilization, Volume 2 |year=1957 |edition=Newman Press |page=224}}</ref><ref>{{cite book|last1=O'Collins|first1=Gerald|title=Catholicism: The Story of Catholic Christianity |year=2003|isbn=978-0-19-925995-3|publisher=Oxford University Press |last2=Farrugia|first2=Maria |page=v (preface)}}</ref> In 610, [[Religion:Islam|Islam]] was founded and quickly became the dominant religion in [[Place:Western Asia|Western Asia]]. The [[Organization:House of Wisdom|House of Wisdom]] was established in Abbasid-era [[Place:Baghdad|Baghdad]], Iraq.<ref>{{Cite web|url=https://www.britannica.com/place/Bayt-al-Hikmah|title=Bayt al-Hikmah|publisher=Encyclopedia Britannica|access-date=November 3, 2016}}</ref> It is considered to have been a major intellectual center during the Islamic Golden Age, where Muslim scholars in [[Place:Baghdad|Baghdad]] and [[Place:Cairo|Cairo]] flourished from the ninth to the thirteenth centuries until the Mongol sack of Baghdad in 1258 AD. In 1054 AD the [[Religion:East–West Schism|Great Schism]] between the Roman Catholic Church and the [[Religion:Eastern Orthodox Church|Eastern Orthodox Church]] led to the prominent cultural differences between [[Place:Western Europe|Western]] and [[Place:Eastern Europe|Eastern Europe]].<ref name="BideleuxJeffries48">{{cite book |title=A history of eastern Europe: crisis and change |last1=Bideleux |first1=Robert |last2=Jeffries |first2=Ian |publisher=[[Company:Routledge|Routledge]] |isbn=978-0-415-16112-1 |page=48 |url=https://books.google.com/books?id=U39AYJm1L94C |year=1998}}</ref>

In the 14th century, the [[Philosophy:Renaissance|Renaissance]] began in Italy with advances in religion, art, and science.{{r|McNeill|page1=317–319}} At that time the Christian Church as a political entity lost much of its power. In 1492, Christopher Columbus reached the Americas, initiating great changes to the new world. European civilization began to change beginning in 1500, leading to the scientific and [[History:Industrial Revolution|industrial]] revolutions. That continent began to exert political and cultural [[Social:Hegemony|dominance]] over human societies around the world, a time known as the Colonial era (also see [[Age of Discovery]]).{{r|McNeill|page1=295–299}} In the 18th century a cultural movement known as the [[Philosophy:Age of Enlightenment|Age of Enlightenment]] further shaped the mentality of Europe and contributed to its [[Philosophy:Secularization|secularization]]. From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in [[Social:World war|world wars]]. Established following [[Social:World War I|World War I]], the League of Nations was a first step in establishing international institutions to settle disputes peacefully. After failing to prevent [[Social:World War II|World War II]], humankind's bloodiest conflict, it was replaced by the {{wipe|United Nations}}. After the war, many new states were formed, declaring or being granted independence in a period of decolonization. The democratic capitalist {{wipe|United States}} and the socialist [[Place:Soviet Union|Soviet Union]] became the world's dominant [[Social:Superpower|superpowers]] for a time, and they held an ideological, often-violent rivalry known as the [[History:Cold War|Cold War]] until the dissolution of the latter. In 1992, several European nations joined in the [[Social:European Union|European Union]]. As transportation and communication improved, the economies and political affairs of nations around the world have become increasingly intertwined. This [[Social:Globalization|globalization]] has often produced both conflict and cooperation.{{citation needed|date=January 2016}}
{{Clear}}

==== Recent events ====
[[File:Aldrin Apollo 11 original.jpg|thumb|Astronaut [[Biography:Buzz Aldrin|Buzz Aldrin]] on the Moon, photographed by Neil Armstrong, 1969]]
{{Main|History:Late modern period}}
Change has continued at a rapid pace from the mid-1940s to today. Technological developments include [[Engineering:Nuclear weapon|nuclear weapon]]s, [[Computer|computer]]s, [[Biology:Genetic engineering|genetic engineering]], and [[Engineering:Nanotechnology|nanotechnology]]. [[Finance:Economic globalization|Economic globalization]], spurred by advances in communication and transportation technology, has influenced everyday life in many parts of the world. Cultural and institutional forms such as [[Social:Democracy|democracy]], [[Philosophy:Capitalism|capitalism]], and [[Biology:Environmentalism|environmentalism]] have increased influence. Major concerns and problems such as [[Disease|disease]], [[Social:War|war]], poverty, violent [[Philosophy:Political radicalism|radicalism]], and recently, human-caused [[Earth:Global warming|climate change]] have risen as the world population increases.{{citation needed|date=January 2016}}

In 1957, the Soviet Union launched [[Engineering:Sputnik 1|the first artificial satellite]] into orbit and, soon afterward, [[Biography:Yuri Gagarin|Yuri Gagarin]] became the first human in space. Neil Armstrong, an American, was the first to set foot on another astronomical object, the Moon. Uncrewed probes have been sent to all the known planets in the Solar System, with some (such as the two [[Astronomy:Voyager program|Voyager]] spacecraft) having left the Solar System. Five space agencies, representing over fifteen countries,{{r|Human}} have worked together to build the [[Astronomy:International Space Station|International Space Station]]. Aboard it, there has been a continuous human presence in space since 2000.{{r|Expedit}} The [[World Wide Web]] became a part of everyday life in the 1990s, and since then has become an indispensable source of information in the [[Finance:Developed country|developed world]].{{Citation needed|date=March 2016}}
{{clear}}

== See also ==
{{div col|colwidth=20em}}
* {{annotated link|Astronomy:Chronology of the universe}}
* {{annotated link|Detailed logarithmic timeline}}
* {{annotated link|Astronomy:Earth phase}}
* {{annotated link|Biology:Evolutionary history of life}}
* {{annotated link|Earth:Future of Earth}}
* {{annotated link|Earth:Geological history of Earth}}
* {{annotated link|Unsolved:Global catastrophic risk}}
* {{annotated link|Biology:Timeline of the evolutionary history of life}}
* {{annotated link|Earth:Timeline of natural history}}
{{div col end}}

==Notes==
{{Reflist|group=nb}}

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<ref name="Best">See Best (2003), pp 612–613; [[Chemistry:Zircon|zircon]]s from the [[Earth:Acasta Gneiss|Acasta Gneiss]] are dated at 4030&nbsp;million years by (among others) Stern & Bleeker (1998)</ref>
-->

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<ref name=Canup>{{cite journal |last1=Canup |first1=R.M. |last2=Asphaug |first2=E. |date=2001 |title=Origin of the Moon in a giant impact near the end of the Earth's formation |journal=Nature |volume=412 |issue=6848 |pages=708–712 |bibcode=2001Natur.412..708C |doi=10.1038/35089010 |pmid=11507633 |s2cid=4413525}}</ref>

<ref name=Cavosie>{{Cite journal |last1=Cavosie |first1=A.J. |first2=J.W. |last2=Valley |first3=S.A. |last3=Wilde |author4=Edinburgh Ion Microprobe Facility (E.I.M.F.) |date=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 |doi=10.1016/j.epsl.2005.04.028 |bibcode=2005E&PSL.235..663C}}</ref>
}}

== Further reading ==
{{Refbegin|30em}}
* {{Cite book |first=G.B. |last=Dalrymple |date=1991 |title=The Age of the Earth |publisher=Stanford University Press |location=California |isbn=978-0-8047-1569-0}}
* {{Cite journal |last=Dalrymple |first=G. Brent |title=The age of the Earth in the twentieth century: a problem (mostly) solved |journal=Geological Society of London, Special Publications |date=2001 |volume=190 |issue=1 |pages=205–221 |url=http://sp.lyellcollection.org/content/190/1/205.abstract |access-date=2012-04-13 |doi=10.1144/GSL.SP.2001.190.01.14 |bibcode=2001GSLSP.190..205D |s2cid=130092094}}
* {{Cite book |last=Dawkins |first=Richard |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Life |date=2004 |publisher=Houghton Mifflin Company |location=Boston |isbn=978-0-618-00583-3 |title-link=The Ancestor's Tale}}
* {{Cite book |editor1-first=F.M. |editor1-last=Gradstein |editor2-first=James George |editor2-last=Ogg |editor3-first=Alan Gilbert |editor3-last=Smith |title=A Geological Time Scale 2004 |date=2004 |publisher={{wipe|Cambridge University Press}} |isbn=978-0-521-78673-7 |others=Reprinted with corrections 2006}}
* {{cite report |first1=Felix M. |last1=Gradstein |first2=James G. |last2=Ogg |first3=Martin |last3=van Kranendonk |title=On the Geological Time Scale 2008 |url=http://www.nysm.nysed.gov/nysgs/resources/images/geologicaltimescale.pdf |date=2008 |publisher=International Commission on Stratigraphy |access-date=20 April 2012 |docket=Fig. 2 |url-status=dead |archive-url=https://web.archive.org/web/20121028022719/http://www.nysm.nysed.gov/nysgs/resources/images/geologicaltimescale.pdf |archive-date=28 October 2012}}
* {{Cite book|last=Levin |first=H.L. |date=2009 |title=The Earth through time |publisher=Saunders College Publishing |edition=9th |isbn= 978-0-470-38774-0}}
* {{Cite book|last=Lunine |first=Jonathan I. |date=1999 |title=Earth: evolution of a habitable world |publisher={{wipe|Cambridge University Press}} |location=United Kingdom |isbn=978-0-521-64423-5}}
* {{Cite book |last=McNeill |first=Willam H. |title=A World History |orig-year=1967 |edition=4th |date=1999 |publisher=[[Organization:Oxford University Press|Oxford University Press]] |location=New York |isbn=978-0-19-511615-1}}
* Melosh, H. J.; Vickery, A. M. & Tonks, W. B. (1993). ''Impacts and the early environment and evolution of the terrestrial planets'', in Levy, H. J. & Lunine, Jonathan I. (eds.): ''Protostars and Planets III'', [[Organization:University of Arizona|University of Arizona]] Press, Tucson, pp.&nbsp;1339–1370.
* {{Cite book |last=Stanley |first=Steven M. |title=Earth system history |date=2005 |publisher=Freeman |location=New York |isbn=978-0-7167-3907-4 |edition=2nd}}
* {{cite journal |last1=Stern |first1=T.W. |last2=Bleeker |first2=W. |date=1998 |title=Age of the world's oldest rocks refined using Canada's SHRIMP: The Acasta Gneiss Complex, Northwest Territories, Canada |journal=Geoscience Canada |volume=25 |pages=27–31}}
* {{Cite journal |last=Wetherill |first=G.W. |title=Occurrence of Earth-Like Bodies in Planetary Systems |journal=Science |date=1991 |volume=253 |issue=5019 |pages=535–538 |doi=10.1126/science.253.5019.535 |pmid=17745185 |bibcode=1991Sci...253..535W |s2cid=10023022}}
{{Refend}}

== External links ==
* [[Biography:Paul Davies|Davies, Paul]]. "[https://www.theguardian.com/technology/2005/dec/20/comment.science Quantum leap of life]". ''{{wipe|The Guardian}}''. 2005 December 20. – discusses speculation on the role of quantum systems in the origin of life
* [http://www.johnkyrk.com/evolution.html Evolution timeline] (uses Flash Player). Animated story of life shows everything from the big bang to the formation of the Earth and the development of bacteria and other organisms to the ascent of man.
* [http://www.bbc.com/earth/bespoke/story/20150123-earths-25-biggest-turning-points/ 25 biggest turning points in Earth History] BBC
* [http://historystack.com/30_Major_Events_in_History_of_the_Earth Evolution of the Earth]. Timeline of the most important events in the evolution of the Earth.
* [https://www.bbc.co.uk/programmes/p005493g Ageing the Earth], BBC Radio 4 discussion with Richard Corfield, Hazel Rymer & Henry Gee (''In Our Time'', Nov. 20, 2003)

{{Earth}}
{{Nature}}

{{DEFAULTSORT:History Of The Earth}}
[[Category:Earth]]
[[Category:Geochronology]]
[[Category:Geology theories]]
[[Category:Historical geology|*]]
[[Category:World history]]

{{Sourceattribution|History of Earth}}