View source for Biology:Biology

{{short description|Science that studies life}}
{{pp-semi-indef}}
[[File:Biology-0001.jpg|thumb|upright=1.35|Biology is the science of life. It spans multiple levels from biomolecules and cells to organisms and populations.]]
'''Biology''' is the [[Science|scientific]] study of life.<ref name="urry2017a">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=Evolution, the themes of biology, and scientific inquiry |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=2–26 |isbn=978-0134093413}}</ref><ref name="hillis2020">{{cite book |last1=Hillis |first1=David M. |last2=Heller |first2= H. Craig |last3=Hacker  |first3=Sally D. |last4=Laskowski |first4=Marta J. |last5=Sadava |first5=David E. |chapter=Studying life |title=Life: The Science of Biology |publisher=W. H. Freeman |edition=12th |date=2020 |isbn=978-1319017644}}</ref><ref name="freeman2017a">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=Biology and the three of life |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, N.J. |pages=1–18 |isbn=978-0321976499}}</ref> It is a [[Natural science|natural science]] with a broad scope but has several unifying themes that tie it together as a single, coherent field.<ref name="urry2017a"/><ref name="hillis2020"/><ref name="freeman2017a"/> For instance, all organisms are made up of [[Biology:Cell|cell]]s that process hereditary information encoded in [[Biology:Gene|gene]]s, which can be transmitted to future generations. Another major theme is [[Biology:Evolution|evolution]], which explains the unity and diversity of life.<ref name="urry2017a"/><ref name="hillis2020"/><ref name="freeman2017a"/> [[Biology:Metabolism|Energy processing]] is also important to life as it allows organisms to [[Biology:Animal locomotion|move]], grow, and reproduce.<ref name="urry2017a"/><ref name="hillis2020"/><ref name="freeman2017a"/> Finally, all organisms are able to regulate their own [[Internal environment|internal environment]]s.<ref name="urry2017a"/><ref name="hillis2020"/><ref name="freeman2017a"/><ref name="modell2015">{{Cite journal |last1=Modell |first1=Harold |last2=Cliff |first2=William |last3=Michael |first3=Joel |last4=McFarland |first4=Jenny |last5=Wenderoth |first5=Mary Pat |last6=Wright |first6=Ann |date=December 2015 |title=A physiologist's view of homeostasis |journal=Advances in Physiology Education |volume=39 |issue=4 |pages=259–266 |doi=10.1152/advan.00107.2015 |pmc=4669363 |pmid=26628646}}</ref><ref name="davies2013">{{cite journal |author1=Davies, PC |author2=Rieper, E |author3=Tuszynski, JA |title=Self-organization and entropy reduction in a living cell |journal=Bio Systems |volume=111 |issue=1 |pages=1–10 |date=January 2013 |pmid=23159919 |pmc=3712629 |doi=10.1016/j.biosystems.2012.10.005 |bibcode=2013BiSys.111....1D }}</ref>

[[Biologist]]s are able to study life at multiple [[Biology:Biological organisation|levels of organization]],<ref name="urry2017a"/> from the [[Biology:Molecular biology|molecular biology]] of a cell to the [[Biology:Anatomy|anatomy]] and [[Biology:Physiology|physiology]] of plants and animals, and evolution of [[Social:Population|population]]s.<ref name="urry2017a"/><ref name=aquarenagloss>Based on definition from: {{cite web |url=http://www.bio.txstate.edu/~wetlands/Glossary/glossary.html |archive-url=https://web.archive.org/web/20040608113114/http://www.bio.txstate.edu/~wetlands/Glossary/glossary.html |archive-date=2004-06-08 |title=Aquarena Wetlands Project glossary of terms |author=<!--Staff writer(s); no by-line.--> |publisher=Texas State University at San Marcos}}</ref> Hence, there are multiple [[Biology:Outline of biology#Branches|subdisciplines within biology]], each defined by the nature of their [[Research question|research question]]s and the [[Biology:List of research methods in biology|tool]]s that they use.<ref>{{Cite book|title=Molecular Biology, Principles of Genome Function|last=Craig|first=Nancy|year=2014|publisher=OUP Oxford |isbn=978-0-19-965857-2}}</ref><ref>{{Cite journal|last1=Mosconi|first1=Francesco |last2=Julou|first2=Thomas|last3=Desprat|first3=Nicolas |last4=Sinha|first4=Deepak Kumar|last5=Allemand|first5=Jean-François|last6=Vincent Croquette |last7=Bensimon|first7=David |date=2008 |title=Some nonlinear challenges in biology |journal=Nonlinearity |volume=21 |issue=8 |page=T131|doi=10.1088/0951-7715/21/8/T03 |bibcode=2008Nonli..21..131M |s2cid=119808230 }}</ref><ref name="AB-20141208">{{cite web|url=https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/|title=How Did Life Become Complex, And Could It Happen Beyond Earth?|last=Howell|first=Elizabeth|date=8 December 2014|work=[[Astronomy:Astrobiology Magazine|Astrobiology Magazine]]|access-date=14 February 2018|archive-date=17 August 2018|archive-url=https://web.archive.org/web/20180817193332/https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/|url-status=usurped}}</ref> Like other [[Scientist|scientist]]s, biologists use the [[Scientific method|scientific method]] to make [[Observation|observation]]s, pose questions, generate [[Hypothesis|hypotheses]], perform [[Experiment|experiment]]s, and form conclusions about the world around them.<ref name="urry2017a"/>

Life on [[Astronomy:Earth|Earth]], which emerged more than 3.7 billion years ago,<ref name="Pearce 343–364">{{cite journal |last1=Pearce |first1=Ben K.D. |last2=Tupper |first2=Andrew S. |last3=Pudritz |first3=Ralph E. |last4=Higgs |first4=Paul G. |display-authors=3 |date=March 1, 2018 |title=Constraining the Time Interval for the Origin of Life on Earth |journal=Astrobiology |volume=18 |issue=3 |pages=343–364 |arxiv=1808.09460 |s2cid=4419671 |bibcode=2018AsBio..18..343P |doi=10.1089/ast.2017.1674 |pmid=29570409}}</ref> is immensely diverse. Biologists have sought to study and classify the various forms of life, from prokaryotic organisms such as [[Biology:Archaea|archaea]] and [[Biology:Bacteria|bacteria]] to [[Biology:Eukaryote|eukaryotic]] organisms such as [[Biology:Protist|protist]]s, [[Biology:Fungi|fungi]], [[Biology:Plant|plant]]s, and [[Biology:Animal|animal]]s. These various organisms contribute to the [[Biology:Biodiversity|biodiversity]] of an [[Earth:Ecosystem|ecosystem]], where they play specialized roles in the [[Biology:Biogeochemical cycle|cycling]] of [[Earth:Nutrient|nutrient]]s and [[Physics:Energy|energy]] through their [[Earth:Biophysical environment|biophysical environment]].

==History==
{{main|Biology:History of biology}}

[[File:Hooke-bluefly.jpg|thumb|upright|alt=A drawing of a fly from facing up, with wing detail|Diagram of a fly from Robert Hooke's innovative ''Micrographia'', 1665]]

The earliest of roots of [[Science|science]], which included [[Medicine:Medicine|medicine]], can be traced to ancient Egypt and [[Place:Mesopotamia|Mesopotamia]] in around 3000 to 1200 [[Common Era|BCE]].<ref name= "Lindberg1">{{cite book |last= Lindberg |first= David C. |year=2007 |chapter=Science before the Greeks |title= The beginnings of Western science: the European Scientific tradition in philosophical, religious, and institutional context |pages=1–20 |edition=2nd |location=Chicago, Illinois |publisher=University of Chicago Press |isbn= 978-0-226-48205-7}}</ref><ref name= "Grant2007a">{{cite book |last= Grant|first= Edward |year=2007 |chapter=Ancient Egypt to Plato |title= A History of Natural Philosophy: From the Ancient World to the Nineteenth Century |url= https://archive.org/details/historynaturalph00gran|url-access= limited|pages=[https://archive.org/details/historynaturalph00gran/page/n16 1]–26 |location=New York|publisher=Cambridge University Press |isbn= 978-052-1-68957-1}}</ref> Their contributions shaped ancient Greek [[Philosophy:Natural philosophy|natural philosophy]].<ref name= "Lindberg1"/><ref name= "Grant2007a"/><ref>{{cite book|last=Magner|first=Lois N.|title=A History of the Life Sciences, Revised and Expanded|url=https://books.google.com/books?id=YKJ6gVYbrGwC|year=2002|publisher=CRC Press|isbn=978-0-203-91100-6|url-status=live|archive-url=https://web.archive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC|archive-date=2015-03-24}}</ref><ref>{{cite book |first=Anthony |last=Serafini |title=The Epic History of Biology |date=2013 |publisher=Springer |url=https://books.google.com/books?id=Z3oECAAAQBAJ&q=biology%20egypt&pg=PA2 |access-date=14 July 2015 |isbn=978-1-4899-6327-7 |archive-date=15 April 2021 |archive-url=https://web.archive.org/web/20210415122005/https://books.google.com/books?id=Z3oECAAAQBAJ&q=biology%20egypt&pg=PA2 |url-status=live }}</ref> {{wipe|Ancient Greece}} philosophers such as [[Biography:Aristotle|Aristotle]] (384–322 BCE) contributed extensively to the development of biological knowledge. He explored biological causation and the diversity of life. His successor, [[Biography:Theophrastus|Theophrastus]], began the scientific study of plants.<ref name="eb1911">{{EB1911|wstitle=Theophrastus|inline=1}}</ref> Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany,<ref name="Fahd-815">{{cite book |last=Fahd |first=Toufic |contribution=Botany and agriculture|page=815|editor-last1=Morelon |editor-first1=Régis |editor-last2=Rashed |editor-first2=Roshdi |year=1996 |title=Encyclopedia of the History of Arabic Science |volume=3 |publisher=[[Company:Routledge|Routledge]] |isbn=978-0-415-12410-2|title-link=Encyclopedia of the History of Arabic Science }}</ref> and Rhazes (865–925) who wrote on [[Biology:Anatomy|anatomy]] and [[Biology:Physiology|physiology]]. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought.

Biology began to quickly develop with Anton van Leeuwenhoek's dramatic improvement of the [[Engineering:Microscope|microscope]]. It was then that scholars discovered spermatozoa, [[Biology:Bacteria|bacteria]], infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in [[Biology:Entomology|entomology]] and helped to develop techniques of microscopic [[Biology:Dissection|dissection]] and [[Staining|staining]].<ref>{{cite book |last=Magner |first=Lois N. |title=A History of the Life Sciences, Revised and Expanded |url=https://books.google.com/books?id=YKJ6gVYbrGwC |year=2002 |publisher=CRC Press |isbn=978-0-203-91100-6 |pages=133–44 |url-status=live |archive-url=https://web.archive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC |archive-date=2015-03-24 }}</ref> Advances in microscopy had a profound impact on biological thinking. In the early 19th century, biologists pointed to the central importance of the [[Biology:Cell|cell]]. In 1838, Schleiden and [[Biography:Theodor Schwann|Schwann]] began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells, continuing to support [[Biology:Spontaneous generation|spontaneous generation]]. However, Robert Remak and Rudolf Virchow were able to reify the third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into [[Biology:Cell theory|cell theory]].<ref>{{cite book |first=Jan |last=Sapp |year=2003 |title=Genesis: The Evolution of Biology |chapter=7 |publisher=Oxford University Press |location=New York |isbn=978-0-19-515618-8 }}</ref><ref>{{cite book |last=Coleman |first=William |year=1977 |title=Biology in the Nineteenth Century: Problems of Form, Function, and Transformation |publisher=Cambridge University Press |location=New York |isbn=978-0-521-29293-1 }}</ref>

Meanwhile, taxonomy and classification became the focus of natural historians. [[Biography:Carl Linnaeus|Carl Linnaeus]] published a basic [[Biology:Taxonomy|taxonomy]] for the natural world in 1735, and in the 1750s introduced [[Biology:Binomial nomenclature|scientific names]] for all his species.<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 4</ref> Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of [[Biology:Common descent|common descent]].<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 7</ref>

[[File:Charles Darwin aged 51.jpg|thumb|upright|In 1842, Charles Darwin penned his first sketch of ''On the Origin of Species''.<ref>* {{cite book |editor-last=Darwin |editor-first=Francis |year=1909 |title=The foundations of The origin of species, a sketch written in 1842 |url=http://darwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf |location=Cambridge |publisher=Printed at the University Press |lccn=61057537 |oclc=1184581 |access-date=27 November 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304111606/http://darwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf |url-status=live |page=53}}</ref>]]

Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck, who presented a coherent theory of evolution.<ref name="Gould 2002 187">[[Biography:Stephen Jay Gould|Gould, Stephen Jay]]. ''The Structure of Evolutionary Theory''. The Belknap Press of Harvard University Press: Cambridge, 2002. {{ISBN|0-674-00613-5}}. p. 187.</ref> The British [[Earth:Natural history|naturalist]] [[Biography:Charles Darwin|Charles Darwin]], combining the biogeographical approach of [[Biography:Alexander von Humboldt|Humboldt]], the uniformitarian geology of Lyell, Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, forged a more successful evolutionary theory based on [[Biology:Natural selection|natural selection]]; similar reasoning and evidence led [[Biography:Alfred Russel Wallace|Alfred Russel Wallace]] to independently reach the same conclusions.<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 10: "Darwin's evidence for evolution and common descent"; and chapter 11: "The causation of evolution: natural selection"</ref><ref>{{cite book |last=Larson |first=Edward J. |title=Evolution: The Remarkable History of a Scientific Theory |chapter-url=https://books.google.com/books?id=xzLRvxlJhzkC |year=2006 |publisher=Random House Publishing Group |isbn=978-1-58836-538-5 |chapter=Ch. 3 |url-status=live |archive-url=https://web.archive.org/web/20150324124009/http://books.google.com/books?id=xzLRvxlJhzkC |archive-date=2015-03-24 }}</ref>

The basis for modern genetics began with the work of [[Biography:Gregor Mendel|Gregor Mendel]] in 1865.<ref>{{Cite book|last=Henig|url=http://archive.org/details/monkingardenlost00heni|title=Op. cit|date=2000|pages=134–138}}</ref> This outlined the principles of biological inheritance.<ref name="miko2008a">{{cite journal |last=Miko |first=Ilona |title=Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. So just what are they? |journal=Nature Education |volume=1 |issue=1 |pages=134 |date=2008 |url=https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/ |access-date=2021-05-13 |archive-date=2019-07-19 |archive-url=https://web.archive.org/web/20190719224056/http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593 |url-status=live }}</ref> However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the [[Biology:Modern synthesis (20th century)|modern synthesis]] reconciled Darwinian evolution with [[Biology:Classical genetics|classical genetics]].<ref name="Futuyma2017a">{{cite book |last1=Futuyma |first1=Douglas J. |last2=Kirkpatrick |first2=Mark |date=2017|pages=3–26 |chapter=Evolutionary Biology |title=Evolution |edition=4th |publisher=Sinauer Associates |location=Sunderland, Mass.}}</ref> In the 1940s and early 1950s, a [[Biology:Hershey–Chase experiment|series of experiments]] by [[Biography:Alfred Hershey|Alfred Hershey]] and Martha Chase pointed to [[Biology:DNA|DNA]] as the component of chromosomes that held the trait-carrying units that had become known as genes. A focus on new kinds of model organisms such as viruses and bacteria, along with the discovery of the double-helical structure of DNA by [[Biography:James Watson|James Watson]] and [[Biography:Francis Crick|Francis Crick]] in 1953, marked the transition to the era of [[Biology:Molecular genetics|molecular genetics]]. From the 1950s onwards, biology has been vastly extended in the molecular domain. The [[Biology:Genetic code|genetic code]] was cracked by [[Biography:Har Gobind Khorana|Har Gobind Khorana]], [[Biography:Robert W. Holley|Robert W. Holley]] and [[Biography:Marshall Warren Nirenberg|Marshall Warren Nirenberg]] after DNA was understood to contain codons. The [[Biology:Human Genome Project|Human Genome Project]] was launched in 1990 to map the human [[Biology:Genome|genome]].<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/2940601.stm |title=Human genome finally complete |access-date=2006-07-22 |date=2003-04-14 |work=BBC News |first=Ivan |last=Noble |url-status=live |archive-url=https://web.archive.org/web/20060614141605/http://news.bbc.co.uk/2/hi/science/nature/2940601.stm |archive-date=2006-06-14 }}</ref>

==Chemical basis==
===Atoms and molecules===
{{Further |Chemistry:Chemistry}}

All organisms are made up of [[Chemistry:Chemical element|chemical element]]s;<ref name="urry2017b">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=The chemical context of life |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=28–43 |isbn=978-0134093413}}</ref> [[Chemistry:Oxygen|oxygen]], [[Chemistry:Carbon|carbon]], [[Chemistry:Hydrogen|hydrogen]], and [[Chemistry:Nitrogen|nitrogen]] account for most (96%) of the mass of all organisms, with [[Chemistry:Calcium|calcium]], [[Chemistry:Phosphorus|phosphorus]], [[Chemistry:Sulfur|sulfur]], [[Chemistry:Sodium|sodium]], [[Chemistry:Chlorine|chlorine]], and [[Chemistry:Magnesium|magnesium]] constituting essentially all the remainder. Different elements can combine to form [[Chemistry:Chemical compound|compound]]s such as water, which is fundamental to life.<ref name="urry2017b"/> [[Biology:Biochemistry|Biochemistry]] is the study of [[Chemistry:Chemical process|chemical process]]es within and relating to living organisms. [[Biology:Molecular biology|Molecular biology]] is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including [[Physics:Biomolecule|molecular]] synthesis, modification, mechanisms, and interactions.

===Water===
[[File:3D model hydrogen bonds in water.svg|right|thumb|Model of hydrogen bonds (1) between molecules of [[Chemistry:Properties of water|water]]]]

Life arose from the Earth's first [[Earth:Ocean|ocean]], which formed some 3.8 billion years ago.<ref name="freeman2017b">{{cite book |last1=Freeman |first1=Scott |title=Biological Science |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael |last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |date=2017 |publisher=Pearson |isbn=978-0321976499 |edition=6th |location=Hoboken, N.J. |pages=55–77 |chapter=Water and carbon: The chemical basis of life}}</ref> Since then, [[Chemistry:Water|water]] continues to be the most abundant molecule in every organism. Water is important to life because it is an effective [[Physics:Solvent|solvent]], capable of dissolving solutes such as sodium and [[Chemistry:Chloride|chloride]] ions or other small molecules to form an [[Chemistry:Aqueous solution|aqueous]] [[Chemistry:Solution|solution]]. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in [[Chemistry:Chemical reaction|chemical reaction]]s that sustain life.<ref name="freeman2017b"/> In terms of its [[Chemistry:Properties of water#Molecular structure|molecular structure]], water is a small [[Chemistry:Chemical polarity#Polar molecules|polar molecule]] with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H<sub>2</sub>O).<ref name="freeman2017b"/> Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge.<ref name="freeman2017b"/> This polar [[Chemistry:Properties of water|property]] of water allows it to attract other water molecules via hydrogen bonds, which makes water [[Chemistry:Cohesion|cohesive]].<ref name="freeman2017b"/> [[Physics:Surface tension|Surface tension]] results from the cohesive force due to the attraction between molecules at the surface of the liquid.<ref name="freeman2017b"/> Water is also [[Chemistry:Adhesion|adhesive]] as it is able to adhere to the surface of any polar or charged non-water molecules.<ref name="freeman2017b"/> Water is [[Physics:Density|denser]] as a [[Chemistry:Water#Liquid water|liquid]] than it is as a solid (or [[Earth:Ice|ice]]).<ref name="freeman2017b"/> This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby [[Physics:Thermal insulation|insulating]] the liquid below from the cold air above.<ref name="freeman2017b"/> Water has the capacity to absorb energy, giving it a higher [[Physics:Specific heat capacity|specific heat capacity]] than other solvents such as [[Chemistry:Ethanol|ethanol]].<ref name="freeman2017b"/> Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into [[Physics:Water vapor|water vapor]].<ref name="freeman2017b"/> As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and hydroxyl ions before reforming into a water molecule again.<ref name="freeman2017b"/> In [[Chemistry:Purified water|pure water]], the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a [[Chemistry:PH|pH]] that is neutral.

===Organic compounds===
{{Further |Chemistry:Organic chemistry}}

[[File:Glucose Haworth.png|thumb|right|Organic compounds such as glucose are vital to organisms.]]

[[Chemistry:Organic compound|Organic compound]]s are molecules that contain carbon bonded to another element such as hydrogen.<ref name="freeman2017b"/> With the exception of water, nearly all the molecules that make up each organism contain carbon.<ref name="freeman2017b"/><ref name="urry2017d">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=Carbon and the molecular diversity of life |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=56–65 |isbn=978-0134093413}}</ref> Carbon can form [[Chemistry:Covalent bond|covalent bond]]s with up to four other atoms, enabling it to form diverse, large, and complex molecules.<ref name="freeman2017b"/><ref name="urry2017d"/> For example, a single carbon atom can form four single covalent bonds such as in [[Chemistry:Methane|methane]], two [[Chemistry:Double bond|double covalent bond]]s such as in [[Chemistry:Carbon dioxide|carbon dioxide]] ({{CO2}}), or a [[Chemistry:Triple bond|triple covalent bond]] such as in [[Chemistry:Carbon monoxide|carbon monoxide]] (CO). Moreover, carbon can form very long chains of interconnecting [[Chemistry:Carbon–carbon bond|carbon–carbon bond]]s such as [[Chemistry:Octane|octane]] or ring-like structures such as [[Chemistry:Glucose|glucose]].

The simplest form of an organic molecule is the [[Chemistry:Hydrocarbon|hydrocarbon]], which is a large family of organic compounds that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as oxygen (O), hydrogen (H), phosphorus (P), and sulfur (S), which can change the chemical behavior of that compound.<ref name="freeman2017b"/> Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called [[Chemistry:Functional group|functional group]]s.<ref name="freeman2017b"/> There are six prominent functional groups that can be found in organisms: [[Chemistry:Amine|amino group]], [[Chemistry:Carboxylic acid|carboxyl group]], [[Chemistry:Carbonyl group|carbonyl group]], [[Chemistry:Hydroxy group|hydroxyl group]], [[Chemistry:Phosphate|phosphate group]], and [[Chemistry:Thiol|sulfhydryl group]].<ref name="freeman2017b"/>

In 1953, the Miller-Urey experiment showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of [[Earth:Early Earth|early Earth]], thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see [[Biology:Abiogenesis|abiogenesis]]).<ref name="hillisetal2014d">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=Carbon and molecular diversity of life |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=56–65 |isbn=978-1464175121}}</ref><ref name="freeman2017b"/>

===Macromolecules===
{{main|Biology:Macromolecule}}

[[File:225 Peptide Bond-01.jpg|thumb |right |The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of a [[Biology:Hemoglobin|hemoglobin]] protein]]

[[Biology:Macromolecule|Macromolecule]]s are large molecules made up of smaller subunits or [[Chemistry:Monomer|monomer]]s.<ref name="freeman2017c">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=Protein structure and function |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, N.J. |pages=78–92 |isbn=978-0321976499}}</ref> Monomers include sugars, amino acids, and nucleotides.<ref name="urry2017e">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=The structure and function of large biological molecules |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=66–92 |isbn=978-0134093413}}</ref> [[Chemistry:Carbohydrate|Carbohydrate]]s include monomers and polymers of sugars.<ref name="freeman2017e">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=An introduction to carbohydrate |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, N.J. |pages=107–118 |isbn=978-0321976499}}</ref>
Lipids are the only class of macromolecules that are not made up of polymers. They include [[Biology:Steroid|steroid]]s, [[Chemistry:Phospholipid|phospholipid]]s, and [[Chemistry:Fat|fat]]s,<ref name="urry2017e"/> largely nonpolar and hydrophobic (water-repelling) substances.<ref name="freeman2017f">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=Lipids, membranes, and the first cells |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, N.J. |pages=119–141|isbn=978-0321976499}}</ref>
Proteins are the most diverse of the macromolecules. They include [[Biology:Enzyme|enzyme]]s, [[Biology:Transport protein|transport protein]]s, large signaling molecules, [[Biology:Antibodies|antibodies]], and structural proteins. The basic unit (or monomer) of a protein is an [[Chemistry:Amino acid|amino acid]].<ref name="freeman2017c"/> Twenty amino acids are used in proteins.<ref name="freeman2017c"/>
Nucleic acids are polymers of [[Biology:Nucleotide|nucleotide]]s.<ref name="freeman2017d">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=Nucleic acids and the RNA world |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, N.J. |pages=93–106 |isbn=978-0321976499}}</ref> Their function is to store, transmit, and express hereditary information.<ref name="urry2017e"/>

==Cells==
{{main|Biology:Cell}}

[[Biology:Cell theory|Cell theory]] states that [[Biology:Cell|cell]]s are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through [[Biology:Cell division|cell division]].<ref name="mazzarello1999">{{cite journal |last=Mazzarello |first=P. |title=A unifying concept: the history of cell theory |journal=Nature Cell Biology |volume=1 |issue=1 |pages=E13–15 |date=May 1999 |pmid=10559875 |doi=10.1038/8964 |s2cid=7338204 }}</ref> Most cells are very small, with diameters ranging from 1 to 100&nbsp;[[Micrometre|micrometer]]s and are therefore only visible under a light or [[Engineering:Electron microscope|electron microscope]].<ref>{{cite book|url=http://www.phschool.com/el_marketing.html|title=Biology: Exploring Life|last1=Campbell|first1=Neil A.|first2=Brad|last2=Williamson|first3=Robin J.|last3=Heyden |publisher=Pearson Prentice Hall|year=2006|isbn=978-0132508827|location=Boston|access-date=2021-05-13|archive-date=2014-11-02|archive-url=https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html |url-status=live}}</ref> There are generally two types of cells: eukaryotic cells, which contain a [[Biology:Cell nucleus|nucleus]], and prokaryotic cells, which do not. Prokaryotes are [[Biology:Unicellular organism|single-celled organisms]] such as [[Biology:Bacterium|bacteria]], whereas eukaryotes can be single-celled or [[Biology:Multicellular organism|multicellular]]. In multicellular organisms, every cell in the organism's body is derived ultimately from a [[Biology:Zygote|single cell]] in a fertilized egg.

===Cell structure===

[[File:biological cell.svg|thumb|right|Structure of an animal cell depicting various organelles]]

Every cell is enclosed within a [[Biology:Cell membrane|cell membrane]] that separates its [[Biology:Cytoplasm|cytoplasm]] from the [[Biology:Extracellular space|extracellular space]].<ref name="urry2017g">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=Membrane structure and function |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=126–142 |isbn=978-0134093413}}</ref> A cell membrane consists of a [[Biology:Lipid bilayer|lipid bilayer]], including [[Chemistry:Cholesterol|cholesterol]]s that sit between phospholipids to maintain their [[Biology:Membrane fluidity|fluidity]] at various temperatures. Cell membranes are [[Physics:Semipermeable membrane|semipermeable]], allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as [[Physics:Ion|ion]]s.<ref name="MBOC">{{cite book |url=https://www.ncbi.nlm.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%20Cell |title=Molecular Biology of the Cell |author1=Alberts, B. |author2=Johnson, A. |author3=Lewis, J. |publisher=Garland Science |year=2002 |isbn=978-0-8153-3218-3 |edition=4th |location=New York |display-authors=0 |url-status=live |archive-url=https://web.archive.org/web/20171220092628/https://www.ncbi.nlm.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%20Cell |archive-date=2017-12-20}}</ref> Cell membranes also contains [[Biology:Membrane protein|membrane protein]]s, including [[Biology:Integral membrane protein|integral membrane protein]]s that go across the membrane serving as membrane transporters, and peripheral proteins that loosely attach to the outer side of the cell membrane, acting as enzymes shaping the cell.<ref name="Tom Herrmann 2019">{{cite journal |author1=Tom Herrmann |author2=Sandeep Sharma |journal=StatPearls |date=March 2, 2019 <!-- |location=1 SIU School of Medicine 2 Baptist Regional Medical Center--> |url=https://www.ncbi.nlm.nih.gov/books/NBK538211/ |pmid=30855799 |title=Physiology, Membrane |access-date=May 14, 2021 |archive-date=February 17, 2022 |archive-url=https://web.archive.org/web/20220217034021/https://www.ncbi.nlm.nih.gov/books/NBK538211/ |url-status=live }}</ref> Cell membranes are involved in various cellular processes such as cell adhesion, [[Biology:Membrane potential|storing electrical energy]], and cell signalling and serve as the attachment surface for several extracellular structures such as a [[Biology:Cell wall|cell wall]], [[Biology:Glycocalyx|glycocalyx]], and [[Biology:Cytoskeleton|cytoskeleton]].

[[File:Plant cell structure-en.svg|thumb|left|Structure of a plant cell]]

Within the cytoplasm of a cell, there are many biomolecules such as [[Biology:Protein|protein]]s and [[Biology:Nucleic acid|nucleic acid]]s.<ref name="Alberts2002">{{Cite journal|last1=Alberts|first1=Bruce|last2=Johnson|first2=Alexander|last3=Lewis|first3=Julian|last4=Raff|first4=Martin|last5=Roberts|first5=Keith|last6=Walter|first6=Peter|date=2002|title=Cell Movements and the Shaping of the Vertebrate Body|url=https://www.ncbi.nlm.nih.gov/books/NBK26863/|journal=Molecular Biology of the Cell|edition=4th|language=en|access-date=2021-05-13|archive-date=2020-01-22|archive-url=https://web.archive.org/web/20200122055346/https://www.ncbi.nlm.nih.gov/books/NBK26863/|url-status=live}} The Alberts text discusses how the "cellular building blocks" move to shape developing [[Biology:Embryo|embryo]]s. It is also common to describe small molecules such as [[Chemistry:Amino acid|amino acid]]s as "[https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 molecular building blocks] {{Webarchive|url=https://web.archive.org/web/20200122055404/https://www.ncbi.nlm.nih.gov/books?cmd=Search&doptcmdl=GenBookHL&term=%22all%2Bcells%22%2BAND%2Bmboc4%5Bbook%5D%2BAND%2B372023%5Buid%5D&rid=mboc4.section.4#23 |date=2020-01-22 }}".</ref> In addition to biomolecules, eukaryotic cells have specialized structures called [[Biology:Organelle|organelle]]s that have their own lipid bilayers or are spatially units.<ref name="hillisetal2014f">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=Cells: The working units of life |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=60–81|isbn=978-1464175121}}</ref> These organelles include the cell nucleus, which contains most of the cell's DNA, or [[Biology:Mitochondrion|mitochondria]], which generates [[Chemistry:Adenosine triphosphate|adenosine triphosphate]] (ATP) to power cellular processes. Other organelles such as endoplasmic reticulum and [[Biology:Golgi apparatus|Golgi apparatus]] play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by [[Biology:Lysosome|lysosome]]s, another specialized organelle. [[Biology:Plant cell|Plant cell]]s have additional organelles that distinguish them from animal cells such as a cell wall that provides support for the plant cell, [[Biology:Chloroplast|chloroplast]]s that harvest sunlight energy to produce sugar, and [[Biology:Vacuole|vacuole]]s that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds.<ref name="hillisetal2014f"/> Eukaryotic cells also have cytoskeleton that is made up of [[Biology:Microtubule|microtubule]]s, [[Biology:Intermediate filament|intermediate filament]]s, and [[Biology:Microfilament|microfilament]]s, all of which provide support for the cell and are involved in the movement of the cell and its organelles.<ref name="hillisetal2014f"/> In terms of their structural composition, the microtubules are made up of [[Biology:Tubulin|tubulin]] (e.g., [[Biology:Tubulin#α-Tubulin|α-tubulin]] and [[Biology:Tubulin#β-Tubulin|β-tubulin]] whereas intermediate filaments are made up of fibrous proteins.<ref name="hillisetal2014f"/> Microfilaments are made up of [[Biology:Actin|actin]] molecules that interact with other strands of proteins.<ref name="hillisetal2014f"/>

===Metabolism===
{{Further |Biology:Bioenergetics}}

[[File:Activation2 updated.svg|right|thumb|Example of an enzyme-catalysed [[Physics:Exothermic reaction|exothermic]] reaction]]

All cells require [[Physics:Energy|energy]] to sustain cellular processes. [[Biology:Metabolism|Metabolism]] is the set of chemical reactions in an organism. The three main purposes of metabolism are: the conversion of food to energy to run cellular processes; the conversion of food/fuel to monomer building blocks; and the elimination of [[Biology:Metabolic waste|metabolic waste]]s. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as catabolic—the breaking down of compounds (for example, the breaking down of glucose to pyruvate by [[Biology:Cellular respiration|cellular respiration]]); or anabolic—the building up ([[Biology:Biosynthesis|synthesis]]) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy. The chemical reactions of metabolism are organized into [[Biology:Metabolic pathway|metabolic pathway]]s, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific enzyme. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require energy that will not occur by themselves, by [[Physics:Coupling|coupling]] them to [[Physics:Spontaneous process|spontaneous reactions]] that release energy. Enzymes act as [[Chemistry:Catalysis|catalysts]]—they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount of [[Chemistry:Activation energy|activation energy]] needed to convert reactants into [[Chemistry:Product|product]]s. Enzymes also allow the [[Biology:Metabolic pathway#Regulation|regulation]] of the rate of a metabolic reaction, for example in response to changes in the cell's environment or to signals from other cells.

===Cellular respiration===
{{Main|Biology:Cellular respiration}}

[[Image:CellRespiration.svg|thumb|300px|Respiration in a [[Biology:Eukaryote|eukaryotic cell]] ]]

Cellular respiration is a set of metabolic reactions and processes that take place in cells to convert [[Physics:Energy#Energy and life|chemical energy]] from nutrients into adenosine triphosphate (ATP), and then release waste products.<ref>{{cite web|last=Bailey|first=Regina|title=Cellular Respiration|url=http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm|url-status=live|archive-url=https://web.archive.org/web/20120505043947/http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm|archive-date=2012-05-05}}</ref> The reactions involved in respiration are [[Biology:Catabolism|catabolic reactions]], which break large molecules into smaller ones, releasing energy. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are [[Chemistry:Redox|redox]] reactions. Although cellular respiration is technically a combustion reaction, it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions.

Sugar in the form of glucose is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: [[Biology:Glycolysis|glycolysis]], [[Biology:Citric acid cycle|citric acid cycle]] (or Krebs cycle), [[Biology:Electron transport chain|electron transport chain]], and [[Biology:Oxidative phosphorylation|oxidative phosphorylation]].<ref name="lodish2008l">{{cite book |last1=Lodish |first1=Harvey |last2=Berk |first2=Arnold. |last3=Kaiser |first3= Chris A.  |last4=Krieger |first4=Monty |last5=Scott |first5=Matthew P. |last6=Bretscher |first6=Anthony |last7=Ploegh |first7=Hidde |last8=Matsudaira |first8=Paul |chapter=Cellular energetics |title=Molecular Cell Biology |publisher=W.H. Freeman and Company |edition=6th |date=2008 |location=New York |pages=479–532 |isbn=978-0716776017}}</ref> Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two [[Chemistry:Pyruvic acid|pyruvate]]s, with two net molecules of ATP being produced at the same time.<ref name="lodish2008l"/> Each pyruvate is then oxidized into [[Chemistry:Acetyl-CoA|acetyl-CoA]] by the [[Biology:Pyruvate dehydrogenase complex|pyruvate dehydrogenase complex]], which also generates [[Chemistry:Nicotinamide adenine dinucleotide|NADH]] and carbon dioxide. Acetyl-Coa enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH<sub>2</sub>, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the [[Biology:Crista|mitochondrial cristae]]. Oxidative phosphorylation comprises the electron transport chain, which is a series of four [[Biology:Protein complex|protein complex]]es that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH<sub>2</sub> that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane ([[Biology:Chemiosmosis|chemiosmosis]]), which generates a [[Biology:Chemiosmosis#Proton-motive force|proton motive force]].<ref name="lodish2008l"/> Energy from the proton motive force drives the enzyme [[Biology:ATP synthase|ATP synthase]] to synthesize more ATPs by [[Biology:Phosphorylation|phosphorylating]] [[Chemistry:Adenosine diphosphate|ADP]]s. The transfer of electrons terminates with molecular oxygen being the final [[Chemistry:Electron acceptor|electron acceptor]].

If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of [[Biology:Fermentation|fermentation]]. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to [[Biology:Cellular waste product|waste products]] that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD<sup>+</sup> so it can be re-used in glycolysis. In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD<sup>+</sup> for glycolysis. This waste product varies depending on the organism. In skeletal muscles, the waste product is [[Chemistry:Lactic acid|lactic acid]]. This type of fermentation is called [[Biology:Lactic acid fermentation|lactic acid fermentation]]. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD<sup>+</sup> regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD<sup>+</sup> attaches to hydrogen from lactate to form ATP. In yeast, the waste products are ethanol and carbon dioxide. This type of fermentation is known as alcoholic or [[Chemistry:Ethanol fermentation|ethanol fermentation]]. The ATP generated in this process is made by [[Biology:Substrate-level phosphorylation|substrate-level phosphorylation]], which does not require oxygen.

===Photosynthesis===

[[File:Simple photosynthesis overview.svg|thumb|Photosynthesis changes sunlight into chemical energy, splits water to liberate O<sub>2</sub>, and fixes CO<sub>2</sub> into sugar.]]

{{Main|Earth:Photosynthesis}}

Photosynthesis is a process used by plants and other organisms to [[Physics:Energy transformation|convert]] light energy into [[Chemistry:Chemical energy|chemical energy]] that can later be released to fuel the organism's metabolic activities via cellular respiration. This chemical energy is stored in carbohydrate molecules, such as [[Chemistry:Sugar|sugar]]s, which are synthesized from carbon dioxide and water.<ref name="OnlineEtDict_photosynthesis">{{cite web |title=photosynthesis |url=http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0 |work=Online Etymology Dictionary |access-date=2013-05-23 |archive-url=https://web.archive.org/web/20130307020959/http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0 |archive-date=2013-03-07 |url-status=live }}</ref><ref name="LSJ1">{{LSJ|fw{{=}}s2|φῶς|ref}}</ref><ref name="LSJ2">{{LSJ|su/nqesis|σύνθεσις|ref}}</ref> In most cases, oxygen is released as a waste product. Most [[Biology:Plant|plant]]s, [[Biology:Algae|algae]], and [[Biology:Cyanobacteria|cyanobacteria]] perform photosynthesis, which is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies most of the energy necessary for life on Earth.<ref name="bryantfrigaard">{{cite journal |author1=Bryant, D. A. |author2=Frigaard, N. U. |title=Prokaryotic photosynthesis and phototrophy illuminated |journal=[[Biology:Trends in Microbiology|Trends in Microbiology]] |volume=14 |issue=11 |pages=488–496 |date=Nov 2006 |pmid=16997562 |doi=10.1016/j.tim.2006.09.001}}</ref>

Photosynthesis has four stages: Light absorption, electron transport, ATP synthesis, and [[Earth:Carbon fixation|carbon fixation]].<ref name="lodish2008l"/> Light absorption is the initial step of photosynthesis whereby light energy is absorbed by [[Biology:Chlorophyll|chlorophyll]] pigments attached to proteins in the [[Biology:Thylakoid#Membrane|thylakoid membrane]]s. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a [[Chemistry:Quinone|quinone]] designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP<sup>+</sup>, which is reduced to NADPH, a process that takes place in a protein complex called [[Biology:Photosystem I|photosystem I]] (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration.<ref name="lodish2008l"/>

During the third stage of photosynthesis, the movement of protons down their [[Physics:Concentration gradient|concentration gradient]]s from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase.<ref name="lodish2008l"/> The NADPH and ATPs generated by the light-dependent reactions in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as ribulose bisphosphate (RuBP) in a sequence of light-independent (or dark) reactions called the [[Biology:Calvin cycle|Calvin cycle]].<ref name="isbn0-321-73975-2">{{cite book |author1=Reece, J. |author2=Urry, L. |author3=Cain, M. |title=Biology |edition=International |publisher=Pearson Education |location=Upper Saddle River, New Jersey |isbn=978-0-321-73975-9 |pages=[https://archive.org/details/isbn_9781256158769/page/235 235, 244] |quote=This initial incorporation of carbon into organic compounds is known as carbon fixation. |year=2011 |url=https://archive.org/details/isbn_9781256158769/page/235 }}</ref>

===Cell signaling===

{{main|Biology:Cell signaling}}

Cell signaling (or communication) is the ability of [[Biology:Cell|cell]]s to receive, process, and transmit signals with its environment and with itself.<ref name="neitzelrasband2021">{{cite journal |last1=Neitzel |first1=James |last2=Rasband |first2=Matthew |title=Cell communication |journal=Nature Education |access-date=29 May 2021 |url=https://www.nature.com/scitable/topic/cell-communication-14122659/ |archive-date=29 September 2010 |archive-url=https://web.archive.org/web/20100929110101/https://www.nature.com/scitable/topic/cell-communication-14122659/ |url-status=live }}</ref><ref name="cellsignalling">{{cite journal |title=Cell signaling |journal=Nature Education |access-date=29 May 2021 |url=https://www.nature.com/scitable/topicpage/cell-signaling-14047077/ |archive-date=31 October 2010 |archive-url=https://web.archive.org/web/20101031053612/https://www.nature.com/scitable/topicpage/cell-signaling-14047077/ |url-status=live }}</ref> Signals can be non-chemical such as light, [[Biology:Action potential|electrical impulses]], and heat, or chemical signals (or [[Chemistry:Ligand|ligand]]s) that interact with [[Biology:Receptor (biochemistry)|receptors]], which can be found [[Biology:Cell surface receptor|embedded]] in the [[Biology:Cell membrane|cell membrane]] of another cell or located deep inside a cell.<ref name="hillisetal2014e">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=Cell membranes and signaling |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=82–104 |isbn=978-1464175121}}</ref><ref name="cellsignalling"/> There are generally four types of chemical signals: [[Biology:Autocrine signaling|autocrine]], [[Biology:Paracrine signaling|paracrine]], [[Biology:Juxtacrine signalling|juxtacrine]], and [[Biology:Hormone|hormone]]s.<ref name="hillisetal2014e"/> In autocrine signaling, the ligand affects the same cell that releases it. [[Medicine:Neoplasm|Tumor]] cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affects them. For example, brain cells called [[Biology:Neuron|neuron]]s release ligands called [[Biology:Neurotransmitter|neurotransmitter]]s that diffuse across a [[Biology:Chemical synapse#Structure|synaptic cleft]] to bind with a receptor on an adjacent cell such as another neuron or [[Biology:Muscle cell|muscle cell]]. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the [[Biology:Circulatory system|circulatory system]]s of animals or vascular systems of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an [[Biology:Ligand-gated ion channel|inotropic receptor]] can alter the excitability of a target cell. Other types of receptors include [[Biology:Protein kinase|protein kinase]] receptors (e.g., [[Biology:Insulin receptor|receptor]] for the hormone [[Biology:Insulin|insulin]]) and [[Biology:G protein-coupled receptor|G protein-coupled receptor]]s. Activation of G protein-coupled receptors can initiate [[Biology:Second messenger system|second messenger]] cascades. The process by which a chemical or physical signal is transmitted through a cell as a [[Biology:Biochemical cascade|series of molecular events]] is called [[Biology:Signal transduction|signal transduction]]

===Cell cycle===

[[File:Meiosis Overview new.svg|thumb|300px|In meiosis, the chromosomes duplicate and the [[Biology:Homologous chromosome|homologous chromosome]]s exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploid gametes.]]

{{Main|Biology:Cell cycle}}

The cell cycle is a series of events that take place in a [[Biology:Cell|cell]] that cause it to divide into two daughter cells. These events include the [[Biology:DNA replication|duplication of its DNA]] and some of its [[Biology:Organelle|organelle]]s, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called [[Biology:Cell division|cell division]].<ref>{{Cite book|title=A dictionary of biology|date=2020 |last1=Martin |first1=E. A. |last2=Hine |first2=R. |publisher=Oxford University Press |isbn=978-0199204625|edition=6th|location=Oxford|oclc=176818780}}</ref> In [[Biology:Eukaryote|eukaryote]]s (i.e., [[Biology:Animal|animal]], [[Biology:Plant|plant]], fungal, and [[Biology:Protist|protist]] cells), there are two distinct types of cell division: [[Biology:Mitosis|mitosis]] and [[Biology:Meiosis|meiosis]].<ref name=":0">{{Cite book |title=Introduction to genetic analysis |date=2012 |publisher=W.H. Freeman |last=Griffiths |first=A. J. |isbn=978-1429229432 |edition=10th |location=New York |oclc=698085201 }}</ref> Mitosis is part of the cell cycle, in which replicated chromosomes are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of [[Biology:Interphase|interphase]] (during which the DNA is replicated) and is often followed by [[Biology:Telophase|telophase]] and [[Biology:Cytokinesis|cytokinesis]]; which divides the [[Biology:Cytoplasm|cytoplasm]], [[Biology:Organelle|organelle]]s and [[Biology:Cell membrane|cell membrane]] of one cell into two new [[Biology:Cell|cells]] containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.<ref>{{Cite web|title=10.2 The Cell Cycle - Biology 2e {{!}} OpenStax|url=https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle|access-date=2020-11-24|website=openstax.org|date=28 March 2018 |language=en|archive-date=2020-11-29|archive-url=https://web.archive.org/web/20201129223722/https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle|url-status=live}}</ref> The cell cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, [[Biology:Skin|skin]], [[Biology:Blood cell|blood cell]]s, and some internal organs are renewed. After cell division, each of the daughter cells begin the [[Biology:Interphase|interphase]] of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.<ref name="freeman2017m">{{cite book |last1=Freeman|first1=Scott |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael | last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |chapter=Meiosis |title=Biological Science |publisher=Pearson |edition=6th |date=2017 |location=Hoboken, New Jersey |pages=271–289 |isbn=978-0321976499}}</ref> [[Biology:Homologous chromosome|Homologous chromosome]]s are separated in the first division ([[Biology:Meiosis#Meiosis I|meiosis I]]), and sister chromatids are separated in the second division ([[Biology:Meiosis#Meiosis II|meiosis II]]). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

[[Biology:Prokaryote|Prokaryote]]s (i.e., [[Biology:Archaea|archaea]] and [[Biology:Bacteria|bacteria]]) can also undergo cell division (or [[Binary fission|binary fission]]). Unlike the processes of [[Biology:Mitosis|mitosis]] and [[Biology:Meiosis|meiosis]] in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a [[Biology:Spindle apparatus|spindle apparatus]] on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by [[Biology:FtsZ|FtsZ]] polymerization and "Z-ring" formation)<ref name=":32046082">{{cite journal |author1=Casiraghi, A. |author2=Suigo, L. |author3=Valoti, E. |author4=Straniero, V. |title=Targeting Bacterial Cell Division: A Binding Site-Centered Approach to the Most Promising Inhibitors of the Essential Protein FtsZ |journal=Antibiotics |volume=9 |issue=2 |pages=69 |date=February 2020 |pmid=32046082 |doi=10.3390/antibiotics9020069 |pmc=7167804 |doi-access=free }}</ref> The new cell wall ([[Biology:Septum (cell biology)|septum]]) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, [[Biology:Ribosome|ribosome]]s, and [[Biology:Plasmid|plasmid]]s.

==Genetics==

===Inheritance===

{{main|Biology:Classical genetics}}

[[File:Punnett square mendel flowers.svg|thumb|right|Punnett square depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms]]

[[Biology:Genetics|Genetics]] is the scientific study of inheritance.<ref name="griffithsetal2014l">{{cite book |last1=Griffiths |first1=Anthony J. |last2=Wessler |first2=Susan R. |last3=Carroll |first3=Sean B. |last4=Doebley |first4=John |chapter=The genetics revolution |title=An Introduction to Genetic Analysis |publisher=W.H. Freeman & Company |edition=11th |date=2015 |location=Sunderland, Massachusetts |pages=1–30 |isbn=978-1464109485}}</ref><ref name=griffiths2000sect60>{{cite book |editor1-first=Anthony J. F. |editor1-last=Griffiths |editor2-first=Jeffrey H. |editor2-last=Miller |editor3-first=David T. |editor3-last=Suzuki |editor4-first=Richard C. |editor4-last=Lewontin |editor5-first=William M.|editor5-last=Gelbart |title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.60 |chapter=Genetics and the Organism: Introduction }}</ref><ref name=Hartl_and_Jones>{{Cite book |author1=Hartl, D. |author2=Jones, E |title=Genetics: Analysis of Genes and Genomes |edition=6th |publisher=Jones & Bartlett |year=2005 |isbn=978-0-7637-1511-3 |url=https://archive.org/details/genetics00dani }}</ref> [[Biology:Mendelian inheritance|Mendelian inheritance]], specifically, is the process by which genes and traits are passed on from parents to offspring.<ref name="miko2008a"/> It has several principles. The first is that genetic characteristics, [[Biology:Allele|allele]]s, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on the [[Biology:Mendelian inheritance#Law of Dominance and Uniformity|law of dominance and uniformity]], which states that some alleles are [[Biology:Dominance (genetics)|dominant]] while others are [[Biology:Dominance (genetics)|recessive]]; an organism with at least one dominant allele will display the [[Biology:Phenotype|phenotype]] of that dominant allele. During gamete formation, the alleles for each gene segregate, so that each gamete carries only one allele for each gene. Heterozygotic individuals produce gametes with an equal frequency of two alleles. Finally, the [[Biology:Mendelian inheritance#Law of Independent Assortment|law of independent assortment]], states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are [[Biology:Sex linkage|sex-linked]]. [[Biology:Test cross|Test cross]]es can be performed to experimentally determine the underlying [[Biology:Genotype|genotype]] of an organism with a dominant phenotype.<ref name="miko2008b">{{Citation |last=Miko |first=Ilona |title=Test crosses |journal=Nature Education |volume=1 |issue=1 |pages=136 |date=2008 |url=https://www.nature.com/scitable/topicpage/test-crosses-585/ |access-date=2021-05-28 |archive-date=2021-05-21 |archive-url=https://web.archive.org/web/20210521003428/https://www.nature.com/scitable/topicpage/test-crosses-585/ |url-status=live }}</ref> A [[Biology:Punnett square|Punnett square]] can be used to predict the results of a test cross. The [[Biology:Boveri–Sutton chromosome theory|chromosome theory of inheritance]], which states that genes are found on chromosomes, was supported by [[Biography:Thomas Hunt Morgan|Thomas Morgans]]'s experiments with [[Biology:Drosophila melanogaster|fruit flies]], which established the [[Biology:Sex linkage|sex linkage]] between eye color and [[Biology:Sex|sex]] in these insects.<ref name="miko2008c">{{Citation |last=Miko |first=Ilona |title=Thomas Hunt Morgan and sex linkage |journal=Nature Education |volume=1 |issue=1 |pages=143 |date=2008 |url=https://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452/ |access-date=2021-05-28 |archive-date=2021-05-20 |archive-url=https://web.archive.org/web/20210520234008/https://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452/ |url-status=live }}</ref>

===Genes and DNA===

[[File:DNA animation.gif|thumb|upright|Bases lie between two spiraling DNA strands.]]
{{further|Biology:Gene|Biology:DNA}}

A gene is a unit of [[Biology:Heredity|heredity]] that corresponds to a region of deoxyribonucleic acid (DNA) that carries genetic information that controls form or function of an organism. DNA is composed of two [[Biology:Polynucleotide|polynucleotide]] chains that coil around each other to form a [[Biology:Nucleic acid double helix|double helix]].<ref name="hillisetal2014i">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=DNA and its role in heredity |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=172–193 |isbn=978-1464175121}}</ref> It is found as linear [[Biology:Chromosome|chromosome]]s in [[Biology:Eukaryote|eukaryote]]s, and circular chromosomes in [[Biology:Prokaryote|prokaryote]]s. The set of chromosomes in a cell is collectively known as its [[Biology:Genome|genome]]. In eukaryotes, DNA is mainly in the [[Biology:Cell nucleus|cell nucleus]].<ref name="russell2001">{{cite book |last= Russell |first= Peter   |title= iGenetics |url= https://archive.org/details/igenetics0000russ_v6o1 |url-access= registration |publisher= Benjamin Cummings |location= New York |year= 2001 |isbn= 0-8053-4553-1}}</ref> In prokaryotes, the DNA is held within the [[Biology:Nucleoid|nucleoid]].<ref>{{cite journal |author1=Thanbichler, M |author2=Wang, SC |author3=Shapiro, L |title=The bacterial nucleoid: a highly organized and dynamic structure |journal=Journal of Cellular Biochemistry |volume=96 |issue=3 |pages=506–21 |date=October 2005 |pmid=15988757 |doi=10.1002/jcb.20519 |s2cid=25355087 |doi-access=free }}</ref> The genetic information is held within genes, and the complete assemblage in an organism is called its [[Biology:Genotype|genotype]].<ref>{{cite web |url=http://www.medterms.com/script/main/art.asp?articlekey=8472 |title=Genotype definition – Medical Dictionary definitions |publisher=Medterms.com |date=2012-03-19 |access-date=2013-10-02 |url-status=live |archive-url=https://web.archive.org/web/20130921054803/http://www.medterms.com/script/main/art.asp?articlekey=8472 |archive-date=2013-09-21 }}</ref>
[[Biology:DNA replication|DNA replication]] is a [[Biology:Semiconservative replication|semiconservative]] process whereby each strand serves as a template for a new strand of DNA.<ref name="hillisetal2014i"/> Mutations are heritable changes in DNA.<ref name="hillisetal2014i"/> They can arise [[Biology:Mutation#Spontaneous mutation|spontaneously]] as a result of replication errors that were not corrected by proofreading or can be [[Biology:Mutation#Induced mutation|induced]] by an environmental [[Physics:Mutagen|mutagen]] such as a chemical (e.g., [[Chemistry:Nitrous acid|nitrous acid]], [[Chemistry:Benzopyrene|benzopyrene]]) or radiation (e.g., [[Physics:X-ray|x-ray]], [[Physics:Gamma ray|gamma ray]], [[Physics:Ultraviolet|ultraviolet radiation]], particles emitted by unstable isotopes).<ref name="hillisetal2014i"/> Mutations can lead to phenotypic effects such as loss-of-function, [[Biology:Gain-of-function research|gain-of-function]], and conditional mutations.<ref name="hillisetal2014i"/>
Some mutations are beneficial, as they are a source of [[Biology:Genetic variation|genetic variation]] for evolution.<ref name="hillisetal2014i"/> Others are harmful if they were to result in a loss of function of genes needed for survival.<ref name="hillisetal2014i"/> Mutagens such as [[Biology:Carcinogen|carcinogen]]s are typically avoided as a matter of [[Medicine:Health policy|public health policy]] goals.<ref name="hillisetal2014i"/>

===Gene expression===

[[File:Extended Central Dogma with Enzymes.jpg|thumb|300px |right |The extended central dogma of molecular biology includes all the processes involved in the flow of genetic information.]]

{{Main|Biology:Gene expression}}

Gene expression is the molecular process by which a [[Biology:Genotype|genotype]] encoded in DNA gives rise to an observable [[Biology:Phenotype|phenotype]] in the proteins of an organism's body. This process is summarized by the [[Biology:Central dogma of molecular biology|central dogma of molecular biology]], which was formulated by [[Biography:Francis Crick|Francis Crick]] in 1958.<ref>{{cite journal |last=Crick |first=Francis H. |title=On protein synthesis |journal=Symposia of the Society for Experimental Biology |volume=12 |pages=138–63 |date=1958 |pmid=13580867 }}</ref><ref>{{cite journal |last=Crick |first=Francis H. |author-link=Francis Crick |title=Central dogma of molecular biology |journal=Nature |volume=227 |issue=5258 |pages=561–3 |date=August 1970 |pmid=4913914 |doi=10.1038/227561a0 |bibcode=1970Natur.227..561C |s2cid=4164029 }}</ref><ref>{{cite journal |title=Central dogma reversed |journal=Nature |volume=226 |issue=5252 |pages=1198–9 |date=June 1970 |pmid=5422595 |doi=10.1038/2261198a0 |bibcode=1970Natur.226.1198. |s2cid=4184060 }}</ref> According to the Central Dogma, genetic information flows from DNA to RNA to protein. There are two gene expression processes: transcription (DNA to RNA) and translation (RNA to protein).<ref name="linelowitz2015">{{cite journal |last1=Lin |first1=Yihan |last2=Elowitz |first2=Michael B. |title=Central Dogma Goes Digital |journal=Molecular Cell |volume=61 |issue=6 |pages=791–792 |date=2016 |doi=10.1016/j.molcel.2016.03.005 |pmid=26990983 |doi-access=free }}</ref>

===Gene regulation===

{{Main|Biology:Regulation of gene expression}}

The regulation of gene expression by environmental factors and during different stages of [[Biology:Developmental biology|development]] can occur at each step of the process such as [[Biology:Transcription|transcription]], [[Biology:RNA splicing|RNA splicing]], [[Biology:Translation|translation]], and [[Biology:Post-translational modification|post-translational modification]] of a protein.<ref name="hillisetal2014k">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=Regulation of gene expression |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=215–233 |isbn=978-1464175121}}</ref> Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called [[Biology:Transcription factor|transcription factor]]s bind to the DNA sequence close to or at a promoter.<ref name="hillisetal2014k"/> A cluster of genes that share the same promoter is called an [[Biology:Operon|operon]], found mainly in prokaryotes and some lower eukaryotes (e.g., ''[[Biology:Caenorhabditis elegans|Caenorhabditis elegans]]'').<ref name="hillisetal2014k"/><ref name="keeneandtenenbaum2002">{{cite journal |last1=Keene |first1=Jack D. |last2=Tenenbaum |first2=Scott A. |title=Eukaryotic mRNPs may represent posttranscriptional operons |journal=Molecular Cell |year=2002 |volume=9 |issue=6 |pages=1161–1167 |doi=10.1016/s1097-2765(02)00559-2|pmid=12086614 |doi-access=free }}</ref> In positive regulation of gene expression, the [[Biology:Activator (genetics)|activator]] is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. Negative regulation occurs when another transcription factor called a [[Biology:Repressor|repressor]] binds to a DNA sequence called an [[Biology:Operon#Operator|operator]], which is part of an operon, to prevent transcription. Repressors can be inhibited by compounds called [[Biology:Inducer|inducer]]s (e.g., [[Chemistry:Allolactose|allolactose]]), thereby allowing transcription to occur.<ref name="hillisetal2014k"/> Specific genes that can be activated by inducers are called [[Biology:Gene expression#Regulation of gene expression|inducible gene]]s, in contrast to [[Biology:Gene expression#Regulation of gene expression|constitutive gene]]s that are almost constantly active.<ref name="hillisetal2014k"/> In contrast to both, [[Biology:Structural gene|structural gene]]s encode proteins that are not involved in gene regulation.<ref name="hillisetal2014k"/> In addition to regulatory events involving the promoter, gene expression can also be regulated by [[Biology:Epigenetics|epigenetic]] changes to [[Biology:Chromatin|chromatin]], which is a complex of DNA and protein found in eukaryotic cells.<ref name="hillisetal2014k"/>

===Genes, development, and evolution===
{{main|Biology:Evolutionary developmental biology}}

[[Biology:Developmental biology|Development]] is the process by which a [[Biology:Multicellular organism|multicellular organism]] ([[Biology:Plant|plant]] or [[Biology:Animal|animal]]) goes through a series of changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.<ref name="hillisetal2014n">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Genes, development, and evolution |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=273–298 |isbn=978-1464175121}}</ref> There are four key processes that underlie development: [[Biology:Cell fate determination|Determination]], [[Biology:Cellular differentiation|differentiation]], [[Biology:Morphogenesis|morphogenesis]], and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as [[Biology:Stem cell|stem cell]]s.<ref>Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.</ref><ref>{{cite journal |last1=Slack |first1=J.M.W. |year=2007 |title=Metaplasia and transdifferentiation: from pure biology to the clinic |journal=Nature Reviews Molecular Cell Biology |volume=8 |issue=5|pages=369–378 |doi=10.1038/nrm2146 |pmid=17377526|s2cid=3353748 }}</ref> Stem cells are [[Biology:Cellular differentiation|undifferentiated]] or partially differentiated [[Biology:Cell|cells]] that can differentiate into various types of cells and [[Biology:Cell proliferation|proliferate]] indefinitely to produce more of the same stem cell.<ref name=":7">{{cite book|last1=Atala|first1=Anthony|last2=Lanza|first2=Robert |url=https://books.google.com/books?id=wm-K_dKpjBAC&pg=RA1-PA451|title=Handbook of Stem Cells|date=2012|publisher=Academic Press|isbn=978-0-12-385943-3|pages=452|language=en|access-date=2021-05-28|archive-date=2021-04-12|archive-url=https://web.archive.org/web/20210412065854/https://books.google.com/books?id=wm-K_dKpjBAC&pg=RA1-PA451|url-status=live}}</ref> Cellular differentiation dramatically changes a cell's size, shape, [[Biology:Membrane potential|membrane potential]], [[Biology:Metabolism|metabolic activity]], and responsiveness to signals, which are largely due to highly controlled modifications in [[Biology:Gene expression|gene expression]] and [[Biology:Epigenetics|epigenetics]]. With a few exceptions, cellular differentiation almost never involves a change in the [[Biology:DNA|DNA]] sequence itself.<ref>{{Cite journal|last1=Yanes|first1=Oscar|last2=Clark|first2=Julie|last3=Wong|first3=Diana M.|last4=Patti|first4=Gary J.|last5=Sánchez-Ruiz|first5=Antonio |last6=Benton|first6=H. Paul|last7=Trauger|first7=Sunia A.|last8=Desponts|first8=Caroline|last9=Ding|first9=Sheng|last10=Siuzdak |first10=Gary |date=June 2010|title=Metabolic oxidation regulates embryonic stem cell differentiation |journal=Nature Chemical Biology |volume=6 |issue=6|pages=411–417|doi=10.1038/nchembio.364|pmid=20436487 |pmc=2873061}}</ref> Thus, different cells can have very different physical characteristics despite having the same [[Biology:Genome|genome]]. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression.<ref name="hillisetal2014n"/> A small fraction of the genes in an organism's genome called the [[Biology:Evo-devo gene toolkit|developmental-genetic toolkit]] control the development of that organism. These toolkit genes are highly conserved among [[Biology:Phylum|phyla]], meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the [[Biology:Hox gene|''Hox'' genes]]. Hox genes determine where repeating parts, such as the many [[Biology:Vertebra|vertebra]]e of [[Biology:Snake|snake]]s, will grow in a developing embryo or larva.<ref name=CarrollNatHist>{{cite web |last1=Carroll |first1=Sean B. |title=The Origins of Form |url=http://www.naturalhistorymag.com/features/061488/the-origins-of-form |website=Natural History |access-date=9 October 2016 |quote=Biologists could say, with confidence, that forms change, and that natural selection is an important force for change. Yet they could say nothing about how that change is accomplished. How bodies or body parts change, or how new structures arise, remained complete mysteries. |archive-date=9 October 2018 |archive-url=https://web.archive.org/web/20181009154501/http://www.naturalhistorymag.com/features/061488/the-origins-of-form |url-status=live }}</ref>

==Evolution==

===Evolutionary processes===

{{main|Biology:Evolutionary biology}}

[[File:Mutation and selection diagram.svg|thumb|right|[[Biology:Natural selection|Natural selection]] for darker traits]]

[[Biology:Evolution|Evolution]] is a central organizing concept in biology. It is the change in [[Biology:Heredity|heritable]] [[Biology:Phenotypic trait|characteristics]] of [[Social:Population|population]]s over successive generations.<ref>{{cite book |last1=Hall |first1=Brian K. |last2=Hallgrímsson |first2=Benedikt |title=Strickberger's Evolution |date=2007 |publisher=Jones & Bartlett Publishers |isbn=978-1-4496-4722-3 |url=https://books.google.com/books?id=jrDD3cyA09kC |pages=4–6 |access-date=2021-05-27 |archive-date=2023-03-26 |archive-url=https://web.archive.org/web/20230326093707/https://books.google.com/books?id=jrDD3cyA09kC |url-status=live }}</ref><ref>{{cite web |title=Evolution Resources |location=Washington, D.C. |publisher=[[Organization:National Academies of Sciences, Engineering, and Medicine|National Academies of Sciences, Engineering, and Medicine]] |year=2016 |url=http://www.nas.edu/evolution/index.html |url-status=live |archive-url=https://web.archive.org/web/20160603230514/http://www.nas.edu/evolution/index.html |archive-date=2016-06-03}}</ref> In artificial selection, animals were selectively bred for specific traits.
<ref name="urry2017v"/> Given that traits are inherited, populations contain a varied mix of traits, and reproduction is able to increase any population, Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits.<ref name="urry2017v"/> Darwin inferred that individuals who possessed heritable traits better adapted to their environments are more likely to survive and produce more offspring than other individuals.<ref name="urry2017v"/> He further inferred that this would lead to the accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment.<ref name="lewontin70">{{cite journal |last=Lewontin |first=Richard C. |date=November 1970 |title=The Units of Selection |url=http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20units%20of%20selection.pdf |journal=Annual Review of Ecology and Systematics |volume=1 |pages=1–18 |doi=10.1146/annurev.es.01.110170.000245 |jstor=2096764  |s2cid=84684420 |url-status=live |archive-url=https://web.archive.org/web/20150206172942/http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20units%20of%20selection.pdf |archive-date=2015-02-06}}</ref><ref>[[Biography:Charles Darwin|Darwin, Charles]] (1859). ''On the Origin of Species'', John Murray.</ref><ref name="futuyma2017a">{{cite book |last1=Futuyma |first1=Douglas J. |last2=Kirkpatrick |first2=Mark |date=2017 |pages=3–26 |chapter=Evolutionary biology |title=Evolution |edition=4th |location=Sunderland, Mass. |publisher=Sinauer Associates }}</ref><ref name="urry2017v">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=Descent with modifications: A Darwinian view of life |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=466–483 |isbn=978-0134093413}}</ref><ref name="hillisetal2014o">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Processes of evolution |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=299–324 |isbn=978-1464175121}}</ref>

===Speciation===
{{main|Biology:Speciation}}

A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other.<ref name="hillisetal2014q">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Speciation |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=343–356 |isbn=978-1464175121}}</ref> For speciation to occur, there has to be [[Biology:Reproductive isolation|reproductive isolation]].<ref name="hillisetal2014q"/> Reproductive isolation can result from incompatibilities between genes as described by [[Biology:Bateson–Dobzhansky–Muller model|Bateson–Dobzhansky–Muller model]]. Reproductive isolation also tends to increase with [[Biology:Genetic divergence|genetic divergence]]. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as [[Biology:Allopatric speciation|allopatric speciation]].<ref name="hillisetal2014q"/>

===Phylogeny===
{{main|Biology:Phylogenetics}}

{{PhylomapB|caption=Phylogenetic tree showing the domains of [[Biology:Bacterium|bacteria]], [[Biology:Archaea|archaea]], and [[Biology:Eukaryote|eukaryote]]s}}

A phylogeny is an evolutionary history of a specific group of organisms or their genes.<ref name="hillisetal2014p">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Reconstructing and using phylogenies |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=325–342 |isbn=978-1464175121}}</ref> It can be represented using a [[Biology:Phylogenetic tree|phylogenetic tree]], a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a [[Biology:Lineage (evolution)|lineage]] of descendants of a particular species or population. When a lineage divides into two, it is represented as a fork or split on the phylogenetic tree.<ref name="hillisetal2014p"/> Phylogenetic trees are the basis for comparing and grouping different species.<ref name="hillisetal2014p"/> Different species that share a feature inherited from a common ancestor are described as having [[Biology:Homology|homologous]] features (or [[Biology:Apomorphy and synapomorphy|synapomorphy]]).<ref name="kitchingetal2001">{{cite encyclopedia |last1=Kitching |first1=Ian J. |last2=Forey |first2=Peter L. |last3=Williams |first3=David M. |editor-last=Levin |editor-first=Simon A. |title=Cladistics |encyclopedia=Encyclopedia of Biodiversity |year=2001 |edition=2nd |pages=33–45 |publisher=Elsevier |doi=10.1016/B978-0-12-384719-5.00022-8 |isbn=9780123847201 |access-date=29 August 2021 |url=https://www.sciencedirect.com/science/article/pii/B9780123847195000228 |archive-date=29 August 2021 |archive-url=https://web.archive.org/web/20210829234556/https://www.sciencedirect.com/science/article/pii/B9780123847195000228 |url-status=live }})</ref><ref name="Futuyma2017p">{{cite book |last1=Futuyma |first1=Douglas J. |last2=Kirkpatrick |first2=Mark |date=2017|pages=401–429 |chapter=Phylogeny: The unity and diversity of life |title=Evolution |edition=4th |publisher=Sinauer Associates |location=Sunderland, Mass.}}</ref><ref name="hillisetal2014p"/> Phylogeny provides the basis of biological classification.<ref name="hillisetal2014p"/> This classification system is rank-based, with the highest rank being the domain followed by [[Biology:Kingdom|kingdom]], [[Biology:Phylum|phylum]], [[Biology:Class|class]], [[Biology:Order|order]], family, [[Biology:Genus|genus]], and [[Biology:Species|species]].<ref name="hillisetal2014p"/> All organisms can be classified as belonging to one of [[Biology:Three-domain system|three domain]]s: [[Biology:Archaea|Archaea]] (originally Archaebacteria); [[Biology:Bacteria|bacteria]] (originally eubacteria), or [[Biology:Eukaryote|eukarya]] (includes the [[Biology:Protist|protist]], [[Biology:Fungi|fungi]], [[Biology:Plant|plant]], and [[Biology:Animal|animal]] kingdoms).<ref name="domain">{{cite journal |author1=Woese, CR |author2=Kandler, O |author3=Wheelis, ML |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=87 |issue=12 |pages=4576–79 |date=June 1990 |pmid=2112744 |pmc=54159 |doi=10.1073/pnas.87.12.4576 |bibcode=1990PNAS...87.4576W |doi-access=free }}</ref>

===History of life===
<!--{{Life timeline}}-->
{{Main|Biology:History of life}}

The history of life on [[Astronomy:Earth|Earth]] traces how organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on [[Astronomy:Earth|Earth]], both living and extinct, descended from a [[Biology:Last universal common ancestor|last universal common ancestor]] that lived about 3.5 billion years ago.<ref>{{cite journal |author1=Montévil, M |author2=Mossio, M |author3=Pocheville, A |author4=Longo, G |title=Theoretical principles for biology: Variation |journal=Progress in Biophysics and Molecular Biology |volume=122 |issue=1 |pages=36–50 |date=October 2016 |pmid=27530930 |doi=10.1016/j.pbiomolbio.2016.08.005 |url=https://www.academia.edu/27942089 |series=From the Century of the Genome to the Century of the Organism: New Theoretical Approaches |s2cid=3671068 |url-status=live |archive-url=https://web.archive.org/web/20180320150224/http://www.academia.edu/27942089/Theoretical_principles_for_biology_Variation |archive-date=2018-03-20 }}</ref><ref name="deduve2002">{{cite book |title=Life Evolving: Molecules, Mind, and Meaning |url=https://archive.org/details/lifeevolvingmole00duve_331 |url-access=limited |last=De Duve |first=Christian |location=New York |publisher=Oxford University Press |year=2002|page=[https://archive.org/details/lifeevolvingmole00duve_331/page/n60 44] |isbn=978-0-19-515605-8}}</ref> [[Geologist]]s have developed a [[Earth:Geologic time scale|geologic time scale]] that divides the history of the Earth into major divisions, starting with four eons ([[Earth:Hadean|Hadean]], [[Earth:Archean|Archean]], [[Earth:Proterozoic|Proterozoic]], and [[Earth:Phanerozoic|Phanerozoic]]), the first three of which are collectively known as the [[Earth:Precambrian|Precambrian]], which lasted approximately 4 billion years.<ref name="hillisetal2014r"/> Each eon can be divided into [[Physics:Era|era]]s, with the Phanerozoic eon that began 539 million years ago<ref>{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |publisher=International Stratigraphic Commission |date=February 2022 |access-date=25 April 2022 |archive-date=2 April 2022 |archive-url=https://web.archive.org/web/20220402100018/https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |url-status=live }}</ref> being subdivided into [[Earth:Paleozoic|Paleozoic]], [[Earth:Mesozoic|Mesozoic]], and [[Earth:Cenozoic|Cenozoic]] eras.<ref name="hillisetal2014r"/> These three eras together comprise eleven [[Earth:Geologic time scale#Terminology|period]]s ([[Earth:Cambrian|Cambrian]], [[Earth:Ordovician|Ordovician]], [[Earth:Silurian|Silurian]], [[Earth:Devonian|Devonian]], [[Earth:Carboniferous|Carboniferous]], [[Earth:Permian|Permian]], [[Earth:Triassic|Triassic]], [[Earth:Jurassic|Jurassic]], [[Earth:Cretaceous|Cretaceous]], [[Earth:Tertiary|Tertiary]], and [[Earth:Quaternary|Quaternary]]).<ref name="hillisetal2014r">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The history of life on Earth | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 357–376 | isbn = 978-1464175121}}</ref>

The similarities among all known present-day [[Biology:Species|species]] indicate that they have diverged through the process of [[Biology:Evolution|evolution]] from their common ancestor.<ref>{{harvnb|Futuyma|2005}}</ref> Biologists regard the ubiquity of the [[Biology:Genetic code|genetic code]] as evidence of universal [[Biology:Common descent|common descent]] for all [[Biology:Bacterium|bacteria]], [[Biology:Archaea|archaea]], and [[Biology:Eukaryote|eukaryote]]s.<ref name="Futuyma">{{cite book |last=Futuyma |first=DJ |title=Evolution |year=2005 |publisher=Sinauer Associates |isbn=978-0-87893-187-3 |oclc=57311264 |url=https://archive.org/details/evolution0000futu }}</ref><ref name="Pearce 343–364"/><ref name="Rosing 674–676">{{cite journal |last=Rosing |first=Minik T. |date=January 29, 1999 |title=<sup>13</sup>C-Depleted Carbon Microparticles in &gt;3700-Ma Sea-Floor Sedimentary Rocks from West Greenland |journal=Science |volume=283 |issue=5402 |pages=674–676 |bibcode=1999Sci...283..674R |doi=10.1126/science.283.5402.674 |pmid=9924024}}</ref><ref name="Ohtomo 25–28">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Earth:Nature Geoscience|Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025}}</ref> [[Biology:Microbial mat|Microbial mat]]s of coexisting bacteria and archaea were the dominant form of life in the early [[Earth:Archean|Archean]] epoch and many of the major steps in early evolution are thought to have taken place in this environment.<ref name="NisbetFowler1999">{{cite journal |last1=Nisbet |first1=Euan G. |last2=Fowler |first2=C.M.R. |date=December 7, 1999 |title=Archaean metabolic evolution of microbial mats |journal=[[Organization:Proceedings of the Royal Society#Proceedings of the Royal Society B|Proceedings of the Royal Society B]] |volume=266 |issue=1436 |pages=2375–2382 |doi=10.1098/rspb.1999.0934 |pmc=1690475}}</ref> The earliest evidence of [[Biology:Eukaryote|eukaryote]]s dates from 1.85 billion years ago,<ref>{{cite journal |last1=Knoll |first1=Andrew H. |last2=Javaux |first2=Emmanuelle J. |last3=Hewitt |first3=David |last4=Cohen |first4=Phoebe |display-authors=3 |date=June 29, 2006 |title=Eukaryotic organisms in Proterozoic oceans |journal=[[Organization:Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=1023–1038 |doi=10.1098/rstb.2006.1843 |pmc=1578724 |pmid=16754612}}</ref><ref name="fedonkin2003">{{cite journal |last=Fedonkin |first=Mikhail A. |author-link=Mikhail Fedonkin |date=March 31, 2003 |title=The origin of the Metazoa in the light of the Proterozoic fossil record |url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |url-status=dead |journal=Paleontological Research |volume=7 |issue=1 |pages=9–41 |doi=10.2517/prpsj.7.9 |s2cid=55178329 |archive-url=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |archive-date=2009-02-26 |access-date=2008-09-02}}</ref> and while they may have been present earlier, their diversification accelerated when they started using oxygen in their [[Biology:Metabolism|metabolism]]. Later, around 1.7 billion years ago, [[Biology:Multicellular organism|multicellular organism]]s began to appear, with [[Biology:Cellular differentiation|differentiated cell]]s performing specialised functions.<ref name="bonner1999">{{cite journal |last=Bonner |first=John Tyler |author-link=John Tyler Bonner |date=January 7, 1998 |title=The origins of multicellularity |journal=[[Biology:Integrative Biology|Integrative Biology]] |volume=1 |issue=1 |pages=27–36 |doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 }}</ref>

Algae-like multicellular land plants are dated back even to about 1 billion years ago,<ref>{{cite journal |last1=Strother |first1=Paul K. |last2=Battison |first2=Leila |last3=Brasier |first3=Martin D. |last4=Wellman |first4=Charles H. |display-authors=3 |date=May 26, 2011 |title=Earth's earliest non-marine eukaryotes |journal=Nature |volume=473 |issue=7348 |pages=505–509 |bibcode=2011Natur.473..505S |doi=10.1038/nature09943 |s2cid=4418860 |pmid=21490597}}</ref> although evidence suggests that [[Biology:Microorganism|microorganism]]s formed the earliest [[Earth:Terrestrial ecosystem|terrestrial ecosystem]]s, at least 2.7 billion years ago.<ref>{{cite journal |last=Beraldi-Campesi |first=Hugo |date=February 23, 2013 |title=Early life on land and the first terrestrial ecosystems |journal=Ecological Processes |volume=2 |issue=1 |pages=1–17 |doi=10.1186/2192-1709-2-1 |doi-access=free |bibcode=2013EcoPr...2....1B }}</ref> Microorganisms are thought to have paved the way for the inception of land plants in the [[Earth:Ordovician|Ordovician]] period. Land plants were so successful that they are thought to have contributed to the [[Earth:Late Devonian extinction|Late Devonian extinction event]].<ref name="AlgeoScheckler1998">{{cite journal |last1=Algeo |first1=Thomas J. |last2=Scheckler |first2=Stephen E. |date=January 29, 1998 |title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events |journal=[[Organization:Philosophical Transactions of the Royal Society B|Philosophical Transactions of the Royal Society B]] |volume=353 |issue=1365 |pages=113–130 |doi=10.1098/rstb.1998.0195  |pmc=1692181}}</ref>

Ediacara biota appear during the [[Earth:Ediacaran|Ediacaran]] period,<ref>{{cite journal |last1=Jun-Yuan |first1=Chen |last2=Oliveri |first2=Paola |last3=Chia-Wei |first3=Li |author4=Gui-Qing Zhou |author5=Feng Gao |last6=Hagadorn |first6=James W. |last7=Peterson |first7=Kevin J. |last8=Davidson |first8=Eric H. |display-authors=3 |date=April 25, 2000 |title=Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=97 |issue=9 |pages=4457–4462 |bibcode=2000PNAS...97.4457C |doi=10.1073/pnas.97.9.4457 |pmid=10781044 |pmc=18256|doi-access=free }}</ref> while [[Biology:Vertebrate|vertebrate]]s, along with most other modern [[Biology:Phylum|phyla]] originated about 525 million years ago during the [[Earth:Cambrian explosion|Cambrian explosion]].<ref name="D-G.Shu et al. 1999">{{cite journal |last1=D-G. |first1=Shu |last2=H-L. |first2=Luo |last3=Conway Morris |first3=Simon |author4=X-L. Zhang |author5=S-X. Hu |author6=L. Chen |author7=J. Han |author8=M. Zhu |author9=Y. Li |author10=L-Z. Chen |display-authors=3 |date=November 4, 1999 |title=Lower Cambrian vertebrates from south China |url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |url-status=dead |journal=Nature |volume=402 |pages=42–46 |doi=10.1038/46965 |issue=6757 |bibcode=1999Natur.402...42S |s2cid=4402854 |archive-url=https://web.archive.org/web/20090226122732/http://www.bios.niu.edu/davis/bios458/Shu1.pdf |archive-date=2009-02-26 |access-date=2015-01-22}}</ref> During the Permian period, [[Biology:Synapsid|synapsid]]s, including the ancestors of [[Biology:Mammal|mammal]]s, dominated the land,<ref>{{cite web |url=http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |title=Synapsid Reptiles |last=Hoyt |first=Donald F. |date=February 17, 1997 |website=ZOO 138 Vertebrate Zoology |publisher=[[Organization:California State Polytechnic University, Pomona|California State Polytechnic University, Pomona]] |location=Pomona, Calif. |type=Lecture |archive-url=https://web.archive.org/web/20090520072737/http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |archive-date=2009-05-20 |access-date=2015-01-22}}</ref> but most of this group became extinct in the [[Earth:Permian–Triassic extinction event|Permian–Triassic extinction event]] 252 million years ago.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |url-status=live |title=The Great Dying |last=Barry |first=Patrick L. |date=January 28, 2002 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=[[Organization:Marshall Space Flight Center|Marshall Space Flight Center]] |archive-url=https://web.archive.org/web/20100410015208/https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |archive-date=2010-04-10 |access-date=2015-01-22}}</ref> During the recovery from this catastrophe, [[Biology:Archosaur|archosaur]]s became the most abundant land vertebrates;<ref name="TannerLucas2004">{{cite journal |last1=Tanner |first1=Lawrence H. |last2=Lucas |first2=Spencer G. |last3=Chapman |first3=Mary G. |date=March 2004 |title=Assessing the record and causes of Late Triassic extinctions |url=http://nmnaturalhistory.org/pdf_files/TJB.pdf |journal=[[Earth:Earth-Science Reviews|Earth-Science Reviews]] |volume=65 |issue=1–2 |pages=103–139 |bibcode=2004ESRv...65..103T |doi=10.1016/S0012-8252(03)00082-5 |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=2007-10-25 |access-date=2007-10-22}}</ref> one archosaur group, the [[Biology:Dinosaur|dinosaur]]s, dominated the Jurassic and Cretaceous periods.<ref>{{cite book |last=Benton |first=Michael J. |year=1997 |title=Vertebrate Palaeontology |edition=2nd |location=London |publisher=Chapman & Hall |isbn=978-0-412-73800-5 |oclc=37378512|title-link=Vertebrate Palaeontology (Benton)}}</ref> After the [[Earth:Cretaceous–Paleogene extinction event|Cretaceous–Paleogene extinction event]] 66 million years ago killed off the non-avian dinosaurs,<ref>{{cite journal |last1=Fastovsky |first1=David E. |last2=Sheehan |first2=Peter M. |date=March 2005 |title=The Extinction of the Dinosaurs in North America |url=https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |url-status=live |journal=GSA Today |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 |archive-url=https://web.archive.org/web/20190322190338/https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |archive-date=2019-03-22 |access-date=2015-01-23}}</ref> mammals increased rapidly in size and diversity.<ref>{{cite news |last=Roach |first=John |date=June 20, 2007 |title=Dinosaur Extinction Spurred Rise of Modern Mammals |url=https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |url-status=dead |work=National Geographic News |location=Washington, D.C. |publisher=[[Organization:National Geographic Society|National Geographic Society]] |archive-url=https://web.archive.org/web/20080511161825/https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |archive-date=2008-05-11 |access-date=2020-02-21}}
* {{cite journal |last1=Wible |first1=John R. |last2=Rougier |first2=Guillermo W. |last3=Novacek |first3=Michael J. |last4=Asher |first4=Robert J. |display-authors=3 |date=June 21, 2007 |title=Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary |journal=Nature |volume=447 |issue=7147 |pages=1003–1006 |bibcode=2007Natur.447.1003W |doi=10.1038/nature05854 |pmid=17581585|s2cid=4334424 }}</ref> Such [[Biology:Extinction event|mass extinction]]s may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<ref name="Van Valkenburgh 1999 463–493">{{cite journal |last=Van Valkenburgh |first=Blaire |date=May 1, 1999 |title=Major Patterns in the History of Carnivorous Mammals |url=https://zenodo.org/record/890156 |journal=[[Earth:Annual Review of Earth and Planetary Sciences|Annual Review of Earth and Planetary Sciences]] |volume=27 |pages=463–493 |bibcode=1999AREPS..27..463V |doi=10.1146/annurev.earth.27.1.463  |access-date=May 15, 2021 |archive-date=February 29, 2020 |archive-url=https://web.archive.org/web/20200229201201/https://zenodo.org/record/890156 |url-status=live }}</ref>

==Diversity==

===Bacteria and Archaea===

{{Further |Biology:Microbiology}}

[[File:Gemmatimonas aurantiaca.jpg|thumb|Bacteria – ''Gemmatimonas aurantiaca'' (-=1 Micrometer)]]

[[Biology:Bacteria|Bacteria]] are a type of [[Biology:Cell|cell]] that constitute a large domain of prokaryotic [[Biology:Microorganism|microorganism]]s. Typically a few [[Micrometre|micrometer]]s in length, bacteria have a [[Biology:Bacterial cell structure#Cell morphology|number of shapes]], ranging from [[Biology:Coccus|spheres]] to [[Biology:Bacillus (shape)|rods]] and [[Biology:Spiral bacteria|spirals]]. Bacteria were among the first life forms to appear on [[Astronomy:Earth|Earth]], and are present in most of its [[Earth:Habitat|habitat]]s. Bacteria inhabit soil, water, [[Earth:Hot spring|acidic hot springs]], [[Physics:Radioactive waste|radioactive waste]],<ref>{{cite journal |author1=Fredrickson, J. K. |author2=Zachara, J. M. |author3=Balkwill, D. L. |title=Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the Hanford site, Washington state |journal=Applied and Environmental Microbiology |volume=70 |issue=7 |pages=4230–41 |date=July 2004 |pmid=15240306 |pmc=444790 |doi=10.1128/AEM.70.7.4230-4241.2004 |bibcode=2004ApEnM..70.4230F }}</ref> and the [[Biology:Deep biosphere|deep biosphere]] of the [[Earth:Earth's crust|Earth's crust]]. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the [[Biology:Bacterial phyla|bacterial phyla]] have species that can be [[Biology:Microbiological culture|grown]] in the laboratory.<ref name=Dudek>{{cite journal |author1=Dudek, N. K. |author2=Sun, C. L. |author3=Burstein, D. |title=Novel Microbial Diversity and Functional Potential in the Marine Mammal Oral Microbiome |journal=Current Biology |volume=27 |issue=24 |pages=3752–3762 |year=2017 |doi=10.1016/j.cub.2017.10.040 |pmid=29153320 |s2cid=43864355 |url=https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pdf?t=pghuwe |doi-access=free |access-date=2021-05-14 |archive-date=2021-03-08 |archive-url=https://web.archive.org/web/20210308145807/https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pdf?t=pghuwe |url-status=live }}</ref>

[[File:Halobacteria.jpg|thumb|Archaea – Halobacteria]]

[[Biology:Archaea|Archaea]] constitute the other domain of prokaryotic cells and were initially [[Biology:Taxonomy|classified]] as [[Biology:Bacteria|bacteria]], receiving the name archaebacteria (in the Archaebacteria [[Biology:Kingdom|kingdom]]), a term that has fallen out of use.<ref>{{cite journal |last=Pace |first=N. R. |title=Time for a change |journal=Nature |volume=441 |issue=7091 |pages=289 |date=May 2006 |pmid=16710401 |doi=10.1038/441289a |bibcode=2006Natur.441..289P |s2cid=4431143 |doi-access=free }}</ref> Archaeal cells have unique properties separating them from the other [[Biology:Three-domain system|two domains]], [[Biology:Bacteria|Bacteria]] and [[Biology:Eukaryote|Eukaryota]]. Archaea are further divided into multiple recognized [[Biology:Phylum|phyla]]. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of ''[[Biology:Haloquadratum walsbyi|Haloquadratum walsbyi]]''.<ref>{{cite journal |last=Stoeckenius |first=W. |title=Walsby's square bacterium: fine structure of an orthogonal procaryote |journal=Journal of Bacteriology |volume=148 |issue=1 |pages=352–60 |date=October 1981 |pmid=7287626 |pmc=216199 |doi=10.1128/JB.148.1.352-360.1981 }}</ref> Despite this [[Biology:Morphology|morphological]] similarity to bacteria, archaea possess [[Biology:Gene|gene]]s and several [[Biology:Metabolic pathway|metabolic pathway]]s that are more closely related to those of eukaryotes, notably for the [[Biology:Enzyme|enzyme]]s involved in transcription and [[Biology:Translation|translation]]. Other aspects of archaeal biochemistry are unique, such as their reliance on [[Chemistry:Ether lipid|ether lipid]]s in their [[Biology:Cell membrane|cell membrane]]s,<ref>{{cite web |title=Archaea Basic Biology |date=March 2018 |url=https://basicbiology.net/micro/microorganisms/archaea |access-date=2021-05-14 |archive-date=2021-04-28 |archive-url=https://web.archive.org/web/20210428221114/https://basicbiology.net/micro/microorganisms/archaea |url-status=live }}</ref> including [[Chemistry:Archaeol|archaeol]]s. Archaea use more energy sources than eukaryotes: these range from organic compounds, such as sugars, to [[Chemistry:Ammonia|ammonia]], [[Physics:Ion|metal ions]] or even [[Chemistry:Hydrogen|hydrogen gas]]. [[Biology:Halophile|Salt-tolerant]] archaea (the [[Biology:Haloarchaea|Haloarchaea]]) use sunlight as an energy source, and other species of archaea [[Earth:Carbon fixation|fix carbon]], but unlike plants and [[Biology:Cyanobacteria|cyanobacteria]], no known species of archaea does both. Archaea reproduce asexually by [[Binary fission|binary fission]], [[Medicine:Fragmentation (reproduction)|fragmentation]], or [[Biology:Budding|budding]]; unlike bacteria, no known species of Archaea form [[Biology:Endospore|endospore]]s.

The first observed archaea were [[Earth:Extremophile|extremophile]]s, living in extreme environments, such as [[Earth:Hot spring|hot spring]]s and [[Chemistry:Salt lake|salt lake]]s with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every [[Earth:Habitat|habitat]], including soil, oceans, and marshlands. Archaea are particularly numerous in the oceans, and the archaea in [[Earth:Plankton|plankton]] may be one of the most abundant groups of organisms on the planet.

Archaea are a major part of Earth's life. They are part of the [[Biology:Microbiota|microbiota]] of all organisms. In the [[Biology:Human microbiome|human microbiome]], they are important in the gut, mouth, and on the skin.<ref name=Bang2015>{{cite journal |author1=Bang, C. |author2=Schmitz, R. A. |title=Archaea associated with human surfaces: not to be underestimated |journal=FEMS Microbiology Reviews |volume=39 |issue=5 |pages=631–48 |date=September 2015 |pmid=25907112 |doi=10.1093/femsre/fuv010 |doi-access=free }}</ref> Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and [[Biology:Syntrophy|syntrophic]] communities, for example.<ref>{{cite journal |author1=Moissl-Eichinger. C. |author2=Pausan, M. |author3=Taffner, J. |author4=Berg, G. |author5=Bang, C. |author6=Schmitz, R. A. |title=Archaea Are Interactive Components of Complex Microbiomes |journal=Trends in Microbiology |volume=26 |issue=1 |pages=70–85 |date=January 2018 |pmid=28826642 |doi=10.1016/j.tim.2017.07.004 }}</ref>

===Eukaryotes===

{{main|Biology:Eukaryote}}

[[File:Euglena diagram.jpg|300px|left|thumb|''[[Biology:Euglena|Euglena]]'', a single-celled eukaryote that can both move and photosynthesize]]

Eukaryotes are hypothesized to have split from archaea, which was followed by their [[Biology:Endosymbiont|endosymbioses]] with bacteria (or [[Biology:Symbiogenesis|symbiogenesis]]) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells.<ref name="hillisetal2014t"/> The major lineages of eukaryotes diversified in the [[Earth:Precambrian|Precambrian]] about 1.5 billion years ago and can be classified into eight major [[Biology:Clade|clade]]s: [[Biology:Alveolate|alveolate]]s, [[Biology:Excavata|excavates]], [[Biology:Stramenopile|stramenopile]]s, [[Biology:Plant|plant]]s, [[Biology:Rhizaria|rhizaria]]ns, [[Biology:Amoebozoa|amoebozoa]]ns, [[Biology:Fungus|fungi]], and [[Biology:Animal|animal]]s.<ref name="hillisetal2014t"/> Five of these clades are collectively known as [[Biology:Protist|protist]]s, which are mostly microscopic eukaryotic organisms that are not plants, fungi, or animals.<ref name="hillisetal2014t">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=The origin and diversification of eukaryotes |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=402–419 |isbn=978-1464175121}}</ref> While it is likely that protists share a [[Biology:Common descent|common ancestor]] (the last eukaryotic common ancestor),<ref name="O'Malley Leger Wideman Ruiz-Trillo pp. 338–344">{{cite journal |last1=O'Malley |first1=Maureen A. |last2=Leger |first2=Michelle M. |last3=Wideman |first3=Jeremy G. |last4=Ruiz-Trillo |first4=Iñaki |s2cid=67790751 |title=Concepts of the last eukaryotic common ancestor |journal=Nature Ecology & Evolution |publisher=Springer Science and Business Media LLC |volume=3 |issue=3 |date=2019-02-18 |doi=10.1038/s41559-019-0796-3 |pages=338–344|pmid=30778187 |bibcode=2019NatEE...3..338O |hdl=10261/201794 |hdl-access=free }}</ref> protists by themselves do not constitute a separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such as [[Biology:Algae|algae]], [[Biology:Invertebrate|invertebrate]]s, or [[Biology:Protozoa|protozoa]]ns, the protist grouping is not a formal taxonomic group but is used for convenience.<ref name="hillisetal2014t"/><ref name="Taylor pp. 1707–1714">{{cite journal |last=Taylor |first=F. J. R. 'M. |title=The collapse of the two-kingdom system, the rise of protistology and the founding of the International Society for Evolutionary Protistology (ISEP) |journal=International Journal of Systematic and Evolutionary Microbiology |publisher=Microbiology Society |volume=53 |issue=6 |date=2003-11-01 |doi=10.1099/ijs.0.02587-0 |pages=1707–1714|pmid=14657097 |doi-access=free }}</ref> Most protists are unicellular; these are called microbial eukaryotes.<ref name="hillisetal2014t"/>

[[Biology:Plant|Plant]]s are mainly multicellular organisms, predominantly photosynthetic eukaryotes of the [[Biology:Kingdom|kingdom]] Plantae, which would exclude [[Biology:Fungi|fungi]] and some [[Biology:Algae|algae]]. Plant cells were derived by endosymbiosis of a cyanobacterium into an early eukaryote about one billion years ago, which gave rise to chloroplasts.<ref name="hillisetal2014u">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=The evolution of plants |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=420–449 |isbn=978-1464175121}}</ref> The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic photosynthetic eukaryotic organisms are collectively described as algae, which is a term of convenience as not all algae are closely related.<ref name="hillisetal2014u"/> Algae comprise several distinct clades such as [[Biology:Glaucophyte|glaucophyte]]s, which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae.<ref name="hillisetal2014u"/> Unlike glaucophytes, the other algal clades such as [[Biology:Red algae|red]] and [[Biology:Green algae|green algae]] are multicellular. Green algae comprise three major clades: chlorophytes, [[Biology:Coleochaetophyceae|coleochaetophyte]]s, and stoneworts.<ref name="hillisetal2014u"/>

[[Biology:Fungus|Fungi]] are eukaryotes that digest foods outside their bodies,<ref name="hillisetal2014v">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=The evolution and diversity of fungi |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=451–468 |isbn=978-1464175121}}</ref> secreting digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are also saprobes, feeding on dead organic matter, making them important [[Biology:Decomposer|decomposer]]s in ecological systems.<ref name="hillisetal2014v"/>

Animals are multicellular eukaryotes. With few exceptions, animals [[Biology:Heterotroph|consume organic material]], [[Biology:Cellular respiration#Aerobic respiration|breathe oxygen]], are [[Biology:Motility|able to move]], can [[Biology:Sexual reproduction|reproduce sexually]], and grow from a hollow sphere of [[Biology:Cell|cells]], the [[Biology:Blastula|blastula]], during [[Biology:Embryogenesis|embryonic development]]. Over 1.5 million living animal [[Biology:Species|species]] have been [[Biology:Species description|described]]—of which around 1 million are [[Biology:Insecta|insects]]—but it has been estimated there are over 7 million animal species in total. They have [[Earth:Ecology|complex interactions]] with each other and their environments, forming intricate [[Biology:Food web|food web]]s.<ref name="hillisetal2014w">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Animal origins and diversity |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=469–519 |isbn=978-1464175121}}</ref>

===Viruses===

{{main|Biology:Virus}}

[[File:Phage.jpg|thumb|right|[[Biology:Bacteriophage|Bacteriophage]]s attached to a bacterial cell wall]]

[[Biology:Virus|Virus]]es are submicroscopic infectious agents that [[Biology:Viral replication|replicate]] inside the [[Biology:Cell|cells]] of organisms.<ref name="NG-20200415">{{cite news |last=Wu |first=K. J. |title=There are more viruses than stars in the universe. Why do only some infect us? – More than a quadrillion quadrillion individual viruses exist on Earth, but most are not poised to hop into humans. Can we find the ones that are? |url=https://www.nationalgeographic.com/science/2020/04/factors-allow-viruses-infect-humans-coronavirus/ |date=15 April 2020 |work=[[Organization:National Geographic Society|National Geographic Society]] |access-date=18 May 2020 |archive-date=28 May 2020 |archive-url=https://web.archive.org/web/20200528154701/https://www.nationalgeographic.com/science/2020/04/factors-allow-viruses-infect-humans-coronavirus/ |url-status=dead }}</ref> Viruses infect all types of life forms, from animals and plants to [[Biology:Microorganism|microorganism]]s, including [[Biology:Bacteria|bacteria]] and [[Biology:Archaea|archaea]].<ref name="pmid16984643">{{cite journal |author1=Koonin, E. V. |author2=Senkevich, T. G. |author3=Dolja, V. V. |title=The ancient Virus World and evolution of cells |journal=Biology Direct |volume=1 |issue=1 |pages=29 |date=September 2006 |pmid=16984643 |pmc=1594570 |doi=10.1186/1745-6150-1-29 |doi-access=free }}</ref><ref name="NYT-20210226">{{cite news |last=Zimmer |first=C. |title=The Secret Life of a Coronavirus - An oily, 100-nanometer-wide bubble of genes has killed more than two million people and reshaped the world. Scientists don't quite know what to make of it. |newspaper=The New York Times |url=https://www.nytimes.com/2021/02/26/opinion/sunday/coronavirus-alive-dead.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/02/26/opinion/sunday/coronavirus-alive-dead.html |archive-date=2021-12-28 |url-access=limited |date=26 February 2021 |access-date=28 February 2021 }}{{cbignore}}</ref> More than 6,000 virus species have been described in detail.<ref name=ictv2019>{{cite web|url=https://ictv.global/taxonomy|title=Virus Taxonomy: 2019 Release|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=25 April 2020|archive-date=20 March 2020|archive-url=https://web.archive.org/web/20200320103754/https://talk.ictvonline.org/taxonomy|url-status=live}}</ref> Viruses are found in almost every [[Earth:Ecosystem|ecosystem]] on Earth and are the most numerous type of biological entity.<ref name="Lawrence">{{cite journal |author1=Lawrence C. M. |author2=Menon S. |author3=Eilers, B. J. |title=Structural and functional studies of archaeal viruses |journal=The Journal of Biological Chemistry |volume=284 |issue=19 |pages=12599–603 |date=May 2009 |pmid=19158076 |pmc=2675988 |doi=10.1074/jbc.R800078200 |doi-access=free }}</ref><ref>{{cite journal |author1=Edwards, R.A. |author2=Rohwer, F. |s2cid=8059643 |title=Viral metagenomics |journal=Nature Reviews. Microbiology |volume=3 |issue=6 |pages=504–10 |date=June 2005 |pmid=15886693 |doi=10.1038/nrmicro1163 }}</ref>

The origins of viruses in the [[Biology:Evolutionary history of life|evolutionary history of life]] are unclear: some may have [[Biology:Evolution|evolved]] from [[Biology:Plasmid|plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of [[Biology:Horizontal gene transfer|horizontal gene transfer]], which increases [[Biology:Genetic diversity|genetic diversity]] in a way analogous to [[Biology:Sexual reproduction|sexual reproduction]].<ref name="Canchaya">{{cite journal |author1=Canchaya, C. |author2=Fournous, G. |author3=Chibani-Chennoufi, S. |title=Phage as agents of lateral gene transfer |journal=Current Opinion in Microbiology |volume=6 |issue=4 |pages=417–24 |date=August 2003 |pmid=12941415 |doi=10.1016/S1369-5274(03)00086-9 }}</ref> Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life",<ref name="ReferenceA">{{cite journal|last=Rybicki |first=E. P. |year=1990|title=The classification of organisms at the edge of life, or problems with virus systematics|journal=South African Journal of Science |volume=86|pages=182–86}}</ref> and as self-replicators.<ref name="kooninstarokadomskyy2016">{{cite journal |author1=Koonin, E. V. |author2=Starokadomskyy, P. |title=Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question |journal=Studies in History and Philosophy of Biological and Biomedical Sciences |volume=59 |pages=125–134 |date=October 2016 |pmid=26965225 |pmc=5406846 |doi=10.1016/j.shpsc.2016.02.016 }}</ref>

==Ecology==

{{Main|Earth:Ecology}}

[[Earth:Ecology|Ecology]] is the study of the distribution and abundance of life, the interaction between organisms and their [[Earth:Natural environment|environment]].<ref>{{cite book |author1=Begon, M |title=Ecology: From individuals to ecosystems |author2=Townsend, CR |author3=Harper, JL |publisher=Blackwell |year=2006 |isbn=978-1-4051-1117-1 |edition=4th}}</ref>

===Ecosystems===

{{Main|Earth:Ecosystem}}

The [[Biology:Community (ecology)|community]] of living ([[Earth:Biotic component|biotic]]) organisms in conjunction with the nonliving ([[Earth:Abiotic component|abiotic]]) components (e.g., water, light, radiation, temperature, [[Physics:Humidity|humidity]], [[Astronomy:Atmosphere|atmosphere]], acidity, and soil) of their environment is called an [[Earth:Ecosystem|ecosystem]].<ref name="habitats_of_the_world">{{cite book |title=Habitats of the world |year=2004 |url=https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238 |publisher=Marshall Cavendish |location=New York |isbn=978-0-7614-7523-1 |page=238 |access-date=2020-08-24 |archive-date=2021-04-15 |archive-url=https://web.archive.org/web/20210415113154/https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238 |url-status=live }}</ref><ref>Tansley (1934); Molles (1999), p. 482; Chapin ''et al.'' (2002), p. 380; Schulze ''et al.'' (2005); p. 400; Gurevitch ''et al.'' (2006), p. 522; Smith & Smith 2012, p. G-5</ref><ref name="hillisetal2014ao">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=The distribution of Earth's ecological systems |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=845–863 |isbn=978-1464175121}}</ref> These biotic and abiotic components are linked together through nutrient cycles and energy flows.<ref name="Odum1971">{{cite book|title=Fundamentals of Ecology|url=https://archive.org/details/fundamentalsofec0000odum|url-access=registration|last=Odum|first=Eugene P|date=1971|publisher=Saunders|isbn=978-0-534-42066-6|edition=3rd|location=New York}}</ref> Energy from the sun enters the system through [[Earth:Photosynthesis|photosynthesis]] and is incorporated into plant tissue. By feeding on plants and on one another, animals move [[Physics:Matter|matter]] and [[Physics:Energy|energy]] through the system. They also influence the quantity of plant and microbial [[Earth:Biomass (ecology)|biomass]] present. By breaking down dead [[Chemistry:Organic matter|organic matter]], [[Biology:Decomposer|decomposer]]s release [[Chemistry:Carbon|carbon]] back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.<ref name="chapinetal2002a">{{cite book |last1=Chapin III |first1=F. Stuart |last2=Matson |first2=Pamela A. |last3=Mooney |first3=Harold A. |chapter=The ecosystem concept |title=Principles of Terrestrial Ecosystem Ecology |date=2002 |pages=10 |publisher=Springer |location=New York |isbn=978-0-387-95443-1}}</ref>

===Populations===

{{main|Biology:Population ecology}}

[[File:Logistic Carrying Capacity.svg|thumb|right|Reaching carrying capacity through a logistic growth curve]]

A [[Social:Population|population]] is the group of organisms of the same [[Biology:Species|species]] that occupies an area and [[Biology:Sexual reproduction|reproduce]] from generation to generation.<ref name="hillisetal2014ap">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4=Price |first4=Mary V. |chapter=Populations|title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=864–897 |isbn=978-1464175121}}</ref><ref name="urry2017ba">{{cite book |last1=Urry |first1=Lisa |last2=Cain |first2=Michael |last3=Wasserman |first3= Steven |last4=Minorsky |first4=Peter | last5=Reece |first5=Jane |chapter=Population ecology |title=Campbell Biology |publisher=Pearson |edition=11th |date=2017 |location=New York |pages=1188–1211 |isbn=978-0134093413}}</ref><ref>{{cite web |title=Population |url=http://www.biology-online.org/dictionary/Population |publisher=Biology Online |access-date=5 December 2012 |archive-date=13 April 2019 |archive-url=https://web.archive.org/web/20190413145351/https://www.biology-online.org/dictionary/Population |url-status=live }}</ref><ref>{{cite web |title=Definition of population (biology) |url=http://oxforddictionaries.com/definition/english/population?q=population |work=Oxford Dictionaries |publisher=Oxford University Press |access-date=5 December 2012 |quote=a community of animals, plants, or humans among whose members interbreeding occurs |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304104019/http://www.oxforddictionaries.com/definition/english/population?q=population |url-status=dead }}</ref><ref>{{cite book |last=Hartl |first=Daniel |title=Principles of Population Genetics |publisher=[[Company:Sinauer Associates|Sinauer Associates]] |year=2007 |isbn=978-0-87893-308-2 |page=45}}</ref> [[Biology:Population size|Population size]] can be estimated by multiplying population density by the area or volume. The carrying capacity of an [[Earth:Natural environment|environment]] is the maximum population size of a [[Biology:Species|species]] that can be sustained by that specific environment, given the food, [[Earth:Habitat|habitat]], water, and other [[Earth:Resource|resource]]s that are available.<ref name=":32">{{Cite journal|date=2018-01-01|title=The flexible application of carrying capacity in ecology|journal=Global Ecology and Conservation|language=en|volume=13|pages=e00365|doi=10.1016/j.gecco.2017.e00365|doi-access=free|last1=Chapman|first1=Eric J.|last2=Byron|first2=Carrie J.}}</ref> The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability resources and the cost of maintaining them. In human populations, new [[Engineering:Technology|technologies]] such as the Green revolution have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the most famous of which was by Thomas Malthus in the 18th century.<ref name="hillisetal2014ap"/>

===Communities===

{{Main|Biology:Community (ecology)}}

[[File:TrophicWeb.jpg|thumb|370px |A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.<ref name="Odum05">{{cite book |last1=Odum |first1=E. P. |last2=Barrett |first2=G. W. |title=Fundamentals of Ecology |edition=5th |publisher=Brooks/Cole, a part of Cengage Learning |year=2005 |isbn=978-0-534-42066-6 |url=http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= |url-status=dead |archive-url=https://web.archive.org/web/20110820163059/http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= |archive-date=2011-08-20 }}</ref>]]

A community is a group of [[Social:Population|population]]s of species occupying the same geographical area at the same time. A [[Biology:Biological interaction|biological interaction]] is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like [[Biology:Pollination|pollination]] and [[Earth:Predation|predation]], or long-term; both often strongly influence the [[Biology:Evolution|evolution]] of the species involved. A long-term interaction is called a [[Earth:Symbiosis|symbiosis]]. Symbioses range from [[Biology:Mutualism|mutualism]], beneficial to both partners, to [[Biology:Competition|competition]], harmful to both partners.<ref>{{cite journal |last1=Wootton |first1=JT |last2=Emmerson |first2=M |title=Measurement of Interaction Strength in Nature |journal=Annual Review of Ecology, Evolution, and Systematics |volume=36|pages=419–44|year=2005|jstor=30033811 |doi=10.1146/annurev.ecolsys.36.091704.175535}}</ref> Every species participates as a consumer, resource, or both in [[Biology:Consumer–resource interactions|consumer–resource interactions]], which form the core of food chains or [[Biology:Food web|food web]]s.<ref name="hillisetal2014aq">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Ecological and evolutionary consequences within and among species |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=882–897|isbn=978-1464175121}}</ref> There are different [[Earth:Trophic level|trophic level]]s within any food web, with the lowest level being the primary producers (or [[Biology:Autotroph|autotroph]]s) such as plants and algae that convert energy and inorganic material into [[Chemistry:Organic compound|organic compound]]s, which can then be used by the rest of the community.<ref name=bryantfrigaard/><ref>{{cite book |author=Smith, AL |title=Oxford dictionary of biochemistry and molecular biology |publisher=Oxford University Press |location=Oxford [Oxfordshire] |year=1997 |page=508 |isbn=978-0-19-854768-6 |quote=Photosynthesis – the synthesis by organisms of organic chemical compounds, esp. carbohydrates, from carbon dioxide using energy obtained from light rather than the oxidation of chemical compounds.}}</ref><ref>{{cite journal |last=Edwards |first=Katrina |title=Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically Active Ridge Flank |journal=Woods Hole Oceanographic Institution }}</ref> At the next level are the [[Biology:Heterotroph|heterotroph]]s, which are the species that obtain energy by breaking apart organic compounds from other organisms.<ref name="hillisetal2014aq"/> Heterotrophs that consume plants are primary consumers (or [[Earth:Herbivore|herbivore]]s) whereas heterotrophs that consume herbivores are secondary consumers (or [[Biology:Carnivore|carnivore]]s). And those that eat secondary consumers are tertiary consumers and so on. [[Biology:Omnivore|Omnivorous]] heterotrophs are able to consume at multiple levels. Finally, there are [[Biology:Decomposer|decomposer]]s that feed on the waste products or dead bodies of organisms.<ref name="hillisetal2014aq"/>
On average, the total amount of energy incorporated into the [[Biology:Biomass|biomass]] of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level.<ref name="hillisetal2014ar">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=Ecological communities |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=898–915|isbn=978-1464175121}}</ref>

===Biosphere===

{{Main|Biology:Biosphere}}

[[File:Carbon cycle.jpg|thumb|250px|right|Fast carbon cycle showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of the slow carbon cycle, such as volcanic and tectonic activity, are not included.<ref name="nasacc">{{cite web|last1=Riebeek|first1=Holli|title=The Carbon Cycle|url=http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|website=Earth Observatory|publisher=NASA|access-date=5 April 2018|date=16 June 2011|archive-url=https://web.archive.org/web/20160305010126/http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|archive-date=5 March 2016|url-status=live|df=dmy-all}}</ref>]]

In the global ecosystem or biosphere, matter exists as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations.<ref name="hillisetal2014as">{{cite book |last1=Hillis |first1=David M. |last2=Sadava |first2=David |last3=Hill |first3=Richard W. |last4= Price |first4=Mary V. |chapter=The distribution of Earth's ecological systems |title=Principles of Life |publisher=Sinauer Associates |edition=2nd |date=2014 |location=Sunderland, Mass. |pages=916–934 |isbn=978-1464175121}}</ref> For example, matter from terrestrial autotrophs are both biotic and accessible to other organisms whereas the matter in rocks and minerals are abiotic and inaccessible. A [[Biology:Biogeochemical cycle|biogeochemical cycle]] is a pathway by which specific [[Chemistry:Chemical element|element]]s of matter are turned over or moved through the biotic ([[Biology:Biosphere|biosphere]]) and the abiotic ([[Earth:Lithosphere|lithosphere]], [[Astronomy:Atmosphere|atmosphere]], and [[Earth:Hydrosphere|hydrosphere]]) compartments of [[Astronomy:Earth|Earth]]. There are biogeochemical cycles for [[Biology:Nitrogen cycle|nitrogen]], [[Earth:Carbon cycle|carbon]], and [[Physics:Water cycle|water]].

===Conservation===

{{Main|Biology:Conservation biology}}

Conservation biology is the study of the conservation of [[Astronomy:Earth|Earth]]'s [[Biology:Biodiversity|biodiversity]] with the aim of protecting [[Biology:Species|species]], their habitats, and ecosystems from excessive rates of [[Biology:Extinction|extinction]] and the erosion of biotic interactions.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M. J |title=Recovery from the most profound mass extinction of all time |journal=Proceedings of the Royal Society B: Biological Sciences |volume=275 |issue=1636 |pages=759–65 |year=2008 |pmid=18198148 |pmc=2596898 |doi=10.1098/rspb.2007.1370 }}</ref><ref name="ConsBiol80">{{cite book |author1=Soulé, Michael E. |author2=Wilcox, Bruce A. |title=Conservation biology: an evolutionary-ecological perspective |publisher=Sinauer Associates |location=Sunderland, Mass. |year=1980 |isbn=978-0-87893-800-1 }}</ref><ref>{{cite journal |last1=Soulé |first1=Michael E. |title=What is Conservation Biology? |journal=BioScience |volume=35 |issue=11 |pages=727–34 |year=1986 |url=http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf |doi=10.2307/1310054 |jstor=1310054 |publisher=American Institute of Biological Sciences |access-date=2021-05-15 |archive-date=2019-04-12 |archive-url=https://web.archive.org/web/20190412085412/http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf |url-status=dead }}</ref> It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender [[Biology:Genetics|genetic]], [[Social:Population|population]], [[Biology:Species|species]], and ecosystem diversity.<ref name="Hunter96">{{cite book |author=Hunter, Malcolm L. |title=Fundamentals of conservation biology |publisher=Blackwell Science |location=Oxford |year=1996 |isbn=978-0-86542-371-8 |url=https://archive.org/details/fundamentalsofco00hunt }}</ref><ref name="Groom06">{{cite book |author1=Meffe, Gary K. |author2=Martha J. Groom |title=Principles of conservation biology |publisher=Sinauer Associates |location=Sunderland, Mass. |year=2006 |isbn=978-0-87893-518-5 |edition=3rd}}</ref><ref name="Dyke08">{{cite book |last=Van Dyke |first=Fred |date=2008 |title=Conservation biology: foundations, concepts, applications |location=New York |publisher=[[Physics:Springer-Verlag|Springer-Verlag]] |edition=2nd |isbn=978-1402068904 |oclc=232001738 |doi=10.1007/978-1-4020-6891-1 |hdl=11059/14777 |url=https://books.google.com/books?id=Evh1UD3ZYWcC |access-date=2021-05-15 |archive-date=2020-07-27 |archive-url=https://web.archive.org/web/20200727115147/https://books.google.com/books?id=Evh1UD3ZYWcC |url-status=live }}</ref><ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M. J. |last3=Ferry |first3=P. A. |title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land |journal=Biology Letters |volume=6 |issue=4 |pages=544–7 |year=2010 |pmid=20106856 |pmc=2936204 |doi=10.1098/rsbl.2009.1024 }}</ref> The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years,<ref name="Koh">{{cite journal |last1=Koh |first1=Lian Pin |last2=Dunn |first2=Robert R. |last3=Sodhi |first3=Navjot S. |last4=Colwell |first4=Robert K. |last5=Proctor |first5=Heather C. |last6=Smith |first6=Vincent S. |title=Species coextinctions and the biodiversity crisis |journal=Science |volume=305 |issue=5690 |pages=1632–4 |year=2004 |pmid=15361627 |doi=10.1126/science.1101101 |bibcode=2004Sci...305.1632K |s2cid=30713492 }}</ref> which has contributed to poverty, starvation, and will reset the course of evolution on this planet.<ref>Millennium Ecosystem Assessment (2005). ''Ecosystems and Human Well-being: Biodiversity Synthesis.'' World Resources Institute, Washington, D.C.[http://www.millenniumassessment.org/documents/document.354.aspx.pdf] {{Webarchive|url=https://web.archive.org/web/20191014033601/http://www.millenniumassessment.org/documents/document.354.aspx.pdf|date=2019-10-14}}</ref><ref name="Jackson">{{cite journal |last1=Jackson |first1=J. B. C. |title=Ecological extinction and evolution in the brave new ocean |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=Suppl 1 |pages=11458–65 |year=2008 |pmid=18695220 |pmc=2556419 |doi=10.1073/pnas.0802812105  |bibcode=2008PNAS..10511458J |doi-access=free }}</ref> [[Biology:Biodiversity|Biodiversity]] affects the functioning of ecosystems, which provide a variety of [[Biology:Ecosystem services|services]] upon which people depend. Conservation biologists research and educate on the trends of [[Biology:Biodiversity loss|biodiversity loss]], species [[Biology:Extinction|extinction]]s, and the negative effect these are having on our capabilities to [[Earth:Sustainability|sustain]] the well-being of human society. Organizations and citizens are responding to the [[Earth:Holocene extinction|current biodiversity crisis]] through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.<ref name="Soule86">{{cite book|last=Soule|first= Michael E. |title=Conservation Biology: The Science of Scarcity and Diversity|year=1986 |publisher=Sinauer Associates|page=584 |isbn=978-0-87893-795-0}}</ref><ref name="Hunter96"/><ref name="Groom06"/><ref name="Dyke08"/>

== See also ==
{{div col|colwidth=20em}}
* Biology in fiction
* Glossary of biology
* List of biological websites
* [[Biography:List of biologists|List of biologists]]
* [[Biology:List of biology journals|List of biology journals]]
* List of biology topics
* [[Biology:List of life sciences|List of life sciences]]
* [[Biology:List of omics topics in biology|List of omics topics in biology]]
* [[Organization:National Association of Biology Teachers|National Association of Biology Teachers]]
* [[Biology:Outline of biology|Outline of biology]]
* Periodic table of life sciences in [[Philosophy:Tinbergen's four questions|Tinbergen's four questions]]
* [[Astronomy:Science tourism|Science tourism]]
* [[:Category:Biology terminology|Terminology of biology]]
{{div col end}}{{Notelist}}

== References ==
{{Reflist}}

== Further reading ==

{{Further |Biology:Bibliography of biology}}

{{refbegin |30em}}
* {{cite book |last1=Alberts |first1=B. |last2=Johnson|first2=A. |last3=Lewis|first3=J. |last4=Raff|first4=M. |last5=Roberts|first5=K. |last6=Walter|first6=P. |title=Molecular Biology of the Cell |url=https://archive.org/details/molecularbiolog000wils |url-access=registration |edition=4th |publisher=Garland |year=2002 |isbn=978-0-8153-3218-3 |oclc=145080076 |ref=none}}
* {{cite book |author1=Begon, M. |author2=Townsend, C. R. |author3=Harper, J. L. |title=Ecology: From Individuals to Ecosystems |edition=4th |publisher=Blackwell Publishing Limited |year=2005 |isbn=978-1-4051-1117-1 |oclc=57639896 |title-link=Ecology: From Individuals to Ecosystems |ref=none}}
* {{cite book |last=Campbell |first=Neil  |title=Biology |edition=7th |publisher=Benjamin-Cummings Publishing Company |year=2004 |isbn=978-0-8053-7146-8 |oclc=71890442 |ref=none}}
* {{cite book |last=Colinvaux |first=Paul |title=Why Big Fierce Animals are Rare: An Ecologist's Perspective |edition=reissue |publisher=Princeton University Press |year=1979 |isbn=978-0-691-02364-9 |oclc=10081738 |url=https://archive.org/details/whybigfierceanim00paul |ref=none}}
* {{cite book |last=Mayr |first=Ernst |title=The Growth of Biological Thought: Diversity, Evolution, and Inheritance |url=https://books.google.com/books?id=pHThtE2R0UQC |year=1982 |publisher=Harvard University Press |isbn=978-0-674-36446-2 |access-date=2015-06-27 |archive-date=2015-10-03 |archive-url=https://web.archive.org/web/20151003080726/https://books.google.com/books?id=pHThtE2R0UQC |url-status=live |ref=none}}
* {{cite book |last=Hoagland |first=Mahlon |title=The Way Life Works |edition=|publisher=Jones and Bartlett Publishers inc |year=2001 |isbn=978-0-7637-1688-2 |oclc=223090105 |ref=none}}
* {{cite book |last=Janovy |first=John |title=On Becoming a Biologist |edition=2nd |publisher=Bison Books |year=2004 |isbn=978-0-8032-7620-8 |oclc=55138571 |ref=none}}
* {{cite book |last=Johnson |first=George B. |title=Biology, Visualizing Life |publisher=Holt, Rinehart, and Winston |year=2005 |isbn=978-0-03-016723-2 |oclc=36306648 |url=https://archive.org/details/holtbiologyvisua00john |ref=none}}
* {{cite book |last1=Tobin |first1=Allan |last2=Dusheck |first2=Jennie |title=Asking About Life |edition=3rd |publisher=Wadsworth |location=Belmont, California |year=2005 |isbn=978-0-534-40653-0 |ref=none}}
{{refend}}

== External links ==
* {{curlie|Science/Biology}}
* [http://phylonames.org/code/ OSU's Phylocode]
* [https://www.biologyonline.com/dictionary Biology Online – Wiki Dictionary]
* [https://ocw.mit.edu/courses/biology/7-012-introduction-to-biology-fall-2004/ MIT video lecture series on biology]
* [https://www.onezoom.org/ OneZoom Tree of Life]

'''Journal links'''
* [https://journals.plos.org/plosbiology/ PLOS Biology] A peer-reviewed, open-access journal published by the Public Library of Science
* [http://www.cell.com/current-biology/ ''Current Biology'']: General journal publishing original research from all areas of biology
* ''[https://royalsocietypublishing.org/journal/rsbl Biology Letters]'': A [[Impact factor|high-impact]] Royal Society journal publishing peer-reviewed biology papers of general interest
* ''[https://www.science.org/collections Science]'': Internationally renowned AAAS science journal – see sections of the life sciences
* ''[https://www.ijbs.com/ International Journal of Biological Sciences]'': A biological journal publishing significant peer-reviewed scientific papers
* ''[https://www.press.jhu.edu/journals/perspectives-biology-and-medicine Perspectives in Biology and Medicine]'': An [[Philosophy:Interdisciplinarity|interdisciplinary]] scholarly journal publishing [[Social:Essay|essay]]s of broad relevance

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