View source for Life

{{Short description|Matter with biological processes}}
{{Other uses|Life (disambiguation)}}
{{Good article}}
{{pp-semi-vandalism|small=yes}}
{{EngvarB|date=August 2024}}
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{{Automatic taxobox
| name = Life <!-- please see section "Biota" on Talk page before reverting-->
| fossil_range = {{Long fossil range|3770|0|earliest=4280}} [[Archean]] – [[Holocene|present]] (possible [[Hadean]] origin)
| image = Coral reef... South end of my area (14119221571).jpg
| image_upright = 1.2
| image_caption = Diverse forms of life on a [[coral reef]]
| taxon = Life
| authority = 
| subdivision_ranks = [[Domain (biology)|Domains]] and [[Kingdom (biology)#Kingdoms of the Eukaryota|Supergroups]]
| subdivision = Life on Earth:
* [[Cellular life]]
** Domain [[Bacteria]]
** Domain [[Archaea]]
** Domain [[Eukaryota]]
*** [[Diaphoretickes]]
**** [[Archaeplastida]] <small>(includes [[plant]]s)</small>
**** [[Haptista]]
**** [[Cryptista]]
**** [[TSAR]]
*** "[[Excavata]]"
*** [[Provora]]
*** [[Ancyromonadida]]
*** [[CRuMs]]
*** [[Hemimastigophora]]
*** [[Amorphea]] <small>(includes [[animal]]s and [[fungi]])</small>
**''[[Parakaryon myojinensis]]'' (''[[incertae sedis]]'')
* [[Non-cellular life]]
** [[Virus]]es{{efn|Viruses are strongly believed not to descend from a common ancestor, with each [[Realm (virology)|realm]] corresponding to separate instances of a virus coming into existence.<ref name=exec >{{cite journal |author=International Committee on Taxonomy of Viruses Executive Committee |date=May 2020 |title=The New Scope of Virus Taxonomy: Partitioning the Virosphere Into 15 Hierarchical Ranks |journal=Nature Microbiology |volume=5 |issue=5 |pages=668–674 |doi=10.1038/s41564-020-0709-x |pmc=7186216 |pmid=32341570}}</ref>}}
** [[Virusoid]]s
** [[Viroid]]s
}}

'''Life''' is a quality that distinguishes [[matter]] that has [[biological process]]es, such as [[Cell signaling|signaling]] and self-sustaining processes, from matter that does not. It is defined descriptively by the capacity for [[homeostasis]], [[Structure#Biological|organisation]], [[metabolism]], [[Cell growth|growth]], [[adaptation]], response to [[stimulus (physiology)|stimuli]], and [[reproduction]]. All life over time eventually reaches a state of [[death]], and none is [[Immortality|immortal]]. Many philosophical definitions of [[living systems]] have been proposed, such as [[self-organizing]] systems. [[Virus]]es in particular make definition difficult as they replicate only in [[Host (biology)|host]] cells. Life exists all over the Earth in air, water, and [[soil]], with many [[ecosystem]]s forming the [[biosphere]]. Some of these are harsh environments occupied only by [[extremophile]]s.

Life has been studied since ancient times, with theories such as [[Empedocles]]'s [[materialism]] asserting that it was composed of [[Classical element|four eternal elements]], and [[Aristotle]]'s [[hylomorphism]] asserting that living things have [[soul]]s and embody both [[Form (Plato)|form]] and matter. [[Origin of life|Life originated]] at least 3.5&nbsp;billion years ago, resulting in a [[universal common ancestor]]. This evolved into all the [[species]] that exist now, by way of many [[extinct]] species, some of which have left traces as [[fossil]]s. Attempts to classify living things, too, [[Aristotle's biology|began with Aristotle]]. Modern [[taxonomy|classification]] began with [[Carl Linnaeus]]'s system of [[binomial nomenclature]] in the 1740s.

Living things are composed of [[Biochemistry|biochemical molecules]], formed mainly from a few core [[chemical element]]s. All living things contain two types of large molecule, [[protein]]s and [[nucleic acid]]s, the latter usually both [[DNA]] and [[RNA]]: these carry the information needed by each species, including the instructions to make each type of protein. The proteins, in turn, serve as the machinery which carries out the many chemical processes of life. The [[Cell (biology)|cell]] is the structural and functional unit of life. Smaller organisms, including [[prokaryote]]s (bacteria and [[archaea]]), consist of small single cells. Larger [[organism]]s, mainly [[eukaryote]]s, can consist of single cells or may be [[multicellular]] with more complex structure. Life is only known to exist on Earth but [[extraterrestrial life]] is [[Fermi paradox|thought probable]]. [[Artificial life]] is being simulated and explored by scientists and engineers.

{{Anchor|Definition}}

== Definitions ==

=== Challenge ===

The definition of life has long been a challenge for scientists and philosophers.<ref name="Definitions 2009">{{cite journal |title=Why Is the Definition of Life So Elusive? Epistemological Considerations |journal=Astrobiology |date=May 2009 |last=Tsokolov |first=Serhiy A. |volume=9 |issue=4 |doi=10.1089/ast.2007.0201 |bibcode=2009AsBio...9..401T |pages=401–412 |pmid=19519215}}</ref><ref name=Emmeche1997>{{cite web |first1=Claus |last1=Emmeche |year=1997 |title=Defining Life, Explaining Emergence |publisher=Niels Bohr Institute |url=http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html |access-date=25 May 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120314095044/http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html |archive-date=14 March 2012 }}</ref><ref name=McKay>{{Cite journal |title=What Is Life—and How Do We Search for It in Other Worlds? |journal=PLOS Biology |date=14 September 2004 |first=Chris P. |last=McKay |pmid=15367939 |volume=2 |issue=9 |pmc=516796 |page=302 |doi=10.1371/journal.pbio.0020302 |doi-access=free }}</ref> This is partially because life is a process, not a substance.<ref name="DefinitionMotivation">{{Cite journal |last=Mautner |first=Michael N. |title=Directed panspermia. 3. Strategies and motivation for seeding star-forming clouds |journal=Journal of the British Interplanetary Society |year=1997 |volume=50 |pages=93–102 |url=http://www.astro-ecology.com/PDFDirectedPanspermia3JBIS1997Paper.pdf |bibcode=1997JBIS...50...93M |url-status=live |archive-url=https://web.archive.org/web/20121102064738/http://www.astro-ecology.com/PDFDirectedPanspermia3JBIS1997Paper.pdf |archive-date=2 November 2012 }}</ref><ref name="SeedingBook">{{Cite book |last=Mautner |first=Michael N. |title=Seeding the Universe with Life: Securing Our Cosmological Future |date=2000 |publisher=Michael Mautner |isbn=978-0-476-00330-9 |url=http://www.astro-ecology.com/PDFSeedingtheUniverse2005Book.pdf |url-status=live |archive-url=https://web.archive.org/web/20121102064713/http://www.astro-ecology.com/PDFSeedingtheUniverse2005Book.pdf |archive-date=2 November 2012 }}</ref><ref>{{cite journal |title=What is life? It's a Tricky, Often Confusing Question |journal=Astrobiology Magazine |date=18 September 2014 |last=McKay |first=Chris}}</ref> This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside Earth.<ref>{{Cite journal |last1=Nealson |first1=K.H. |last2=Conrad |first2=P.G. |title=Life: past, present and future |journal=[[Philosophical Transactions of the Royal Society of London B]] |volume=354 |issue=1392 |pages=1923–1939 |date=December 1999 |pmid=10670014 |pmc=1692713 |doi=10.1098/rstb.1999.0532 |url=https://royalsociety.org/journals/|archive-date=3 January 2016 |archive-url=https://wayback.archive-it.org/all/20160103000925/https://royalsociety.org/journals/ |url-status=live }}</ref><ref name="Bioethics">{{Cite journal |last=Mautner |first=Michael N. |title=Life-centered ethics, and the human future in space |journal=Bioethics |volume=23 |pages=433–440 |year=2009 |doi=10.1111/j.1467-8519.2008.00688.x |pmid=19077128 |url=http://www.astro-ecology.com/PDFLifeCenteredBioethics2009Paper.pdf |issue=8 |s2cid=25203457 |url-status=live |archive-url=https://web.archive.org/web/20121102064743/http://www.astro-ecology.com/PDFLifeCenteredBioethics2009Paper.pdf |archive-date=2 November 2012 }}</ref> Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.<ref name=Jeuken1975>{{cite journal|title=The biological and philosophical defitions of life |author=Jeuken M |journal=Acta Biotheoretica |volume=24 |issue=1–2 |pages=14–21 |year=1975 |doi=10.1007/BF01556737|pmid=811024 |s2cid=44573374 }}</ref> [[Legal death|Legal definitions]] of life have been debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.<ref name=Capron1978>{{cite journal|title=Legal definition of death |author=Capron AM |journal=Annals of the New York Academy of Sciences |year=1978 |doi=10.1111/j.1749-6632.1978.tb50352.x |pmid=284746 |volume=315 |issue=1 |pages=349–362 |bibcode=1978NYASA.315..349C |s2cid=36535062 }}</ref> At least 123 definitions of life have been compiled.<ref name="JBSD-20110317">{{cite journal |last=Trifonov |first=Edward N. |title=Vocabulary of Definitions of Life Suggests a Definition |date=17 March 2011 |journal=Journal of Biomolecular Structure and Dynamics |volume=29 |issue=2 |pages=259–266 |doi=10.1080/073911011010524992 |pmid=21875147 |doi-access=free }}</ref>

=== Descriptive ===

{{further|Organism}}

Since there is no consensus for a definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This implies all or most of the following traits:<ref name=McKay/><ref name=Koshland>{{Cite journal |title=The Seven Pillars of Life |journal=Science |date=22 March 2002 | first=Daniel E. Jr. | last=Koshland |volume=295 |issue=5563 |pages=2215–2216 |doi=10.1126/science.1068489 |pmid=11910092 |doi-access=free }}</ref><ref>{{Cite book |title=The American Heritage Dictionary of the English Language |publisher=Houghton Mifflin |year=2006 |isbn=978-0-618-70173-5 |edition=4th |chapter=life }}</ref><ref name=merriamwebster>{{cite web|url=http://www.merriam-webster.com/dictionary/life|title=Life|publisher=Merriam-Webster Dictionary|access-date=25 July 2022|url-status=live|archive-url=https://web.archive.org/web/20211213211541/https://www.merriam-webster.com/dictionary/life|archive-date=13 December 2021}}</ref><ref>{{cite web |url=http://phoenix.lpl.arizona.edu/mars141.php |title=Habitability and Biology: What are the Properties of Life? |access-date=6 June 2013 |website=Phoenix Mars Mission |publisher=The University of Arizona |url-status=live |archive-url=https://web.archive.org/web/20140416114923/http://phoenix.lpl.arizona.edu/mars141.php |archive-date=16 April 2014 }}</ref><ref name="JBS-2012Feb">{{cite journal |last=Trifonov |first=Edward N. |title=Definition of Life: Navigation through Uncertainties |journal=Journal of Biomolecular Structure & Dynamics |volume=29 |issue=4 |pages=647–650 |doi=10.1080/073911012010525017 |pmid=22208269 |year=2012 |s2cid=8616562 |doi-access=free }}</ref>

# [[Homeostasis]]: regulation of the internal environment to maintain a constant state; for example, [[sweating]] to reduce temperature.
# [[Organisation]]: being structurally composed of one or more [[cell (biology)|cells]]&nbsp;– the basic units of life.
# [[Metabolism]]: transformation of energy, used to convert chemicals into cellular components ([[anabolism]]) and to decompose organic matter ([[catabolism]]). Living things [[bioenergetics|require energy]] for homeostasis and other activities.
# [[Cell growth|Growth]]: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size and structure.
# [[Adaptation]]: the evolutionary process whereby an organism becomes better able to live in its [[habitat]].<ref>{{cite book |last=Dobzhansky |first=Theodosius |author-link=Theodosius Dobzhansky |chapter=On Some Fundamental Concepts of Darwinian Biology |date=1968 |chapter-url=http://dx.doi.org/10.1007/978-1-4684-8094-8_1 |title=Evolutionary Biology |pages=1–34 |place=Boston, MA |publisher=Springer US |doi=10.1007/978-1-4684-8094-8_1 |isbn=978-1-4684-8096-2 |access-date=23 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730033922/https://link.springer.com/chapter/10.1007/978-1-4684-8094-8_1 |url-status=live }}</ref><ref>{{Cite book |last=Wang |first=Guanyu |url=https://www.worldcat.org/oclc/868928102 |title=Analysis of complex diseases : a mathematical perspective |date=2014 |publisher=CRC Press |isbn=978-1-4665-7223-2|oclc=868928102 |access-date=23 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730033921/https://www.worldcat.org/title/analysis-of-complex-diseases-a-mathematical-perspective/oclc/868928102 |url-status=live }}</ref><ref>{{Cite book |url=https://www.worldcat.org/oclc/906025831 |title=Climate change impact on livestock : adaptation and mitigation |date=2015 |publisher=Springer |editor-last1=Sejian |editor-first1=Veerasamy |editor-last2=Gaughan |editor-first2=John |editor-last3=Baumgard |editor-first3=Lance |editor-last4=Prasad |editor-first4=C. S. |isbn=978-81-322-2265-1 |oclc=906025831 |access-date=23 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730033921/https://www.worldcat.org/title/climate-change-impact-on-livestock-adaptation-and-mitigation/oclc/906025831 |url-status=live }}</ref>
# Response to [[stimulus (physiology)|stimuli]]: such as the contraction of a [[unicellular organism]] away from external chemicals, the complex reactions involving all the senses of [[multicellular organisms]], or the motion of the leaves of a plant turning toward the sun ([[phototropism]]), and [[chemotaxis]].
# [[Reproduction]]: the ability to produce new individual organisms, either [[asexual reproduction|asexually]] from a single parent organism or [[sexual reproduction|sexually]] from two parent organisms.

=== Physics ===

{{further|Entropy and life}}

From a [[physics]] perspective, an organism is a [[thermodynamic system]] with an organised molecular structure that can reproduce itself and evolve as survival dictates.<ref name="Luttermoser-1">{{cite web |last1=Luttermoser |first1=Donald G. |title=ASTR-1020: Astronomy II Course Lecture Notes Section XII |url=http://www.etsu.edu/physics/lutter/courses/astr1020/a1020chap12.pdf |publisher=[[East Tennessee State University]] |access-date=28 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120322185054/http://www.etsu.edu/physics/lutter/courses/astr1020/a1020chap12.pdf |archive-date=22 March 2012 }}</ref><ref name="Luttermoser-2">{{cite web |last1=Luttermoser |first1=Donald G. |title=Physics 2028: Great Ideas in Science: The Exobiology Module |url=http://www.etsu.edu/physics/lutter/courses/phys2028/p2028exobnotes.pdf |date=Spring 2008 |publisher=[[East Tennessee State University]] |access-date=28 August 2011 |url-status=dead |archive-url=https://web.archive.org/web/20120322185041/http://www.etsu.edu/physics/lutter/courses/phys2028/p2028exobnotes.pdf |archive-date=22 March 2012 }}</ref> Thermodynamically, life has been described as an open system which makes use of gradients in its surroundings to create imperfect copies of itself.<ref name="Review 2009">{{cite journal |title=What makes a planet habitable? |journal=The Astronomy and Astrophysics Review |year=2009 |last1=Lammer |first1=H. |last2=Bredehöft |first2=J.H. |last3=Coustenis |first3=A. |author3-link=Athena Coustenis |last4=Khodachenko |first4=M.L. |volume=17 |issue=2 |pages=181–249 |doi=10.1007/s00159-009-0019-z |url=http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |access-date=3 May 2016 |quote=Life as we know it has been described as a (thermodynamically) open system (Prigogine et al. 1972), which makes use of gradients in its surroundings to create imperfect copies of itself. |url-status=dead |archive-url=https://web.archive.org/web/20160602235333/http://veilnebula.jorgejohnson.me/uploads/3/5/8/7/3587678/lammer_et_al_2009_astron_astro_rev-4.pdf |archive-date=2 June 2016 |bibcode=2009A&ARv..17..181L|s2cid=123220355 }}</ref> Another way of putting this is to define life as "a self-sustained chemical system capable of undergoing [[Darwinian evolution]]", a definition adopted by a [[NASA]] committee attempting to define life for the purposes of [[exobiology]], based on a suggestion by [[Carl Sagan]].<ref>{{Cite journal |last=Benner |first=Steven A. |date=December 2010 |title=Defining Life |journal=Astrobiology |volume=10 |issue=10 |pages=1021–1030 |doi=10.1089/ast.2010.0524 |pmc=3005285 |pmid=21162682 |bibcode=2010AsBio..10.1021B}}</ref><ref>{{cite book |first1=Gerald F. |title=Extraterrestrials |last1=Joyce |author-link=Gerald Joyce |pages=139–151 |publisher=Cambridge University Press |date=1995 |doi=10.1017/CBO9780511564970.017 |isbn=978-0-511-56497-0 |chapter=The RNA World: Life before DNA and Protein |hdl=2060/19980211165 |s2cid=83282463 }}</ref> This definition, however, has been widely criticised because according to it, a single sexually reproducing individual is not alive as it is incapable of evolving on its own.<ref>{{Cite journal |last=Benner |first=Steven A. |date=December 2010 |title=Defining Life |journal=Astrobiology |volume=10 |issue=10 |pages=1021–1030 |doi=10.1089/ast.2010.0524 |pmc=3005285 |pmid=21162682|bibcode=2010AsBio..10.1021B }}</ref>

=== Living systems ===

{{main|Living systems}}

Others take a [[living systems theory]] viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are [[self-organizing]] and [[autopoietic]] (self-producing). Variations of this include [[Stuart Kauffman]]'s definition as an [[autonomous agent]] or a [[multi-agent system]] capable of reproducing itself, and of completing at least one [[thermodynamic cycle|thermodynamic work cycle]].<ref>{{Cite book |first1=Stuart |last1=Kaufmann |title=Science and Ultimate Reality |date=2004 |chapter=Autonomous agents |editor1-first=John D. |editor1-last=Barrow |editor2-last=Davies |editor3-first=C.L. |editor3-last=Harper, Jr. |pages=654–666 |doi=10.1017/CBO9780511814990.032 |isbn=978-0-521-83113-0 |chapter-url=https://books.google.com/books?id=K_OfC0Pte_8C&pg=PA654 |editor2-first=P.C.W. |access-date=10 August 2023 |archive-date=5 November 2023 |archive-url=https://web.archive.org/web/20231105190205/https://books.google.com/books?id=K_OfC0Pte_8C&pg=PA654#v=onepage&q&f=false |url-status=live }}</ref> This definition is extended by the evolution of novel functions over time.<ref>{{Cite book |last1=Longo |first1=Giuseppe |last2=Montévil |first2=Maël |last3=Kauffman |first3=Stuart |title=Proceedings of the 14th annual conference companion on Genetic and evolutionary computation |chapter=No entailing laws, but enablement in the evolution of the biosphere |date=1 January 2012 |url=https://www.academia.edu/11720588 |series=GECCO '12 |pages=1379–1392 |doi=10.1145/2330784.2330946 |isbn=978-1-4503-1178-6 |url-status=live |archive-url=https://web.archive.org/web/20170511103757/http://www.academia.edu/11720588/No_entailing_laws_but_enablement_in_the_evolution_of_the_biosphere |archive-date=11 May 2017 |arxiv=1201.2069 |citeseerx=10.1.1.701.3838 |bibcode=2012arXiv1201.2069L |s2cid=15609415 }}</ref>

=== Death ===

{{main|Death}}

[[File:Male Lion and Cub Chitwa South Africa Luca Galuzzi 2004.JPG|right|thumb|Animal corpses, like this [[African buffalo]], are recycled by the [[ecosystem]], providing energy and nutrients for living organisms.]]

Death is the termination of all vital functions or life processes in an organism or cell.<ref>{{cite encyclopedia |title=Definition of death |url=http://encarta.msn.com/dictionary_1861602899/death.html |archive-url=https://web.archive.org/web/20091103065510/http://encarta.msn.com/dictionary_1861602899/death.html |archive-date=3 November 2009 |url-status=dead}}</ref><ref name=define_death>{{cite web |title=Definition of death |website=Encyclopedia of Death and Dying |publisher=Advameg, Inc. |url=http://www.deathreference.com/Da-Em/Definitions-of-Death.html |access-date=25 May 2012 |url-status=dead |archive-url=https://web.archive.org/web/20070203141750/http://www.deathreference.com/Da-Em/Definitions-of-Death.html |archive-date=3 February 2007 }}</ref>
One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment life ends, or when the state that follows life begins.<ref name=define_death/> However, determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems.<ref>{{cite magazine |title=Crossing Over: How Science Is Redefining Life and Death |url=https://www.nationalgeographic.com/magazine/2016/04/dying-death-brain-dead-body-consciousness-science/ |author=Henig, Robin Marantz |author-link=Robin Marantz Henig |magazine=[[National Geographic]] |date=April 2016 |access-date=23 October 2017 |url-status=dead |archive-url=https://web.archive.org/web/20171101071129/https://www.nationalgeographic.com/magazine/2016/04/dying-death-brain-dead-body-consciousness-science/ |archive-date=1 November 2017 }}</ref> Such determination, therefore, requires drawing conceptual lines between life and death. This is problematic because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of [[afterlife]] or [[reincarnation]] for the [[soul]], or [[resurrection]] of the body at a later date.<ref>{{Cite web|title=How the Major Religions View the Afterlife|url=https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/how-major-religions-view-afterlife|access-date=4 February 2022|website=Encyclopedia.com|archive-date=4 February 2022|archive-url=https://web.archive.org/web/20220204201436/https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/how-major-religions-view-afterlife|url-status=live}}</ref>

=== Viruses ===

{{main|Virus}}

[[File:Adenovirus transmission electron micrograph B82-0142 lores.jpg|thumb|right|[[Adenovirus]]es as seen under an electron microscope]]

Whether or not viruses should be considered as alive is controversial.<ref>{{Cite web |title=Virus |url=https://www.genome.gov/genetics-glossary/Virus |access-date=25 July 2022 |website=Genome.gov |archive-date=11 May 2022 |archive-url=https://web.archive.org/web/20220511064713/https://www.genome.gov/genetics-glossary/Virus |url-status=live }}</ref><ref>{{Cite web |title=Are Viruses Alive? |url=https://serc.carleton.edu/microbelife/yellowstone/viruslive.html |access-date=25 July 2022 |website=Yellowstone Thermal Viruses |archive-date=14 June 2022 |archive-url=https://web.archive.org/web/20220614031640/https://serc.carleton.edu/microbelife/yellowstone/viruslive.html |url-status=live }}</ref> They are most often considered as just [[gene coding]] [[DNA replication|replicators]] rather than forms of life.<ref>{{cite journal |title=Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question |journal=Studies in the History and Philosophy of Biology and Biomedical Science |volume=59 |pages=125–134 |date=7 March 2016 |last1=Koonin |first1=E.V. |last2=Starokadomskyy |first2=P. |doi=10.1016/j.shpsc.2016.02.016 |pmid=26965225 |pmc=5406846}}</ref> They have been described as "organisms at the edge of life"<ref>{{Cite journal |last=Rybicki |first=EP |year=1990 |url=https://journals.co.za/doi/pdf/10.10520/AJA00382353_6229 |title=The classification of organisms at the edge of life, or problems with virus systematics |journal=S Afr J Sci |volume=86 |pages=182–186 |access-date=5 November 2023 |archive-date=21 September 2021 |archive-url=https://web.archive.org/web/20210921114412/https://journals.co.za/doi/pdf/10.10520/AJA00382353_6229 |url-status=live }}</ref> because they possess [[gene]]s, evolve by natural selection,<ref name="pmid17914905">{{Cite journal |last1=Holmes |first1=E.C. |title=Viral evolution in the genomic age |journal=PLOS Biol. |volume=5 |issue=10 |page=e278 |date=October 2007 |pmid=17914905 |pmc=1994994 |doi=10.1371/journal.pbio.0050278 |doi-access=free }}</ref><ref name="Forterre 2010">{{cite journal |title=Defining Life: The Virus Viewpoint |journal=Orig Life Evol Biosph |date=3 March 2010 |first=Patrick |last=Forterre |volume=40 |issue=2 |pages=151–160 |doi=10.1007/s11084-010-9194-1 |bibcode=2010OLEB...40..151F |pmc=2837877 |pmid=20198436}}</ref> and replicate by making multiple copies of themselves through self-assembly. However, viruses do not metabolise and they require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the [[origin of life]], as it may support the hypothesis that life could have started as self-assembling [[organic molecules]].<ref name="pmid16984643">{{Cite journal |last1=Koonin |first1=E.V. |author1-link=Eugene Koonin |last2=Senkevich |first2=T.G. |last3=Dolja |first3=V.V. |title=The ancient Virus World and evolution of cells |journal=Biology Direct |volume=1 |page=29 |year=2006 |pmid=16984643 |pmc=1594570 |doi=10.1186/1745-6150-1-29 |doi-access=free }}</ref><ref>{{cite web |url=http://www.mcb.uct.ac.za/tutorial/virorig.html#Virus%20Origins |title=Origins of Viruses |last=Rybicki |first=Ed |date=November 1997 |archive-url=https://web.archive.org/web/20090509094459/http://www.mcb.uct.ac.za/tutorial/virorig.html|archive-date=9 May 2009 |url-status=dead |access-date=12 April 2009}}</ref>

== History of study ==

=== Materialism ===

{{main|Materialism}}

Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. [[Empedocles]] (430 BC) argued that everything in the universe is made up of a combination of [[Classical element|four eternal "elements"]] or "roots of all": earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements. The various forms of life are caused by an appropriate mixture of elements.<ref>{{cite web |first1=Richard |last1=Parry |date=4 March 2005 |title=Empedocles |website=Stanford Encyclopedia of Philosophy |url=http://plato.stanford.edu/entries/empedocles/ |access-date=25 May 2012 |archive-date=13 May 2012 |archive-url=https://web.archive.org/web/20120513201301/http://plato.stanford.edu/entries/empedocles/ |url-status=live }}</ref>
[[Democritus]] (460 BC) was an [[atomist]]; he thought that the essential characteristic of life was having a [[soul]] (''psyche''), and that the soul, like everything else, was composed of fiery atoms. He elaborated on fire because of the apparent connection between life and heat, and because fire moves.<ref name=democritus>{{cite web |first1=Richard |last1=Parry |date=25 August 2010 |title=Democritus |website=Stanford Encyclopedia of Philosophy |url=http://plato.stanford.edu/entries/democritus/#4 |access-date=25 May 2012 |archive-date=30 August 2006 |archive-url=https://web.archive.org/web/20060830030642/http://plato.stanford.edu/entries/democritus/#4 |url-status=live }}</ref>
[[Plato]], in contrast, held that the world was organised by permanent [[Theory of Forms|forms]], reflected imperfectly in matter; forms provided direction or intelligence, explaining the regularities observed in the world.<ref>{{cite book |title=Cause and Explanation in Ancient Greek Thought |last=Hankinson |first=R.J. |publisher=Oxford University Press |date=1997 |isbn=978-0-19-924656-4 |url=https://books.google.com/books?id=iwfy-n5IWL8C |page=125  |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194747/https://books.google.com/books?id=iwfy-n5IWL8C |url-status=live }}</ref> The [[mechanism (philosophy)|mechanistic]] materialism that originated in [[ancient Greece]] was revived and revised by the French philosopher [[René Descartes]] (1596–1650), who held that animals and humans were assemblages of parts that together functioned as a machine. This idea was developed further by [[Julien Offray de La Mettrie]] (1709–1750) in his book ''L'Homme Machine''.<ref>{{cite book |last=de la Mettrie |first=J.J.O. |date=1748 |title=L'Homme Machine |trans-title=Man a machine |publisher=Elie Luzac |place=Leyden }}</ref> In the 19th century the advances in [[cell theory]] in biological science encouraged this view. The [[evolution]]ary theory of [[Charles Darwin]] (1859) is a mechanistic explanation for the origin of species by means of [[natural selection]].<ref>{{cite book |first1=Paul |last1=Thagard |title=The Cognitive Science of Science: Explanation, Discovery, and Conceptual Change |publisher=MIT Press |date=2012 |isbn=978-0-262-01728-2 |pages=204–205 |url=https://books.google.com/books?id=HrJIV19_nZYC&pg=PA204 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194751/https://books.google.com/books?id=HrJIV19_nZYC&pg=PA204 |url-status=live }}</ref> At the beginning of the 20th century [[Stéphane Leduc]] (1853–1939) promoted the idea that biological processes could be understood in terms of physics and chemistry, and that their growth resembled that of inorganic crystals immersed in solutions of sodium silicate. His ideas, set out in his book ''La biologie synthétique'',<ref>{{cite book |last=Leduc |first=Stéphane |author-link=Stéphane Leduc |date=1912 |title=La Biologie Synthétique |trans-title =Synthetic Biology |publisher=Poinat |place =Paris}}</ref> were widely dismissed during his lifetime, but has incurred a resurgence of interest in the work of Russell, Barge and colleagues.<ref>{{cite journal |doi=10.1089/ast.2013.1110 |title=The Drive to Life on Wet and Icy Worlds|year=2014|last1=Russell |first1=Michael J. |last2=Barge |first2=Laura M. |last3=Bhartia |first3=Rohit |last4=Bocanegra |first4=Dylan |last5=Bracher |first5=Paul J. |last6=Branscomb |first6=Elbert |last7=Kidd |first7=Richard |last8=McGlynn |first8=Shawn |last9=Meier |first9=David H. |last10=Nitschke |first10=Wolfgang |last11=Shibuya |first11=Takazo |last12=Vance |first12=Steve |last13=White |first13=Lauren |last14=Kanik |first14=Isik  |journal=Astrobiology |volume=14 |issue=4 |pages=308–343 |pmid=24697642 |pmc=3995032 |bibcode=2014AsBio..14..308R}}</ref>

=== Hylomorphism ===

{{Main|Hylomorphism}}

[[File:Aristotelian Soul.png|thumb|upright=1.5|The [[Soul#Aristotle|structure of the souls]] of plants, animals, and humans, according to [[Aristotle]]]]

Hylomorphism is a theory first expressed by the Greek philosopher [[Aristotle]] (322 BC). The application of hylomorphism to biology was important to Aristotle, and [[Aristotle's biology|biology is extensively covered in his extant writings]]. In this view, everything in the material universe has both matter and form, and the form of a living thing is its [[soul]] (Greek ''psyche'', Latin ''anima''). There are three kinds of souls: the ''vegetative soul'' of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the ''animal soul'', which causes animals to move and feel; and the ''rational soul'', which is the source of consciousness and reasoning, which (Aristotle believed) is found only in man.<ref>{{Cite book |title=On the Soul |last=Aristotle |pages=Book II |no-pp=y |title-link=On the Soul }}</ref> Each higher soul has all of the attributes of the lower ones. Aristotle believed that while matter can exist without form, form cannot exist without matter, and that therefore the soul cannot exist without the body.<ref>{{cite book |first1=Don |last1=Marietta |page=104 |title=Introduction to ancient philosophy |publisher=M.E. Sharpe |date=1998 |isbn=978-0-7656-0216-9 |url=https://books.google.com/books?id=Gz-8PsrT32AC |access-date=25 August 2020 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194754/https://books.google.com/books?id=Gz-8PsrT32AC |url-status=live }}</ref>

This account is consistent with [[Teleology in biology|teleological explanations of life]], which account for phenomena in terms of purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality (from the future to the past) is in contradiction with the scientific evidence for natural selection, which explains the consequence in terms of a prior cause. Biological features are explained not by looking at future optimal results, but by looking at the past [[evolutionary history]] of a species, which led to the natural selection of the features in question.<ref name=stewert_williams2010>{{Cite book |first1=Steve |last1=Stewart-Williams |date=2010 |title=Darwin, God and the meaning of life: how evolutionary theory undermines everything you thought you knew of life |publisher=Cambridge University Press |isbn=978-0-521-76278-6 |pages=193–194 |url=https://books.google.com/books?id=KBp69los_-oC&pg=PA193 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194752/https://books.google.com/books?id=KBp69los_-oC&pg=PA193 |url-status=live }}</ref>

=== Spontaneous generation ===

{{Main|Spontaneous generation}}

Spontaneous generation was the belief that living organisms can form without descent from similar organisms. Typically, the idea was that certain forms such as fleas could arise from inanimate matter such as dust or the supposed seasonal generation of mice and insects from mud or garbage.<ref>{{Cite book |title=Origines Sacrae |last=Stillingfleet |first=Edward |publisher=Cambridge University Press |year=1697 }}</ref>

The theory of spontaneous generation was proposed by [[Aristotle]],<ref>{{cite book |author=André Brack |editor=André Brack |title=The Molecular Origins of Life |access-date=7 January 2009 |year=1998 |publisher=Cambridge University Press |isbn=978-0-521-56475-5 |page=[https://archive.org/details/molecularorigins0000brac/page/1 1] |chapter=Introduction |chapter-url=http://assets.cambridge.org/97805215/64755/excerpt/9780521564755_excerpt.pdf |url=https://archive.org/details/molecularorigins0000brac/page/1 }}</ref> who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it was considered the best explanation for two millennia. It was decisively dispelled by the experiments of [[Louis Pasteur]] in 1859, who expanded upon the investigations of predecessors such as [[Francesco Redi]].<ref>{{cite web |last1=Levine |first1=Russell |last2=Evers |first2=Chris |title=The Slow Death of Spontaneous Generation (1668–1859) |url=http://www.ncsu.edu/project/bio183de/Black/cellintro/cellintro_reading/Spontaneous_Generation.html |website=North Carolina State University |publisher=National Health Museum |url-status=dead |archive-url=https://web.archive.org/web/20151009044415/http://www.ncsu.edu/project/bio183de/Black/cellintro/cellintro_reading/Spontaneous_Generation.html |archive-date=9 October 2015 |access-date=6 February 2016 }}</ref><ref>{{Cite book |title=Fragments of Science |last=Tyndall |first=John |publisher=P.F. Collier |year=1905 |volume=2 |location=New York |pages=Chapters IV, XII, and XIII |no-pp=y }}</ref> Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.<ref name="Bernal 1967">{{cite book |last=Bernal |first=J.D. |year=1967 |orig-year=Reprinted work by [[Alexander Oparin|A.I. Oparin]] originally published 1924; Moscow: [[Publishing houses in the Soviet Union|The Moscow Worker]] |title=The Origin of Life |url=https://archive.org/details/originoflife0000bern |url-access=registration |series=The Weidenfeld and Nicolson Natural History |others=Translation of Oparin by Ann Synge |location=London |publisher=[[Weidenfeld & Nicolson]] |lccn=67098482}}</ref><ref>{{Cite book |title=Origins of Life: On Earth and in the Cosmos |last=Zubay |first=Geoffrey |publisher=Academic Press |year=2000 |isbn=978-0-12-781910-5 |edition=2nd }}</ref><ref name= "Szathmary">{{cite book |author1=Smith, John Maynard |author2=Szathmary, Eors |title=The Major Transitions in Evolution |publisher=Oxford University Press |location=Oxford Oxfordshire |year=1997 |isbn=978-0-19-850294-4}}</ref>

=== Vitalism ===

{{Main|Vitalism}}

Vitalism is the belief that there is a non-material life-principle. This originated with [[Georg Ernst Stahl]] (17th century), and remained popular until the middle of the 19th century. It appealed to philosophers such as [[Henri Bergson]], [[Friedrich Nietzsche]], and [[Wilhelm Dilthey]],<ref>{{cite book |first1=Sanford |last1=Schwartz |title=C.S. Lewis on the Final Frontier: Science and the Supernatural in the Space Trilogy |publisher=Oxford University Press |date=2009 |isbn=978-0-19-988839-9 |page=56 |url=https://books.google.com/books?id=4hQLdPtJe9EC&pg=PA56 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194800/https://books.google.com/books?id=4hQLdPtJe9EC&pg=PA56 |url-status=live }}</ref> anatomists like [[Xavier Bichat]], and chemists like [[Justus von Liebig]].<ref name=Wilkinson>{{cite journal |first1=Ian |last1=Wilkinson |title=History of Clinical Chemistry&nbsp;– Wöhler & the Birth of Clinical Chemistry |journal=The Journal of the International Federation of Clinical Chemistry and Laboratory Medicine |volume=13 |issue=4 |year=1998 |url=http://www.ifcc.org/ifccfiles/docs/130304003.pdf |access-date=27 December 2015 |url-status=dead |archive-url=https://web.archive.org/web/20160105031229/http://www.ifcc.org/ifccfiles/docs/130304003.pdf |archive-date=5 January 2016 }}</ref> Vitalism included the idea that there was a fundamental difference between organic and inorganic material, and the belief that [[organic material]] can only be derived from living things. This was disproved in 1828, when [[Friedrich Wöhler]] prepared [[urea]] from inorganic materials.<ref>{{cite journal |title=Ueber künstliche Bildung des Harnstoffs |author=Friedrich Wöhler |journal=[[Annalen der Physik und Chemie]] |volume=88 |issue=2 |pages=253–256 |year=1828 |doi=10.1002/andp.18280880206 |url=http://gallica.bnf.fr/ark:/12148/bpt6k15097k/f261.chemindefer |bibcode=1828AnP....88..253W |url-status=live |archive-url=https://web.archive.org/web/20120110094705/http://gallica.bnf.fr/ark:/12148/bpt6k15097k/f261.chemindefer |archive-date=10 January 2012 |author-link=Friedrich Wöhler }}</ref> This [[Wöhler synthesis]] is considered the starting point of modern [[organic chemistry]]. It is of historical significance because for the first time an [[organic compound]] was produced in [[inorganic]] reactions.<ref name=Wilkinson/>

During the 1850s [[Hermann von Helmholtz]], anticipated by [[Julius Robert von Mayer]], demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle.<ref>{{cite book |first1=Anson |last1=Rabinbach |title=The Human Motor: Energy, Fatigue, and the Origins of Modernity |publisher=University of California Press |date=1992 |isbn=978-0-520-07827-7 |pages=124–125 |url=https://books.google.com/books?id=e5ZBNv-zTlQC&pg=PA124 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194755/https://books.google.com/books?id=e5ZBNv-zTlQC&pg=PA124 |url-status=live }}</ref> These results led to the abandonment of scientific interest in vitalistic theories, especially after [[Eduard Buchner]]'s demonstration that alcoholic fermentation could occur in cell-free extracts of yeast.<ref>{{cite book | isbn= 978-8437-033280 | title= New Beer in an Old Bottle. Eduard Buchner and the Growth of Biochemical Knowledge | editor= Cornish-Bowden Athel | year=1997 | publisher = Universitat de València | place=Valencia, Spain}}</ref> Nonetheless, belief still exists in [[pseudoscientific]] theories such as [[homoeopathy]], which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.<ref>{{cite web |url=http://www.ncahf.org/pp/homeop.html |title=NCAHF Position Paper on Homeopathy |date=February 1994 |publisher=National Council Against Health Fraud |access-date=12 June 2012 |archive-date=25 December 2018 |archive-url=https://web.archive.org/web/20181225185228/https://www.ncahf.org/pp/homeop.html |url-status=live }}</ref>

== Development ==

{{align|right|{{Life timeline}}}}

=== Origin of life ===

{{Main|Abiogenesis}}

The [[age of Earth]] is about 4.54 [[Bya|billion years]].<ref>{{cite journal |last=Dalrymple |first=G. Brent |title=The age of the Earth in the twentieth century: a problem (mostly) solved |journal=Special Publications, Geological Society of London |year=2001 |volume=190 |issue=1 |pages=205–221 |doi=10.1144/GSL.SP.2001.190.01.14 |bibcode=2001GSLSP.190..205D|s2cid=130092094 }}</ref> Life on Earth has existed for at least 3.5 billion years,<ref name="PNAS-20151014-pdf">{{cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnike |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |date=19 October 2015 |title=Potentially biogenic carbon preserved in a 4.1&nbsp;billion-year-old zircon |url=http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf |journal=PNAS |doi=10.1073/pnas.1517557112 |pmid=26483481 |pmc=4664351 |volume=112 |issue=47 |pages=14518–14521 |bibcode=2015PNAS..11214518B |url-status=live |archive-url=https://web.archive.org/web/20151106021508/http://www.pnas.org/content/early/2015/10/14/1517557112.full.pdf |archive-date=6 November 2015 |doi-access=free }}</ref><ref>{{Cite journal |title=Fossil evidence of Archaean life |journal=Philos. Trans. R. Soc. Lond. B Biol. Sci. |volume=361 |issue=1470 |pmc=1578735|doi=10.1098/rstb.2006.1834 |pmid=16754604 |date=June 2006 |pages=869–885 | last1 = Schopf | first1 = J.W.}}</ref><ref name="RavenJohnson2002">{{cite book |first1=Peter |last1=Hamilton Raven |first2=George |last2=Brooks Johnson |title=Biology |url=https://archive.org/details/biologyrave00rave |url-access=registration |date=2002 |publisher=McGraw-Hill Education |isbn=978-0-07-112261-0 |page=[https://archive.org/details/biologyrave00rave/page/68 68] |access-date=7 July 2013 }}</ref><ref>{{cite book |first1=Clare |last1=Milsom |first2=Sue |last2=Rigby |author2-link=Sue Rigby |title=Fossils at a Glance |edition=2nd |publisher=John Wiley & Sons |date=2009 |isbn=978-1-4051-9336-8 |page=134 |url=https://books.google.com/books?id=OdrCdxr7QdgC&pg=PA134 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194758/https://books.google.com/books?id=OdrCdxr7QdgC&pg=PA134 |url-status=live }}</ref> with the oldest physical [[Trace fossil|traces]] of life dating back 3.7&nbsp;billion years.<ref name="NG-20131208">{{cite journal |first1=Yoko |last1=Ohtomo |first2=Takeshi |last2=Kakegawa |first3=Akizumi |last3=Ishida |first4=Toshiro |last4=Nagase |first5=Minik T. |last5=Rosing |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Nature Geoscience]] |doi=10.1038/ngeo2025 |date=8 December 2013 |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O}}</ref><ref name="AST-20131108">{{cite journal |last1=Noffke |first1=Nora |author-link=Nora Noffke |last2=Christian |first2=Daniel |last3=Wacey |first3=David |last4=Hazen |first4=Robert M. |title=Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia |date=8 November 2013 |journal=[[Astrobiology (journal)|Astrobiology]] |volume=13 |issue=12 |pages=1103–1124 |doi=10.1089/ast.2013.1030 |bibcode=2013AsBio..13.1103N |pmid=24205812 |pmc=3870916}}</ref> Estimates from molecular clocks, as summarised in the [[TimeTree]] public database, place the origin of life around 4.0 billion years ago.<ref>{{cite book |last=Hedges |first=S. B. Hedges |chapter=Life |pages=89–98 |title=The Timetree of Life |editor1=S. B. Hedges |editor2=S. Kumar |publisher=Oxford University Press |year=2009 |isbn=978-0-1995-3503-3}}</ref> Hypotheses on the origin of life attempt to explain the formation of a [[universal common ancestor]] from simple [[organic molecule]]s via pre-cellular life to [[protocell]]s and metabolism.<ref>{{cite web |url=http://phoenix.lpl.arizona.edu/mars145.php |title=Habitability and Biology: What are the Properties of Life? |access-date=6 June 2013 |website=Phoenix Mars Mission |publisher=The University of Arizona |url-status=live |archive-url=https://web.archive.org/web/20140417155949/http://phoenix.lpl.arizona.edu/mars145.php |archive-date=17 April 2014 }}</ref> In 2016, a set of 355 [[gene]]s from the [[last universal common ancestor]] was tentatively identified.<ref name="NYT-20160725">{{cite news |last=Wade |first=Nicholas |author-link=Nicholas Wade |title=Meet Luca, the Ancestor of All Living Things |url=https://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html |date=25 July 2016 |work=[[The New York Times]] |access-date=25 July 2016 |url-status=live |archive-url=https://web.archive.org/web/20160728053822/http://www.nytimes.com/2016/07/26/science/last-universal-ancestor.html |archive-date=28 July 2016 }}</ref>

The biosphere is postulated to have developed, from the origin of life onwards, at least some 3.5&nbsp;billion years ago.<ref name="Campbell 2006">{{cite book |last=Campbell |first=Neil A. |author2=Brad Williamson |author3=Robin J. Heyden |title=Biology: Exploring Life |publisher=Pearson Prentice Hall |year=2006 |location=Boston, Massachusetts |url=http://www.phschool.com/el_marketing.html |isbn=978-0-13-250882-7 |url-status=dead |archive-url=https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html |archive-date=2 November 2014 |access-date=15 June 2016 }}</ref> The earliest evidence for life on Earth includes [[biogenic]] [[graphite]] found in 3.7&nbsp;billion-year-old [[Metasediment|metasedimentary rocks]] from [[Western Greenland]]<ref name="NG-20131208"/> and [[microbial mat]] [[fossils]] found in 3.48&nbsp;billion-year-old [[sandstone]] from [[Western Australia]].<ref name="AST-20131108"/> More recently, in 2015, "remains of [[biotic life]]" were found in 4.1&nbsp;billion-year-old rocks in Western Australia.<ref name="PNAS-20151014-pdf"/> In 2017, putative fossilised [[microorganism]]s (or [[Micropaleontology#Microfossils|microfossils]]) were announced to have been discovered in [[hydrothermal vent|hydrothermal vent precipitates]] in the [[Nuvvuagittuq Belt]] of Quebec, Canada that were as old as 4.28&nbsp;billion years, the oldest record of life on Earth, suggesting "an almost instantaneous emergence of life" after [[Origin of water on Earth#History of water on Earth|ocean formation 4.4&nbsp;billion years ago]], and not long after the [[Age of the Earth|formation of the Earth]] 4.54&nbsp;billion years ago.<ref name="NAT-20170301">{{cite journal |last1=Dodd |first1=Matthew S. |last2=Papineau |first2=Dominic |last3=Grenne |first3=Tor |last4=Slack |first4=John F. |last5=Rittner |first5=Martin |last6=Pirajno |first6=Franco |last7=O'Neil |first7=Jonathan |last8=Little |first8=Crispin T.S.  |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates |url=http://eprints.whiterose.ac.uk/112179/ |journal=[[Nature (journal)|Nature]] |date=1 March 2017 |volume=543 |issue=7643 |pages=60–64 |doi=10.1038/nature21377 |pmid=28252057 |access-date=2 March 2017 |bibcode=2017Natur.543...60D |url-status=live |archive-url=https://web.archive.org/web/20170908201821/http://eprints.whiterose.ac.uk/112179/ |archive-date=8 September 2017 |doi-access=free }}</ref>

=== Evolution ===

{{main|Evolution}}

[[Evolution]] is the change in [[heritable]] [[Phenotypic trait|characteristics]] of biological populations over successive generations. It results in the appearance of new species and often the disappearance of old ones.<ref>{{cite book |last1=Hall |first1=Brian K. |author-link1=Brian K. Hall |last2=Hallgrímsson |first2=Benedikt |title=Strickberger's Evolution |url=https://archive.org/details/strickbergersevo0000hall |url-access=registration |year=2008 |edition=4th |location=Sudbury, Massachusetts |publisher=Jones and Bartlett Publishers |isbn=978-0-7637-0066-9 |lccn=2007008981 |oclc=85814089 |pages=[https://books.google.com/books?id=jrDD3cyA09kC&pg=PA4 4–6]}}</ref><ref>{{cite web |title=Evolution Resources |location=Washington, DC |publisher=[[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=3 June 2016}}</ref> Evolution occurs when evolutionary processes such as [[natural selection]] (including [[sexual selection]]) and [[genetic drift]] act on genetic variation, resulting in certain characteristics increasing or decreasing in frequency within a population over successive generations.<ref name="Scott-Phillips">{{cite journal |last1=Scott-Phillips |first1=Thomas C. |last2=Laland |first2=Kevin N. |author2-link=Kevin Laland |last3=Shuker |first3=David M. |last4=Dickins |first4=Thomas E. |last5=West |first5=Stuart A. |author-link5=Stuart West |date=May 2014 |title=The Niche Construction Perspective: A Critical Appraisal |journal=[[Evolution (journal)|Evolution]] |volume=68 |issue=5 |pages=1231–1243 |doi=10.1111/evo.12332 |pmid=24325256 |pmc=4261998 |quote=Evolutionary processes are generally thought of as processes by which these changes occur. Four such processes are widely recognized: natural selection (in the broad sense, to include sexual selection), genetic drift, mutation, and migration (Fisher 1930; Haldane 1932). The latter two generate variation; the first two sort it.}}</ref> The process of evolution has given rise to [[biodiversity]] at every level of [[biological organisation]].<ref>{{harvnb|Hall|Hallgrímsson|2008|pp=3–5}}</ref><ref name="Voet2016a">{{cite book |last1=Voet |first1=Donald |author-link1=Donald Voet |last2=Voet |first2=Judith G. |author-link2=Judith G. Voet|last3=Pratt |first3=Charlotte W. |author-link3=Charlotte W. Pratt|year=2016 |title=Fundamentals of Biochemistry: Life at the Molecular Level |edition=Fifth |location=[[Hoboken, New Jersey]] |publisher=[[John Wiley & Sons]] |isbn=978-1-118-91840-1 |lccn=2016002847 |oclc=939245154 |at=Chapter 1: Introduction to the Chemistry of Life, pp. 1–22}}</ref>

=== Fossils ===

{{main|Fossils}}

Fossils are the preserved remains or [[trace fossil|traces]] of organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in layers ([[strata]]) of [[sedimentary rock]] is known as the ''fossil record''. A preserved specimen is called a fossil if it is older than the arbitrary date of 10,000 years ago.<ref>{{cite web|url=http://www.sdnhm.org/science/paleontology/resources/frequent/|title=Frequently Asked Questions|publisher=San Diego Natural History Museum|access-date=25 May 2012|url-status=dead|archive-url=https://web.archive.org/web/20120510101706/http://sdnhm.org/science/paleontology/resources/frequent/|archive-date=10 May 2012}}</ref> Hence, fossils range in age from the youngest at the start of the [[Holocene]] Epoch to the oldest from the [[Archean|Archaean]] Eon, up to 3.4 [[1000000000 (number)|billion]] years old.<ref>{{cite news |first1=Brian |last1=Vastag |title=Oldest 'microfossils' raise hopes for life on Mars |date=21 August 2011 |url=https://www.washingtonpost.com/national/health-science/oldest-microfossils-hail-from-34-billion-years-ago-raise-hopes-for-life-on-mars/2011/08/19/gIQAHK8UUJ_story.html?hpid=z3 |newspaper=The Washington Post |access-date=21 August 2011 |url-status=live |archive-url=https://web.archive.org/web/20111019000458/http://www.washingtonpost.com/national/health-science/oldest-microfossils-hail-from-34-billion-years-ago-raise-hopes-for-life-on-mars/2011/08/19/gIQAHK8UUJ_story.html?hpid=z3 |archive-date=19 October 2011 }}</ref><ref>{{cite news |first=Nicholas |last=Wade |title=Geological Team Lays Claim to Oldest Known Fossils |date=21 August 2011 |url=https://www.nytimes.com/2011/08/22/science/earth/22fossil.html?_r=1&partner=rss&emc=rss&src=ig |work=The New York Times |access-date=21 August 2011 |url-status=live |archive-url=https://web.archive.org/web/20130501085118/http://www.nytimes.com/2011/08/22/science/earth/22fossil.html?_r=1&partner=rss&emc=rss&src=ig |archive-date=1 May 2013 }}</ref>

=== Extinction ===

{{Main|Extinction}}

Extinction is the process by which a [[species]] dies out.<ref>{{cite encyclopedia |title=Extinction&nbsp;– definition |url=http://encarta.msn.com/dictionary_1861609974/extinction.html |archive-url=https://web.archive.org/web/20090926011523/http://encarta.msn.com/dictionary_1861609974/extinction.html |archive-date=26 September 2009 |url-status=dead}}</ref> The moment of extinction is the death of the last individual of that species. Because a species' potential [[range (biology)|range]] may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. Species become extinct when they are no longer able to survive in changing [[habitat]] or against superior competition. Over 99% of all the species that have ever lived are now extinct.<ref>{{cite web |url=http://palaeo.gly.bris.ac.uk/Palaeofiles/Triassic/extinction.htm |title=What is an extinction? |website=Late Triassic |publisher=Bristol University |access-date=27 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120901011807/http://palaeo.gly.bris.ac.uk/palaeofiles/triassic/extinction.htm |archive-date=1 September 2012 }}</ref><ref name="Book-Biology">{{cite book |editor1=Kunin, W.E. |editor2=Gaston, Kevin |author=McKinney, Michael L. |chapter=How do rare species avoid extinction? A paleontological view |title=The Biology of Rarity: Causes and consequences of rare—common differences |chapter-url=https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110 |date=1996 |publisher=Springer |isbn=978-0-412-63380-5 |access-date=26 May 2015 |archive-date=3 February 2023 |archive-url=https://web.archive.org/web/20230203051637/https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110 |url-status=live }}</ref><ref name="StearnsStearns2000">{{cite book |last1=Stearns |first1=Beverly Peterson |last2=Stearns |first2=Stephen C. |title=Watching, from the Edge of Extinction |url=https://books.google.com/books?id=0BHeC-tXIB4C&q=99+percent |year=2000 |publisher=[[Yale University Press]] |isbn=978-0-300-08469-6 |page=x |access-date=30 May 2017 |archive-date=5 November 2023 |archive-url=https://web.archive.org/web/20231105190204/https://books.google.com/books?id=0BHeC-tXIB4C&q=99+percent#v=snippet&q=99%20percent&f=false |url-status=live }}</ref><ref name="NYT-20141108-MJN">{{cite news |last=Novacek |first=Michael J. |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |date=8 November 2014 |work=[[The New York Times]] |access-date=25 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20141229225657/http://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |archive-date=29 December 2014 }}</ref> [[Mass extinction]]s may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<ref>{{Cite journal |last=Van Valkenburgh |first=B. |author-link=Blaire Van Valkenburgh |year=1999 |title=Major patterns in the history of carnivorous mammals |journal=Annual Review of Earth and Planetary Sciences |volume=27 |pages=463–493 |doi=10.1146/annurev.earth.27.1.463 |bibcode=1999AREPS..27..463V |url=https://zenodo.org/record/890156 |access-date=29 June 2019 |archive-date=29 February 2020 |archive-url=https://web.archive.org/web/20200229201201/https://zenodo.org/record/890156 |url-status=live }}</ref>

== Environmental conditions ==

[[File:20100422 235222 Cyanobacteria.jpg|thumb|upright=0.9|[[Cyanobacteria]] [[oxygen catastrophe|dramatically changed]] the composition of life forms on Earth by leading to the near-extinction of [[Anaerobic organism|oxygen-intolerant organisms]].]]

The diversity of life on Earth is a result of the dynamic interplay between [[genetic opportunity]], metabolic capability, [[environment (biophysical)|environmental]] challenges,<ref name=astrobiology>{{cite web |url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-url=https://web.archive.org/web/20120329092237/http://astrobiology.arc.nasa.gov/roadmap/g5.html |archive-date=29 March 2012 |url-status=dead |title=Understand the evolutionary mechanisms and environmental limits of life |access-date=13 July 2009 |last=Rothschild |first=Lynn |author-link=Lynn J. Rothschild |date=September 2003 |publisher=NASA}}</ref> and [[symbiosis]].<ref>{{Cite journal |title=Symbiosis and the origin of life |journal=Origins of Life and Evolution of Biospheres |date=April 1977 |first=G.A.M. |last=King |volume=8 |issue=1 |pages=39–53 |doi=10.1007/BF00930938 |pmid=896191 |bibcode=1977OrLi....8...39K|s2cid=23615028 }}</ref><ref>{{Cite book |last=Margulis |first=Lynn |author-link=Lynn Margulis |title=The Symbiotic Planet: A New Look at Evolution |publisher=Orion Books |date=2001 |location=London|isbn=978-0-7538-0785-9}}</ref><ref>{{Cite book |last1=Futuyma |first1=D.J. |author1-link=Douglas J. Futuyma |author2=Janis Antonovics |title=Oxford surveys in evolutionary biology: Symbiosis in evolution |publisher=Oxford University Press |date=1992 |volume=8 |location=London, England |pages=347–374 |isbn=978-0-19-507623-3}}</ref> For most of its existence, Earth's habitable environment has been dominated by [[microorganism]]s and subjected to their metabolism and evolution. As a consequence of these microbial activities, the physical-chemical environment on Earth has been changing on a [[geologic time scale]], thereby affecting the path of evolution of subsequent life.<ref name=astrobiology/> For example, the release of molecular [[oxygen]] by [[cyanobacteria]] as a by-product of [[photosynthesis]] induced global changes in the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this posed novel evolutionary challenges, and ultimately resulted in the formation of Earth's major animal and plant species. This interplay between organisms and their environment is an inherent feature of living systems.<ref name=astrobiology/>

=== Biosphere ===

{{main|Biosphere}}

[[File:Deinococcus geothermalis cells.jpg|thumb|''[[Deinococcus geothermalis]]'', a bacterium that thrives in [[geothermal springs]] and deep ocean subsurfaces<ref>{{Cite journal |last1=Liedert |first1=Christina |last2=Peltola |first2=Minna |last3=Bernhardt |first3=Jörg |last4=Neubauer |first4=Peter |last5=Salkinoja-Salonen |first5=Mirja |date=15 March 2012 |title=Physiology of Resistant Deinococcus geothermalis Bacterium Aerobically Cultivated in Low-Manganese Medium |journal=Journal of Bacteriology |language=en |volume=194 |issue=6 |pages=1552–1561 |doi=10.1128/JB.06429-11 |pmc=3294853 |pmid=22228732}}</ref>|left]]
The [[biosphere]] is the global sum of all ecosystems. It can also be termed as the zone of life on Earth, a closed system (apart from solar and cosmic radiation and heat from the interior of the Earth), and largely self-regulating.<ref>{{Cite encyclopedia |encyclopedia=The Columbia Encyclopedia|edition=6th |publisher=Columbia University Press |year=2004 |url=https://www.questia.com/library/encyclopedia/biosphere.jsp |url-status= |archive-url=https://web.archive.org/web/20111027194858/http://www.questia.com/library/encyclopedia/biosphere.jsp |archive-date=27 October 2011 |title=Biosphere }}</ref> Organisms exist<!--not necessarily metabolising--> in every part of the biosphere, including [[soil]], [[hot spring]]s, [[endolith|inside rocks]] at least {{convert|12|mi|km|order=flip|abbr=on}} deep underground, the deepest parts of the ocean, and at least {{convert|40|mi|km|order=flip|abbr=on}} high in the atmosphere.<ref name="SD-19980625-UG">{{cite web |author=University of Georgia |title=First-Ever Scientific Estimate Of Total Bacteria On Earth Shows Far Greater Numbers Than Ever Known Before |url=https://www.sciencedaily.com/releases/1998/08/980825080732.htm |date=25 August 1998 |website=[[Science Daily]] |access-date=10 November 2014 |url-status=live |archive-url=https://web.archive.org/web/20141110162101/https://www.sciencedaily.com/releases/1998/08/980825080732.htm |archive-date=10 November 2014 }}</ref><ref name="ABM-20150112">{{cite web |last=Hadhazy |first=Adam |title=Life Might Thrive a Dozen Miles Beneath Earth's Surface |url=http://www.astrobio.net/extreme-life/life-might-thrive-dozen-miles-beneath-earths-surface/ |date=12 January 2015 |website=[[Astrobiology Magazine]] |access-date=11 March 2017 |url-status=dead |archive-url=https://web.archive.org/web/20170312065614/http://www.astrobio.net/extreme-life/life-might-thrive-dozen-miles-beneath-earths-surface/ |archive-date=12 March 2017 }}</ref><ref name="BBC-20151124">{{cite web |last=Fox-Skelly |first=Jasmin |title=The Strange Beasts That Live in Solid Rock Deep Underground |url=http://www.bbc.com/earth/story/20151124-meet-the-strange-creatures-that-live-in-solid-rock-deep-underground |date=24 November 2015 |publisher=[[BBC]] |access-date=11 March 2017 |url-status=live |archive-url=https://web.archive.org/web/20161125013248/http://www.bbc.com/earth/story/20151124-meet-the-strange-creatures-that-live-in-solid-rock-deep-underground |archive-date=25 November 2016 }}</ref> For example, spores of ''[[Aspergillus niger]]'' have been detected in the [[mesosphere]] at an altitude of 48 to 77 km.<ref>{{Cite journal |last1=Imshenetsky |first1=AA |last2=Lysenko |first2=SV |last3=Kazakov |first3=GA |date=June 1978 |title=Upper boundary of the biosphere |journal=Applied and Environmental Microbiology |volume=35 |issue=1 |pages=1–5 |doi=10.1128/aem.35.1.1-5.1978 |pmc=242768 |pmid=623455|bibcode=1978ApEnM..35....1I }}</ref> Under test conditions, life forms have been observed to survive in the vacuum of space.<ref name="Dose">{{cite journal |title=ERA-experiment "space biochemistry" |journal=Advances in Space Research |volume=16 |issue=8 |year=1995 |pages=119–129 |doi=10.1016/0273-1177(95)00280-R |pmid=11542696 |last1=Dose |first1=K. |last2=Bieger-Dose |first2=A. |last3=Dillmann |first3=R. |last4=Gill |first4=M. |last5=Kerz |first5=O. |last6=Klein |first6=A. |last7=Meinert |first7=H. |last8=Nawroth |first8=T. |last9=Risi |first9=S. | last10=Stridde | first10=C. |bibcode=1995AdSpR..16h.119D}}</ref><ref name="Horneck">{{cite journal |title=Biological responses to space: results of the experiment "Exobiological Unit" of ERA on EURECA I |journal=Adv. Space Res. |year=1995 |author1=Horneck G. |author2=Eschweiler, U. |author3=Reitz, G. |author4=Wehner, J. |author5=Willimek, R. |author6=Strauch, K. |volume=16 |issue=8 |pages=105–118 |pmid=11542695 |bibcode=1995AdSpR..16h.105H |doi=10.1016/0273-1177(95)00279-N}}</ref> Life forms thrive in the deep [[Mariana Trench]],<ref name="NG-20130317">{{cite journal |last1=Glud |first1=Ronnie |last2=Wenzhöfer |first2=Frank |last3=Middelboe |first3=Mathias |last4=Oguri |first4=Kazumasa |last5=Turnewitsch |first5=Robert |last6=Canfield |first6=Donald E. |last7=Kitazato |first7=Hiroshi |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth |doi=10.1038/ngeo1773 |date=17 March 2013 |journal=[[Nature Geoscience]] |volume=6 |issue=4 |pages=284–288 |bibcode=2013NatGe...6..284G}}</ref> and inside rocks up to {{convert|580|m|ft mi|abbr=on}} below the sea floor under {{convert|2590|m|ft mi|abbr=on}} of ocean off the coast of the northwestern United States,<ref name="LS-20130317">{{cite web |last=Choi |first=Charles Q. |title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |date=17 March 2013 |publisher=[[LiveScience]] |access-date=17 March 2013 |url-status=live |archive-url=https://web.archive.org/web/20130402234623/http://www.livescience.com/27954-microbes-mariana-trench.html |archive-date=2 April 2013 }}</ref><ref name="LS-20130314">{{cite web |last=Oskin |first=Becky |title=Intraterrestrials: Life Thrives in Ocean Floor |url=http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |date=14 March 2013 |publisher=[[LiveScience]] |access-date=17 March 2013 |url-status=live |archive-url=https://web.archive.org/web/20130402235647/http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |archive-date=2 April 2013 }}</ref> and {{convert|2400|m|ft mi|abbr=on}} beneath the seabed off Japan.<ref name="BBC-20141215-RM">{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |title=Microbes discovered by deepest marine drill analysed |url=https://www.bbc.com/news/science-environment-30489814 |date=15 December 2014 |work=[[BBC News]] |access-date=15 December 2014 |url-status=live |archive-url=https://web.archive.org/web/20141216185424/http://www.bbc.com/news/science-environment-30489814 |archive-date=16 December 2014 }}</ref> In 2014, life forms were found living {{convert|800|m|ft mi|abbr=on}} below the ice of Antarctica.<ref name="NAT-20140820">{{cite journal |last=Fox |first=Douglas |title=Lakes under the ice: Antarctica's secret garden |date=20 August 2014 |journal=[[Nature (journal)|Nature]] |volume=512 |issue=7514 |pages=244–246 |doi=10.1038/512244a |bibcode=2014Natur.512..244F |pmid=25143097 |doi-access=free }}</ref><ref name="FRB-20140820">{{cite web |last=Mack |first=Eric |title=Life Confirmed Under Antarctic Ice; Is Space Next? |url=https://www.forbes.com/sites/ericmack/2014/08/20/life-confirmed-under-antarctic-ice-is-space-next/ |date=20 August 2014 |website=[[Forbes]] |access-date=21 August 2014 |url-status=live |archive-url=https://web.archive.org/web/20140822002442/http://www.forbes.com/sites/ericmack/2014/08/20/life-confirmed-under-antarctic-ice-is-space-next/ |archive-date=22 August 2014 }}</ref> Expeditions of the [[International Ocean Discovery Program]] found [[unicellular]] life in 120&nbsp;°C sediment 1.2&nbsp;km below seafloor in the [[Nankai Trough]] [[subduction]] zone.<ref>{{Cite journal |last1=Heuer |first1=Verena B. |last2=Inagaki |first2=Fumio |last3=Morono |first3=Yuki |last4=Kubo |first4=Yusuke |last5=Spivack |first5=Arthur J. |last6=Viehweger |first6=Bernhard |last7=Treude |first7=Tina |last8=Beulig |first8=Felix |last9=Schubotz |first9=Florence |last10=Tonai |first10=Satoshi |last11=Bowden |first11=Stephen A.|date=4 December 2020 |title=Temperature limits to deep subseafloor life in the Nankai Trough subduction zone |journal=Science |volume=370 |issue=6521 |pages=1230–1234 |doi=10.1126/science.abd7934 |pmid=33273103 |bibcode=2020Sci...370.1230H |hdl=2164/15700 |s2cid=227257205 |url=https://escholarship.org/uc/item/5b65v425 |hdl-access=free |access-date=5 November 2023 |archive-date=26 September 2022 |archive-url=https://web.archive.org/web/20220926003958/https://escholarship.org/uc/item/5b65v425 |url-status=live }}</ref> According to one researcher, "You can find [[microbe]]s everywhere—they're extremely adaptable to conditions, and survive wherever they are."<ref name="LS-20130317" />{{-}}

=== Range of tolerance ===

The inert components of an ecosystem are the physical and chemical factors necessary for life—energy (sunlight or [[biochemistry|chemical energy]]), water, heat, [[Earth's atmosphere|atmosphere]], [[gravitational biology|gravity]], [[nutrient]]s, and [[ultraviolet]] [[ozone layer|solar radiation protection]].<ref>{{cite web |url=http://cmapsnasacmex.ihmc.us/servlet/SBReadResourceServlet?rid=1025200161109_2045745605_1714&partName=htmltext |title=Essential requirements for life |access-date=14 July 2009 |publisher=CMEX-NASA |url-status=dead |archive-url=https://web.archive.org/web/20090817100436/http://cmapsnasacmex.ihmc.us/servlet/SBReadResourceServlet?rid=1025200161109_2045745605_1714&partName=htmltext |archive-date=17 August 2009 }}</ref> In most ecosystems, the conditions vary during the day and from one season to the next. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called the "range of tolerance".<ref name=tolerance>{{Cite book |last=Chiras |first=Daniel C. |edition=6th |title=Environmental Science&nbsp;– Creating a Sustainable Future |date=2001 |isbn=978-0-7637-1316-4 |url=https://archive.org/details/environmentalsci0000chir |publisher=Sudbury, MA : Jones and Bartlett }}</ref> Outside that are the "zones of physiological stress", where the survival and reproduction are possible but not optimal. Beyond these zones are the "zones of intolerance", where survival and reproduction of that organism is unlikely or impossible. Organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.<ref name=tolerance/>

=== Extremophiles ===

{{further|Extremophile}}

[[File:Deinococcus radiodurans.jpg|thumb|upright|''[[Deinococcus radiodurans]]'' is an [[extremophile]] that can resist extremes of cold, dehydration, vacuum, acid, and radiation exposure.]]

To survive, some microorganisms have evolved to withstand [[psychrophile|freezing]], [[xerophile|complete desiccation]], [[oligotroph|starvation]], high levels of [[radioresistance|radiation exposure]], and other physical or chemical challenges. These [[extremophile]] microorganisms may survive exposure to such conditions for long periods.<ref name=astrobiology/><ref name="NYT-20160912">{{cite news |last=Chang |first=Kenneth |title=Visions of Life on Mars in Earth's Depths |url=https://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |date=12 September 2016 |work=[[The New York Times]] |access-date=12 September 2016 |url-status=live |archive-url=https://web.archive.org/web/20160912225220/http://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |archive-date=12 September 2016 }}</ref> They excel at exploiting uncommon sources of energy. Characterization of the [[morphology (biology)|structure]] and metabolic diversity of microbial communities in such [[extreme environment]]s is ongoing.<ref>{{cite journal |first1=Pabulo Henrique |last1=Rampelotto |year=2010 |volume=2 |issue=6 |pages=1602–1623 |title=Resistance of microorganisms to extreme environmental conditions and its contribution to astrobiology |doi=10.3390/su2061602 |bibcode=2010Sust....2.1602R |journal=Sustainability|url=http://urlib.net/dpi.inpe.br/plutao@80/2010/06.29.20.11 |doi-access=free }}</ref>

== Classification ==

{{Main|Biological classification}}

=== Antiquity ===

{{main|Aristotle's biology}}

The first classification of organisms was made by the Greek philosopher Aristotle (384–322 BC), who grouped living things as either plants or animals, based mainly on their ability to move. He distinguished animals with blood from animals without blood, which can be compared with the concepts of [[vertebrate]]s and [[invertebrate]]s respectively, and divided the blooded animals into five groups: viviparous quadrupeds ([[mammal]]s), oviparous quadrupeds (reptiles and [[amphibian]]s), birds, fishes and [[Cetacea|whales]]. The bloodless animals were divided into five groups: [[cephalopod]]s, [[crustacean]]s, insects (which included the spiders, [[scorpion]]s, and [[centipede]]s), shelled animals (such as most [[mollusc]]s and [[echinoderm]]s), and "[[zoophyte]]s" (animals that resemble plants). This theory remained dominant for more than a thousand years.<ref>{{Cite web |url=http://www.ucmp.berkeley.edu/history/aristotle.html |title=Aristotle |publisher=University of California Museum of Paleontology |access-date=15 November 2016 |url-status=dead |archive-url=https://web.archive.org/web/20161120124920/http://www.ucmp.berkeley.edu/history/aristotle.html |archive-date=20 November 2016}}</ref>

=== Linnaean ===

In the late 1740s, [[Carl Linnaeus]] introduced his system of [[binomial nomenclature]] for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and precisely defining their meaning.<ref>{{Cite journal |author1-last=Knapp |author1-first=Sandra |author1-link=Sandra Knapp |author2-last=Lamas |author2-first=Gerardo |author3-last=Lughadha |author3-first=Eimear Nic |author4-last=Novarino |author4-first=Gianfranco |title=Stability or stasis in the names of organisms: the evolving codes of nomenclature |journal=[[Philosophical Transactions of the Royal Society of London B]] |volume=359 |issue=1444 |pages=611–622 |date=April 2004 |pmid=15253348 |pmc=1693349 |doi=10.1098/rstb.2003.1445}}</ref>

The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon [[Vermes]] in Animalia, but later placed them back in Plantae. [[Herbert Copeland (biologist)|Herbert Copeland]] classified the Fungi in his [[Protoctista]], including them with single-celled organisms and thus partially avoiding the problem but acknowledging their special status.<ref>{{Cite journal |title=The Kingdoms of Organisms |journal=Quarterly Review of Biology |volume=13|issue=4 |doi=10.1086/394568 |year=1938 |page=383 | last1 = Copeland | first1 = Herbert F.|s2cid=84634277 }}</ref> The problem was eventually solved by [[Robert Whittaker (ecologist)|Whittaker]], when he gave them their own [[Kingdom (biology)|kingdom]] in his [[five-kingdom system]]. [[Evolutionary history]] shows that the fungi are more closely related to animals than to plants.<ref>{{Cite journal |last1=Whittaker |first1=R.H. |title=New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms |journal=Science |volume=163 |issue=3863 |pages=150–160 |date=January 1969 |pmid=5762760 |doi=10.1126/science.163.3863.150 |bibcode=1969Sci...163..150W |citeseerx=10.1.1.403.5430}}</ref>

As advances in [[microscopy]] enabled detailed study of [[cell (biology)|cells]] and microorganisms, new groups of life were revealed, and the fields of [[cell biology]] and [[microbiology]] were created. These new organisms were originally described separately in [[protozoa]] as animals and [[thallophyte|protophyta/thallophyta]] as plants, but were united by [[Ernst Haeckel]] in the kingdom [[Protista]]; later, the [[prokaryote]]s were split off in the kingdom [[Monera]], which would eventually be divided into two separate groups, the Bacteria and the [[Archaea]]. This led to the [[six-kingdom system]] and eventually to the current [[three-domain system]], which is based on evolutionary relationships.<ref name="Woese1990"/> However, the classification of eukaryotes, especially of protists, is still controversial.<ref name="Adl 05">{{Cite journal |last1=Adl |first1=S.M. |last2=Simpson |first2=A.G. |last3=Farmer |first3=M.A. |title=The new higher level classification of eukaryotes with emphasis on the taxonomy of protists |journal=Journal of Eukaryotic Microbiology |volume=52 |issue=5 |pages=399–451 |year=2005 |pmid=16248873 |doi=10.1111/j.1550-7408.2005.00053.x|s2cid=8060916 |doi-access=free }}</ref>

As microbiology developed, viruses, which are non-cellular, were discovered. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses have been classed into "species" based on their [[genetics]], but many aspects of such a classification remain controversial.<ref>{{Cite journal |last=Van Regenmortel |first=M.H. |title=Virus species and virus identification: past and current controversies |journal=Infection, Genetics and Evolution |volume=7 |issue=1 |pages=133–144 |date=January 2007 |pmid=16713373 |doi=10.1016/j.meegid.2006.04.002|bibcode=2007InfGE...7..133V |s2cid=86179057 }}</ref>

The original Linnaean system has been modified many times, for example as follows:<!--the table is potentially highly misleading: it is not the case that Cavalier-Smith represents the latest thinking and indeed his classification of the Eukaryotes is not widely accepted-->

{{Biological systems}}

The attempt to organise the Eukaryotes into a small number of kingdoms has been challenged. The Protozoa do not form a [[clade]] or natural grouping<!--i.e. they're polyphyletic or paraphyletic-->,<ref name="SimpsonRoger2004">{{cite journal |title=The real 'kingdoms' of eukaryotes |last1=Simpson |first1=Alastair G.B. |last2=Roger |first2=Andrew J. |journal=[[Current Biology]] |volume=14 |issue=17 |pages=R693–R696 |doi=10.1016/j.cub.2004.08.038 |pmid=15341755|year=2004 |s2cid=207051421 |doi-access=free|bibcode=2004CBio...14.R693S }}</ref> and nor do the [[Chromista]] (Chromalveolata).<ref>{{cite journal |last1=Harper |first1=J.T. |last2=Waanders |first2=E. |last3=Keeling |first3=P.J. |year=2005 |title=On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes |journal=[[International Journal of Systematic and Evolutionary Microbiology]] |volume=55 |issue=Pt 1 |pmid=15653923 |pages=487–496 |doi=10.1099/ijs.0.63216-0  |doi-access=free}}</ref>

=== Metagenomic ===

The ability to sequence large numbers of complete [[genome]]s has allowed biologists to take a [[metagenomic]] view of the [[phylogeny]] of the whole [[Tree of life (biology)|tree of life]]. This has led to the realisation that the majority of living things are bacteria, and that all have a common origin.<ref name="Woese1990"/><!--"Woese1990" is defined in {{biological systems}}--yes, I know what you think of that--><ref name="NM-20160411">{{cite journal |last1=Hug |first1=Laura A. |last2=Baker |first2=Brett J. |last3=Anantharaman |first3=Karthik |last4=Brown |first4=Christopher T. |last5=Probst |first5=Alexander J. |last6=Castelle |first6=Cindy J. |last7=Butterfield |first7=Cristina N. |last8=Hernsdorf |first8=Alex W. |last9=Amano |first9=Yuki |last10=Ise |first10=Kotaro |last11=Suzuki |first11=Yohey |last12=Dudek |first12=Natasha |last13=Relman |first13=David A. |last14=Finstad |first14=Kari M. |last15=Amundson |first15=Ronald |date=11 April 2016 |title=A new view of the tree of life |journal=[[Nature Microbiology]] |volume=1 |issue=5 |at=16048 |doi=10.1038/nmicrobiol.2016.48 |pmid=27572647 |doi-access=free |last16=Thomas |first16=Brian C. |last17=Banfield |first17=Jillian F.}}</ref>

<gallery class=center mode="nolines" heights="300" widths="300">File:Phylogenetic tree of life LUCA.svg|[[Phylogenetic]] tree based on [[rRNA]] [[gene]]s data (Woese ''et al.'', 1990)<ref name="Woese1990"/> showing the 3&nbsp;life [[Domain (biology)|domains]], with the [[last universal common ancestor|last universal common ancestor (LUCA)]] at its root
File:A Novel Representation Of The Tree Of Life.svg|A 2016 [[metagenomic]] representation of the [[Tree of life (biology)|tree of life]], unrooted, using [[ribosomal protein]] sequences. Bacteria are at top (left and right); [[Archaea]] at bottom; [[Eukaryote]]s in green at bottom right.<ref name="NM-20160411"/>
</gallery>

== Composition ==

=== Chemical elements ===

All life forms require certain core [[chemical element]]s for their [[biochemical]] functioning. These include [[carbon]], [[hydrogen]], [[nitrogen]], [[oxygen]], [[phosphorus]], and [[sulfur]]—the elemental [[nutrient|macronutrients]] for all organisms.<ref name=wsj20101203>{{cite news |first1=Robert Lee |last1=Hotz |title=New link in chain of life |work=[[The Wall Street Journal]] |date=3 December 2010 |url=https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |quote=Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA. |url-status=live |archive-url=https://web.archive.org/web/20170817163835/https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |archive-date=17 August 2017 }}</ref> Together these make up [[nucleic acid]]s, proteins and [[lipid]]s, the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur. The latter is a component of the amino acids [[cysteine]] and [[methionine]]. The most abundant of these elements in organisms is carbon, which has the desirable attribute of forming multiple, stable [[covalent bond]]s. This allows carbon-based (organic) molecules to form the immense variety of chemical arrangements described in [[organic chemistry]].<ref name="Lipkus Yuan Lucas Funk 2008 pp. 4443–4451">{{cite journal | last1=Lipkus | first1=Alan H. | last2=Yuan | first2=Qiong | last3=Lucas | first3=Karen A. | last4=Funk | first4=Susan A. | last5=Bartelt | first5=William F. | last6=Schenck | first6=Roger J. | last7=Trippe | first7=Anthony J.| title=Structural Diversity of Organic Chemistry. A Scaffold Analysis of the CAS Registry | journal=The Journal of Organic Chemistry | publisher=American Chemical Society (ACS) | volume=73 | issue=12 |year=2008 | doi=10.1021/jo8001276 | pages=4443–4451| pmid=18505297 | doi-access=free }}</ref>
Alternative [[hypothetical types of biochemistry]] have been proposed that eliminate one or more of these elements, swap out an element for one not on the list, or change required [[Chirality (chemistry)|chiralities]] or other chemical properties.<ref>{{cite book |author1=Committee on the Limits of Organic Life in Planetary Systems |author2=Committee on the Origins and Evolution of Life |author3=National Research Council |date=2007 |publisher=National Academy of Sciences |title=The Limits of Organic Life in Planetary Systems |isbn=978-0-309-66906-1 |url=http://www.nap.edu/catalog.php?record_id=11919 |access-date=3 June 2012 |url-status=live |archive-url=https://web.archive.org/web/20120510213123/http://www.nap.edu/catalog.php?record_id=11919 |archive-date=10 May 2012 }}</ref><ref>{{cite journal |first1=Steven A. |last1=Benner |first2=Alonso |last2=Ricardo |first3=Matthew A. |last3=Carrigan |journal=Current Opinion in Chemical Biology |title=Is there a common chemical model for life in the universe? |volume=8 |issue=6 |date=December 2004 |pages=672–689 |doi=10.1016/j.cbpa.2004.10.003 |url=http://www.fossildna.com/articles/benner_commonmodelforlife.pdf |archive-url=https://web.archive.org/web/20121016220349/http://www.fossildna.com/articles/benner_commonmodelforlife.pdf |archive-date=16 October 2012 |url-status=dead |access-date=3 June 2012 |pmid=15556414}}</ref>

=== DNA ===

{{main|DNA}}

Deoxyribonucleic acid or [[DNA]] is a [[molecule]] that carries most of the [[genetics|genetic]] instructions used in the growth, development, functioning and [[reproduction]] of all known living [[organism]]s and many viruses. DNA and [[RNA]] are [[nucleic acid]]s; alongside [[protein]]s and [[Polysaccharide|complex carbohydrates]], they are one of the three major types of [[macromolecule]] that are essential for all known forms of life. Most DNA molecules consist of two [[biopolymer]] strands coiled around each other to form a [[double helix]]. The two DNA strands are known as [[polynucleotide]]s since they are composed of [[monomer|simpler units]] called [[nucleotide]]s.<ref>{{cite web |url=http://basicbiology.net/micro/genetics/dna |title=DNA |date=5 February 2016 |website=Basic Biology |access-date=15 November 2016 |last1=Purcell |first1=Adam |url-status=dead |archive-url=https://web.archive.org/web/20170105045651/http://basicbiology.net/micro/genetics/dna/ |archive-date=5 January 2017 }}</ref> Each nucleotide is composed of a [[nitrogenous base|nitrogen-containing]] [[nucleobase]]—either [[cytosine]] (C), [[guanine]] (G), [[adenine]] (A), or [[thymine]] (T)—as well as a [[monosaccharide|sugar]] called [[deoxyribose]] and a [[phosphate group]]. The nucleotides are joined to one another in a chain by [[covalent bond]]s between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating [[backbone chain|sugar-phosphate backbone]]. According to [[base pair]]ing rules (A with T, and C with G), [[hydrogen bond]]s bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. This has the key property that each strand contains all the information needed to recreate the other strand, enabling the information to be preserved during reproduction and cell division.<ref name="NYT-20150718-rn">{{cite news |last=Nuwer |first=Rachel |author-link=Rachel Nuwer|date=18 July 2015 |title=Counting All the DNA on Earth |url=https://www.nytimes.com/2015/07/21/science/counting-all-the-dna-on-earth.html |work=The New York Times |location=New York |access-date=18 July 2015 |url-status=live |archive-url=https://web.archive.org/web/20150718153742/http://www.nytimes.com/2015/07/21/science/counting-all-the-dna-on-earth.html |archive-date=18 July 2015 }}</ref> Within cells, DNA is organised into long structures called [[chromosome]]s. During [[cell division]] these chromosomes are duplicated in the process of [[DNA replication]], providing each cell its own complete set of chromosomes. Eukaryotes store most of their DNA inside the [[cell nucleus]].<ref>{{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=978-0-8053-4553-7}}</ref> <!--DNA was first isolated by [[Friedrich Miescher]] in 1869.<ref>{{cite journal |last=Dahm |first=R. |title=Discovering DNA: Friedrich Miescher and the early years of nucleic acid research |journal=Hum. Genet. |volume=122 |issue=6 |pages=565–581 |year=2008 |pmid=17901982 |doi=10.1007/s00439-007-0433-0|s2cid=915930 }}</ref> Its molecular structure was identified by [[James Watson]] and [[Francis Crick]] in 1953, whose model-building efforts were guided by [[X-ray diffraction]] data acquired by [[Rosalind Franklin]].<ref name="pmid24840850">{{cite journal |last=Portin |first=P. |title=The birth and development of the DNA theory of inheritance: sixty years since the discovery of the structure of DNA |journal=Journal of Genetics |volume=93 |issue=1 |pages=293–302 |year=2014 |pmid=24840850 |doi=10.1007/s12041-014-0337-4 |s2cid=8845393 }}</ref>-->

=== Cells ===

{{main|Cell (biology)}}

Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by [[Cell division|division]].<ref>{{cite web |date=2 June 2019 |title=2.2: The Basic Structural and Functional Unit of Life: The Cell |url=https://med.libretexts.org/Courses/American_Public_University/APUS%3A_An_Introduction_to_Nutrition_(Byerley)/Text/03%3A_Nutrition_and_the_Human_Body/2.2%3A_The_Basic_Structural_and_Functional_Unit_of_Life%3A_The_Cell |url-status=live |archive-url=https://web.archive.org/web/20200329060227/https://med.libretexts.org/Courses/American_Public_University/APUS:_An_Introduction_to_Nutrition_(Byerley)/Text/03:_Nutrition_and_the_Human_Body/2.2:_The_Basic_Structural_and_Functional_Unit_of_Life:_The_Cell |archive-date=29 March 2020 |access-date=29 March 2020 |publisher=LibreTexts}}</ref><ref>{{cite web |last=Bose |first=Debopriya |date=14 May 2019 |title=Six Main Cell Functions |url=https://sciencing.com/six-main-cell-functions-6891800.html |url-status=live |archive-url=https://web.archive.org/web/20200329060221/https://sciencing.com/six-main-cell-functions-6891800.html |archive-date=29 March 2020 |access-date=29 March 2020 |publisher=Leaf Group Ltd./Leaf Group Media}}</ref> [[Cell theory]] was formulated by [[Henri Dutrochet]], [[Theodor Schwann]], [[Rudolf Virchow]] and others during the early nineteenth century, and subsequently became widely accepted.<ref name=sapp2003>{{cite book |first1=Jan |last1=Sapp |title=Genesis: The Evolution of Biology |publisher=Oxford University Press |date=2003 |isbn=978-0-19-515619-5 |pages=[https://archive.org/details/genesisevolution00sapp/page/75 75]–78 |url=https://archive.org/details/genesisevolution00sapp |url-access=registration }}</ref> The activity of an organism depends on the total activity of its cells, with [[Cellular respiration|energy flow]] occurring within and between them. Cells contain hereditary information that is carried forward as a [[genetics|genetic]] code during cell division.<ref>{{cite journal |last1=Lintilhac |first1=P.M. |title=Thinking of biology: toward a theory of cellularity—speculations on the nature of the living cell |journal=BioScience |date=Jan 1999 |volume=49 |issue=1 |pages=59–68 |pmid=11543344 |url=https://www.rz.uni-karlsruhe.de/~db45/Studiendekanat/Lehre/Master/Module/Botanik_1/M1401/Evolution_Zellbiologie/Lintilhac%202003.pdf |access-date=2 June 2012 |doi=10.2307/1313494 |jstor=1313494 |url-status=dead |archive-url=https://web.archive.org/web/20130406043511/https://www.rz.uni-karlsruhe.de/~db45/Studiendekanat/Lehre/Master/Module/Botanik_1/M1401/Evolution_Zellbiologie/Lintilhac%202003.pdf |archive-date=6 April 2013}}</ref>

There are two primary types of cells, reflecting their evolutionary origins. [[Prokaryote]] cells lack a [[Cell nucleus|nucleus]] and other membrane-bound [[organelle]]s, although they have circular DNA and [[ribosome]]s. Bacteria and [[Archaea]] are two [[domain (biology)|domains]] of prokaryotes. The other primary type is the [[eukaryote]] cell, which has a distinct nucleus bound by a nuclear membrane and membrane-bound organelles, including [[mitochondria]], [[chloroplasts]], [[lysosomes]], rough and smooth [[endoplasmic reticulum]], and [[vacuoles]]. In addition, their DNA is organised into [[chromosome]]s. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though with a wide diversity of [[protist]] [[microorganism]]s.<ref>{{Cite journal |last1=Whitman |first1=W. |last2=Coleman |first2=D. |last3=Wiebe |first3=W. |title=Prokaryotes: The unseen majority |doi=10.1073/pnas.95.12.6578 |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=95 |issue=12 |pages=6578–6583 |year=1998 |pmid=9618454 |pmc=33863 |bibcode=1998PNAS...95.6578W|doi-access=free }}</ref> The conventional model is that eukaryotes evolved from prokaryotes, with the main organelles of the eukaryotes forming through [[endosymbiosis]] between bacteria and the progenitor eukaryotic cell.<ref>{{cite journal |first1=Norman R. |last1=Pace |title=Concept Time for a change |journal=Nature |volume=441 |page=289 |date=18 May 2006 |doi=10.1038/441289a |url=http://coursesite.uhcl.edu/NAS/Kang/BIOL3231/Week3-Pace_2006.pdf |archive-url=https://web.archive.org/web/20121016220349/http://coursesite.uhcl.edu/NAS/Kang/BIOL3231/Week3-Pace_2006.pdf |archive-date=16 October 2012 |url-status=dead |access-date=2 June 2012 |pmid=16710401 |bibcode=2006Natur.441..289P |issue=7091|s2cid=4431143 }}</ref>

The molecular mechanisms of [[cell biology]] are based on [[protein]]s. Most of these are synthesised by the ribosomes through an [[Enzyme catalysis|enzyme-catalyzed]] process called [[protein biosynthesis]]. A sequence of amino acids is assembled and joined based upon [[gene expression]] of the cell's nucleic acid.<ref>{{cite web |title=Scientific background |website=The Nobel Prize in Chemistry 2009 |publisher=Royal Swedish Academy of Sciences |url=https://www.nobelprize.org/nobel_prizes/chemistry/laureates/2009/advanced.html |access-date=10 June 2012 |url-status=live |archive-url=https://web.archive.org/web/20120402150754/http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2009/advanced.html |archive-date=2 April 2012 }}</ref> In eukaryotic cells, these proteins may then be transported and processed through the [[Golgi apparatus]] in preparation for dispatch to their destination.<ref name="pmid20605430">{{cite journal|last1=Nakano |first1=A. |last2=Luini |first2=A. |year=2010 |title=Passage through the Golgi |journal=Current Opinion in Cell Biology |volume=22 |issue=4 |pages=471–478 |doi=10.1016/j.ceb.2010.05.003 |pmid=20605430 }}</ref>

Cells reproduce through a process of [[cell division]] in which the parent cell divides into two or more daughter cells. For prokaryotes, cell division occurs through a process of [[Fission (biology)|fission]] in which the DNA is replicated, then the two copies are attached to parts of the cell membrane. In [[eukaryote]]s, a more complex process of [[mitosis]] is followed. However, the result is the same; the resulting cell copies are identical to each other and to the original cell (except for [[mutations]]), and both are capable of further division following an [[interphase]] period.<ref>{{cite book |first1=Joseph |last1=Panno |title=The Cell |series=Facts on File science library |publisher=Infobase Publishing |date=2004 |isbn=978-0-8160-6736-7 |pages=60–70 |url=https://books.google.com/books?id=sYgKY6zz20YC&pg=PA60 |access-date=10 August 2023 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194758/https://books.google.com/books?id=sYgKY6zz20YC&pg=PA60 |url-status=live }}</ref>

=== Multicellular structure ===

[[Multicellular organism]]s may have first evolved through the formation of [[Colony (biology)|colonies]] of identical cells. These cells can form group organisms through [[cell adhesion]]. The individual members of a colony are capable of surviving on their own, whereas the members of a true multi-cellular organism have developed specialisations, making them dependent on the remainder of the organism for survival. Such organisms are formed [[Clone (cell biology)|clonally]] or from a single [[germ cell]] that is capable of forming the various specialised cells that form the adult organism. This specialisation allows multicellular organisms to exploit resources more efficiently than single cells.<ref>{{cite book |first1=Bruce |last1=Alberts |first2=Dennis |last2=Bray |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=James D. |last6=Watson |chapter=From Single Cells to Multicellular Organisms |title=Molecular Biology of the Cell |edition=3rd |location=New York |publisher=Garland Science |date=1994 |isbn=978-0-8153-1620-6 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK28332/ |access-date=12 June 2012 |url-access=registration |url=https://archive.org/details/molecularbiology00albe }}</ref> About 800 million years ago, a minor genetic change in a single molecule, the [[enzyme]] [[GK-PID]], may have allowed organisms to go from a single cell organism to one of many cells.<ref name="NYT-20160107">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Genetic Flip Helped Organisms Go From One Cell to Many |url=https://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |date=7 January 2016 |work=[[The New York Times]] |access-date=7 January 2016 |url-status=live |archive-url=https://web.archive.org/web/20160107204432/http://www.nytimes.com/2016/01/12/science/genetic-flip-helped-organisms-go-from-one-cell-to-many.html |archive-date=7 January 2016 }}</ref>

Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability. [[Cell signalling]] coordinates cellular activities, and hence governs the basic functions of multicellular organisms. Signaling between cells can occur through direct cell contact using [[juxtacrine signalling]], or indirectly through the exchange of agents as in the [[endocrine system]]. In more complex organisms, coordination of activities can occur through a dedicated [[nervous system]].<ref name=alberts2002>{{cite book |first1=Bruce |last1=Alberts |first2=Alexander |last2=Johnson |first3=Julian |last3=Lewis |first4=Martin |last4=Raff |first5=Keith |last5=Roberts |first6=Peter |last6=Walter |chapter=General Principles of Cell Communication |title=Molecular Biology of the Cell |location=New York |publisher=Garland Science |date=2002 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK26813/ |access-date=12 June 2012 |isbn=978-0-8153-3218-3 |url-status=live |archive-url=https://web.archive.org/web/20150904000612/http://www.ncbi.nlm.nih.gov/books/NBK26813/ |archive-date=4 September 2015 }}</ref>

== In the universe ==
{{main|Extraterrestrial life|Astrobiology|Astroecology}}

Though life is confirmed only on Earth, many think that [[extraterrestrial life]] is not only plausible, but probable or inevitable,<ref name="RaceRandolph2002">{{cite journal |last1=Race |first1=Margaret S. |last2=Randolph |first2=Richard O. |title=The need for operating guidelines and a decision making framework applicable to the discovery of non-intelligent extraterrestrial life |journal=Advances in Space Research |volume=30 |issue=6 |year=2002 |pages=1583–1591 |doi=10.1016/S0273-1177(02)00478-7 |quote=There is growing scientific confidence that the discovery of extraterrestrial life in some form is nearly inevitable |bibcode=2002AdSpR..30.1583R |citeseerx=10.1.1.528.6507 }}</ref><ref>{{cite news |url=http://www.newser.com/story/50874/alien-life-inevitable-astronomer.html |title=Alien Life 'Inevitable': Astronomer |last=Cantor |first=Matt |date=15 February 2009 |work=Newser |quote=Scientists now believe there could be as many habitable planets in the cosmos as there are stars, and that makes life's existence elsewhere "inevitable" over billions of years, says one. |access-date=3 May 2013 |archive-url=https://web.archive.org/web/20130523141853/http://www.newser.com/story/50874/alien-life-inevitable-astronomer.html |url-status=dead |archive-date=23 May 2013 }}</ref> possibly resulting in a [[biophysical cosmology]] instead of a mere [[physical cosmology]].<ref name="v237">{{cite book | last=Dick | first=Steven J. | title=Space, Time, and Aliens | chapter=The Biophysical Cosmology: The Place of Bioastronomy in the History of Science | publisher=Springer International Publishing | publication-place=Cham | date=2020 | isbn=978-3-030-41613-3 | doi=10.1007/978-3-030-41614-0_4 | pages=53–58}}</ref> Other planets and [[moons]] in the [[Solar System]] and other [[planetary system]]s are being examined for evidence of having once supported simple life, and projects such as [[SETI]] are trying to detect radio transmissions from possible alien civilisations. Other locations within the [[Solar System]] that may host [[microbial]] life include the subsurface of [[Life on Mars (planet)|Mars]], the upper atmosphere of [[Life on Venus|Venus]],<ref>{{Cite journal |last1=Schulze-Makuch |first1=Dirk |last2=Dohm |first2=James M. |last3=Fairén |first3=Alberto G. |last4=Baker |first4=Victor R. |last5=Fink |first5=Wolfgang |last6=Strom |first6=Robert G. | title=Venus, Mars, and the Ices on Mercury and the Moon: Astrobiological Implications and Proposed Mission Designs |journal=Astrobiology |volume=5 |issue=6 |pages=778–795 |date=December 2005 |doi=10.1089/ast.2005.5.778 |pmid=16379531 |bibcode=2005AsBio...5..778S |s2cid=13539394 }}</ref> and subsurface oceans on some of the [[Natural satellite habitability|moons]] of the [[giant planet]]s.<ref name="WRD-20150127">{{cite journal |last=Woo |first=Marcus |title=Why We're Looking for Alien Life on Moons, Not Just Planets |url=https://www.wired.com/2015/01/looking-alien-life-moons-just-planets/ |date=27 January 2015 |journal=[[Wired (website)|Wired]] |access-date=27 January 2015 |url-status=live |archive-url=https://web.archive.org/web/20150127120332/http://www.wired.com/2015/01/looking-alien-life-moons-just-planets/ |archive-date=27 January 2015 }}</ref><ref>{{cite web |first1=Daniel |last1=Strain |date=14 December 2009 |title=Icy moons of Saturn and Jupiter may have conditions needed for life |publisher=The University of Santa Cruz |url=http://news.ucsc.edu/2009/12/3443.html |access-date=4 July 2012 |url-status=live |archive-url=https://web.archive.org/web/20121231111334/http://news.ucsc.edu/2009/12/3443.html |archive-date=31 December 2012 }}</ref>

Investigation of the tenacity and versatility of life on Earth,<ref name="NYT-20160912"/> as well as an understanding of the molecular systems that some organisms utilise to survive such extremes, is important for the search for extraterrestrial life.<ref name=astrobiology/> For example, [[lichen]] could survive for a month in a [[Life on Earth under Martian conditions|simulated Martian environment]].<ref name="Skymania-20120426">{{cite web |last=Baldwin |first=Emily |title=Lichen survives harsh Mars environment |url=http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html |date=26 April 2012 |publisher=Skymania News |access-date=27 April 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120528145425/http://www.skymania.com/wp/2012/04/lichen-survives-harsh-martian-setting.html/ |archive-date=28 May 2012 }}</ref><ref name="EGU-20120426">{{cite journal |last1=de Vera |first1=J.-P. |last2=Kohler |first2=Ulrich |title=The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars |journal=EGU General Assembly Conference Abstracts |volume=14 |page=2113 |url=http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |archive-url=https://web.archive.org/web/20120504224706/http://media.egu2012.eu/media/filer_public/2012/04/05/10_solarsystem_devera.pdf |archive-date=4 May 2012 |url-status=dead |date=26 April 2012 |access-date=27 April 2012|bibcode=2012EGUGA..14.2113D }}</ref>

Beyond the Solar System, the region around another [[main-sequence star]] that could support Earth-like life on an Earth-like planet is known as the [[habitable zone]]. The inner and outer radii of this zone vary with the luminosity of the star, as does the time interval during which the zone survives. Stars more massive than the Sun have a larger habitable zone, but remain on the Sun-like "main sequence" of [[stellar evolution]] for a shorter time interval. Small [[red dwarf]]s have the opposite problem, with a smaller habitable zone that is subject to higher levels of magnetic activity and the effects of [[tidal locking]] from close orbits. Hence, stars in the intermediate mass range such as the Sun may have a greater likelihood for Earth-like life to develop.<ref name=selis2006>{{Cite book |first1=Frank |last1=Selis |date=2006 |chapter=Habitability: the point of view of an astronomer |title=Lectures in Astrobiology |volume=2 |editor1-first=Muriel |editor1-last=Gargaud |editor2-first=Hervé |editor2-last=Martin |editor3-first=Philippe |editor3-last=Claeys |publisher=Springer |isbn=978-3-540-33692-1 |pages=210–214 |chapter-url=https://books.google.com/books?id=3uYmP0K5PXEC&pg=PA210 |access-date=10 August 2023 |archive-date=5 November 2023 |archive-url=https://web.archive.org/web/20231105190206/https://books.google.com/books?id=3uYmP0K5PXEC&pg=PA210#v=onepage&q&f=false |url-status=live }}</ref> The location of the star within a galaxy may also affect the likelihood of life forming. Stars in regions with a greater abundance of heavier elements that can form planets, in combination with a low rate of potentially [[habitat]]-damaging [[supernova]] events, are predicted to have a higher probability of hosting planets with complex life.<ref name=science303_5654_59>{{Cite journal |last1=Lineweaver |first1=Charles H. |last2=Fenner |first2=Yeshe |last3=Gibson |first3=Brad K. |date=January 2004 |title=The Galactic Habitable Zone and the age distribution of complex life in the Milky Way |journal=Science |volume=303 |issue=5654 |pages=59–62 |doi=10.1126/science.1092322 |bibcode=2004Sci...303...59L |pmid=14704421 |arxiv=astro-ph/0401024 |s2cid=18140737 |url=https://cds.cern.ch/record/704101 |access-date=30 August 2018 |archive-date=31 May 2020 |archive-url=https://web.archive.org/web/20200531022432/https://cds.cern.ch/record/704101 |url-status=live }}</ref> The variables of the [[Drake equation]] are used to discuss the conditions in planetary systems where civilisation is most likely to exist, within wide bounds of uncertainty.<ref name=vakoch_harrison2011>{{Cite book |first1=Douglas A. |last1=Vakoch |first2=Albert A. |last2=Harrison |title=Civilizations beyond Earth: extraterrestrial life and society |series=Berghahn Series |pages=37–41 |publisher=Berghahn Books |date=2011 |url=https://books.google.com/books?id=BVJzsvqWip0C&pg=PA37 |isbn=978-0-85745-211-5 |access-date=25 August 2020 |archive-date=13 April 2023 |archive-url=https://web.archive.org/web/20230413194802/https://books.google.com/books?id=BVJzsvqWip0C&pg=PA37 |url-status=live }}</ref> A "Confidence of Life Detection" scale (CoLD) for reporting evidence of life beyond Earth has been proposed.<ref name="NAT-20211027">{{cite journal |first1=James |last1=Green |first2=Tori |last2=Hoehler |first3=Marc |last3=Neveu |first4=Shawn |last4=Domagal-Goldman |first5=Daniella |last5=Scalice |first6=Mary |last6=Voytek |author6-link=Mary Voytek |title=Call for a framework for reporting evidence for life beyond Earth |url=https://www.nature.com/articles/s41586-021-03804-9 |date=27 October 2021 |journal=[[Nature (journal)|Nature]] |volume=598 |issue=7882 |pages=575–579 |doi=10.1038/s41586-021-03804-9 |pmid=34707302 |arxiv=2107.10975 |bibcode=2021Natur.598..575G |s2cid=236318566 |accessdate=1 November 2021 |archive-date=1 November 2021 |archive-url=https://web.archive.org/web/20211101023448/https://www.nature.com/articles/s41586-021-03804-9 |url-status=live }}</ref><ref name="COS-20211030">{{cite news |last=Fuge |first=Lauren |title=NASA proposes playbook for communicating the discovery of alien life – Sensationalising aliens is so 20th century, according to NASA scientists. |url=https://cosmosmagazine.com/space/astrobiology/what-happens-when-we-find-aliens/ |date=30 October 2021 |work=[[Cosmos (Australian magazine)|Cosmos]] |accessdate=1 November 2021 |archive-date=31 October 2021 |archive-url=https://web.archive.org/web/20211031221719/https://cosmosmagazine.com/space/astrobiology/what-happens-when-we-find-aliens/ |url-status=live }}</ref>

== Artificial ==

{{main|Artificial life |Synthetic biology}}

Artificial life is the [[simulation]] of any aspect of life, as through computers, [[robotics]], or [[biochemistry]].<ref>{{Cite web|url=http://www.dictionary.com/browse/artificial--life|title=Artificial life|website=Dictionary.com|access-date=15 November 2016|url-status=dead|archive-url=https://web.archive.org/web/20161116021041/http://www.dictionary.com/browse/artificial--life|archive-date=16 November 2016}}</ref> [[Synthetic biology]] is a new area of [[biotechnology]] that combines science and [[biological engineering]]. The common goal is the design and construction of new biological functions and systems not found in nature. Synthetic biology includes the broad redefinition and expansion of biotechnology, with the ultimate goals of being able to design and build engineered biological systems that process information, manipulate chemicals, fabricate materials and structures, produce energy, provide food, and maintain and enhance human health and the environment.<ref>{{Cite journal |volume=6 |last=Chopra |first=Paras |author2=Akhil Kamma |title=Engineering life through Synthetic Biology |journal=In Silico Biology |access-date=9 June 2008 |url=http://www.bioinfo.de/isb/2006/06/0038/ |url-status=live |archive-url=https://web.archive.org/web/20080805175817/http://www.bioinfo.de/isb/2006/06/0038/ |archive-date=5 August 2008 }}</ref>

== See also ==
{{div col|colwidth=30}}
* [[Biology]], the study of life
* [[Biosignature]]
* [[Carbon-based life]]
* [[Central dogma of molecular biology]]
* [[History of life]]
* [[Lists of organisms by population]]
* [[Viable system theory]]

{{div col end}}

== Notes ==

{{Notelist}}

== References ==

{{Reflist}}

== External links ==
* [https://www.biolib.cz/en/taxon/id14772 Vitae] (BioLib)
* [[species:Main Page|Wikispecies]] – a free directory of life
* [https://web.archive.org/web/20140715055239/http://taxonomicon.taxonomy.nl/TaxonTree.aspx?id=1&src=0 Biota] (Taxonomicon) (archived 15 July 2014)
* [http://plato.stanford.edu/entries/life/ Entry] on the ''[[Stanford Encyclopedia of Philosophy]]''
* [https://archive.today/20231205121742/https://www.theatlantic.com/science/archive/2023/12/defining-life-existentialism-scientific-theory/676238/ What Is Life?] – by Jaime Green, ''[[The Atlantic]]'' (archived 5 December 2023)
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