Life Warning: You are not logged in. Your IP address will be publicly visible if you make any edits. If you log in or create an account, your edits will be attributed to your username, along with other benefits.Anti-spam check. Do not fill this in! == 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 |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, [[Perspiration|sweating]] to reduce temperature. # [[Biological organization|Organisation]]: being structurally composed of one or more [[cell (biology)|cells]] – 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 |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 |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 criticized 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> The reason for this potential flaw is that "NASA's definition" refers to life as a phenomenon, not a living individual, which makes it incomplete.<ref name="Piast-2019">{{Cite journal |last=Piast |first=Radosław W. |date=June 2019 |title=Shannon's information, Bernal's biopoiesis and Bernoulli distribution as pillars for building a definition of life |journal=Journal of Theoretical Biology |volume=470 |pages=101–107 |doi=10.1016/j.jtbi.2019.03.009 |pmid=30876803 |bibcode=2019JThBi.470..101P |s2cid=80625250 }}</ref> Alternative definitions based on the notion of life as a phenomenon and a living individual have been proposed as [[Continuum mechanics|continuum]] of a self-maintainable information, and a distinct element of this continuum, respectively. A major strength of this approach is that it defines life in terms of mathematics and physics, avoiding biological vocabulary.<ref name="Piast-2019" /> === 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-organization|self-organizing]] and [[autopoiesis|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 (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> Summary: Please note that all contributions to Christianpedia may be edited, altered, or removed by other contributors. 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