View source for Life

{{Other uses|Life (disambiguation)}}
{{pp-move-indef}}{{automatic taxobox |color=limegreen |name='''Life (''[[Biota (taxonomy)|Biota]]'')''' |image=Ruwenpflanzen.jpg |image_width=250px |subdivision_ranks=[[Domain (biology)|Domains]] and [[Kingdom (biology)|Kingdoms]] |subdivision=Life on Earth:
* [[Non-cellular life]] ([[virus]]es) [[Paraphyly|<span title="May be paraphyletic as the 'evolution' of viruses and other similar forms is still uncertain, cellular life might have evolved from non-cellular life.">*</span>]] [[Polyphyly|<span title="May be polyphyletic as the 'evolution' of viruses and other similar forms is still uncertain, the most recent common ancestor might not be included.">*</span>]]
* [[Cellular life]]
** [[Bacteria]]
** [[Archaea]]
** [[Eukarya]]
*** [[Protista]]
*** [[Fungi]]
*** [[Plantae]]
*** [[Animalia]]}}

'''Life''' (cf. [[Biota (ecology)|biota]]) is a characteristic that distinguishes [[physical body|objects]] that have [[biological process|signaling and self-sustaining processes]] (biology) from those that do not,<ref name=Koshland>{{Cite journal |title=The Seven Pillars of Life |journal=Science |date=March 22, 2002 |first=Daniel E. |last=Koshland Jr |volume=295 |issue=5563 |pages=2215–2216 |doi= 10.1126/science.1068489 |url=http://www.sciencemag.org/cgi/content/full/295/5563/2215 |accessdate=2009-05-25 |pmid=11910092}}</ref><ref name=AHDLife>The [[American Heritage Dictionary]] of the English Language, 4th edition, published by Houghton Mifflin Company, via [http://www.answers.com/topic/life Answers.com]:
* "The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the [[Environment (biophysical)|environment]] originating from within the organism."
* "The characteristic state or condition of a living organism."</ref> either because such functions have ceased ([[death]]), or else because they lack such functions and are classified as [[wikt:inanimate|inanimate]].<ref>[http://wordnetweb.princeton.edu/perl/webwn?s=inanimate Definition of ''inanimate''.] WordNet Search by Princeton University.</ref>

In [[biology]], the science of living organisms, life is the condition which distinguishes active [[organism]]s from [[inorganic compound|inorganic matter]].<ref>{{Cite web |url=http://www.merriam-webster.com/dictionary/life |title=Merriam-Webster Dictionary |accessdate=2009-06-21 |publisher=Merriam-Webster Dictionary}}</ref> Living organisms undergo [[metabolism]], maintain [[homeostasis]], possess a capacity to [[cell growth|grow]], respond to [[stimulus (physiology)|stimuli]], [[reproduce]] and, through [[natural selection]], adapt to their environment in successive generations. More complex living organisms can communicate through various means.<ref name=Koshland/><ref name=Chambers>{{cite encyclopedia |encyclopedia=Chambers 21st Century Dictionary |edition=online |year=1999 |title=organism}}</ref> A diverse array of living organisms (life forms) can be found in the [[biosphere]] on [[Earth]], and the properties common to these organisms—[[plant]]s, [[animal]]s, [[fungus|fungi]], [[protist]]s, [[archaea]], and [[bacteria]]—are a [[carbon-based life|carbon]]- and [[Water#Effects on life|water]]-based [[cell (biology)|cellular]] form with complex [[organization]] and heritable [[gene]]tic information.

In [[philosophy]] and [[religion]], the [[concept]]ion of life and its nature varies. Both offer interpretations as to how life relates to [[existence]] and [[consciousness]], and both touch on many related issues, including [[life stance]], [[purpose]], [[Conceptions of god|conception of a god or gods]], a [[soul]] or an [[afterlife]].
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==Early theories about life==

===Materialism===
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[[Image:Hoh rain forest trees.jpg|thumb|[[Plant]] life]]
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[[Image:Herds Maasi Mara (cropped and straightened).jpg|thumb|Herds of zebra and impala gathering on the [[Masai Mara]] plain]]
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[[Image:Grand prismatic spring.jpg|thumb|An aerial photo of microbial mats around the [[Grand Prismatic Spring]] of
[[Yellowstone National Park]].]]
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Some of the earliest theories of life were [[materialist]], holding that all that exists is matter, and that all life is merely a complex form or arrangement of matter. [[Empedocles]] (430 BC) argued that every thing 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. For example, growth in plants is explained by the natural downward movement of earth and the natural upward movement of fire.<ref>[http://plato.stanford.edu/entries/empedocles SEP]</ref>

[[Democritus]] (460 BC), the disciple of [[Leucippus]], thought that the essential characteristic of life is having a soul (''psyche''). In common with other ancient writers, he used the term to mean the principle of living things that causes them to function as a living thing. He thought the soul was composed of fire atoms, because of the apparent connection between life and heat, and because fire moves.<ref>[http://plato.stanford.edu/entries/democritus/#4 SEP]</ref> He also suggested that humans originally lived like animals, gradually developing communities to help one another, originating language, and developing crafts and agriculture.<ref>''Ibidem''</ref>

In the [[Scientific Revolution|scientific revolution of the 17th century]], mechanistic ideas were revived by philosophers like [[Descartes]].

===Hylomorphism===
[[Hylomorphism]] is the theory (originating with [[Aristotle]] (322 BC)) that all things are a combination of matter and form. Aristotle was one of the first ancient writers to approach the subject of life in a scientific way. Biology was one of his main interests, and there is extensive biological material in his extant writings. According to him, all things in the material universe have both matter and form. 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>Aristotle, ''De Anima'', Book II</ref> Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.<ref>''Introduction to Ancient Philosophy'', Don Marietta, p. 104.</ref>

Consistent with this account is a [[teleological]] explanation of life. A teleological explanation accounts for phenomena in terms of their purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its ''purpose'' of camouflage. The direction of causality is the other way round from materialistic science, which explains the consequence in terms of a prior cause. Modern biologists now reject this functional view in terms of a material and causal one: biological features are to be explained not by looking ''forward'' to future optimal results, but by looking ''backwards'' to the past evolutionary history of a species, which led to the [[natural selection]] of the features in question.

===Vitalism===
[[Vitalism]] is the belief that the life-principle is essentially immaterial. This originated with [[Georg Ernst Stahl|Stahl]] (17th century), and held sway until the middle of the 19th century. It appealed to philosophers such as [[Henri Bergson]], [[Nietzsche]], [[Wilhelm Dilthey]], anatomists like [[Marie François Xavier Bichat|Bichat]], and chemists like [[Justus von Liebig|Liebig]].

Vitalism underpinned the idea of a fundamental separation of 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. This so-called [[Wöhler synthesis]] is considered the starting point of modern [[organic chemistry]]. It is of great historical significance because for the first time an [[organic compound]] was produced from [[inorganic compound|inorganic]] reactants.

Later, [[Hermann von Helmholtz|Helmholtz]], anticipated by [[Julius Robert von Mayer|Mayer]], demonstrated that no energy is lost in muscle movement, suggesting that there were no ''vital forces'' necessary to move a muscle. These empirical results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in non-scientific theories such as [[homeopathy]], which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.
<!--No references have come forward:
 ===Hylozoism and panpsychism===
In a more drastic philosophic view, life itself is completely put on a par with other things; all or nearly all matter that exists in the [[universe]] is said to be living.{{Cn|date=September 2010}} This view is further subdivided into [[hylozoism]] and [[panpsychism]]. -->

==Definitions==
It is still a challenge for scientists and philosophers to define life in unequivocal terms.<ref>[http://www.astrobio.net/exclusive/226/defining-life Defining Life : Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond]</ref><ref>[http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html Defining Life, Explaining Emergence]</ref><ref>{{Cite web |url= http://artsandsciences.colorado.edu/magazine/2009/03/can-we-define-life/ |title=Can We Define Life |accessdate=2009-06-22 |year=2009 |publisher=Colorado Arts & Sciences}}</ref> Defining life is difficult—in part—because life is a process, not a pure substance.<ref name=McKay>{{Cite journal |title=What Is Life—and How Do We Search for It in Other Worlds? |journal=PLoS Biol. |date=September 14, 2004 |first=Chris P. |last=McKay |pmid=15367939 |volume=2 |issue=2(9) |pmc=516796 |pages=302 |doi=10.1371/journal.pbio.0020302 |url=http://www.ncbi.nlm.nih.gov/pmc/articles/PMC516796/?tool=pubmed |accessdate=2010-02-02}}</ref> Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from earthly life.<ref>{{Cite journal |author=Nealson KH, Conrad PG |title=Life: past, present and future |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=354 |issue=1392 |pages=1923–39 |date= December 1999 |pmid=10670014 |pmc=1692713 |doi=10.1098/rstb.1999.0532 |url=http://journals.royalsociety.org/content/7r10hqn3rp1g1vag/fulltext.pdf}}</ref>

===Biology===
Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a characteristic of organisms that exhibit all or most of the following [[phenomena]]:<ref name=McKay/><ref>{{Cite web |url=http://www2.una.edu/pdavis/BI%20101/Overview%20Fall%202004.htm |title=How to Define Life |accessdate=2008-10-17 |last=Davison |first=Paul G. |publisher=The University of North Alabama }}</ref>
# '''[[Homeostasis]]''': Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.
# '''[[Biological organization|Organization]]''': Being structurally composed of one or more [[cell (biology)|cells]], which are the basic units of life.
# '''[[Metabolism]]''': Transformation of energy by converting chemicals and energy into cellular components ([[anabolism]]) and decomposing organic matter ([[catabolism]]). Living things require [[bioenergetics|energy]] to maintain internal organization ([[homeostasis]]) and to produce the other phenomena associated with life.
# '''[[Cell growth|Growth]]''': Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
# '''[[Adaptation]]''': The ability to change over a period of time in response to the environment. This ability is fundamental to the process of [[evolution]] and is determined by the organism's [[heredity]] as well as the composition of metabolized substances, and external factors present.
# '''Response to [[stimulus (physiology)|stimuli]]''': A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun ([[phototropism]]) and by [[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.

====Proposed====
To reflect the minimum phenomena required, some have proposed other biological definitions of life:
* Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.{{Cn|date=October 2010}}
* A network of inferior negative feedbacks (regulatory mechanisms) subordinated to a superior positive feedback (potential of expansion, reproduction).<ref>Korzeniewski, Bernard (2001). "[http://www.ncbi.nlm.nih.gov/pubmed/11312589 Cybernetic formulation of the definition of life]". ''Journal of Theoretical Biology''. 2001 April 7. 209 (3) pp. 275–86.</ref>
* A [[Living systems theory|systemic]] definition of life is that living things are [[self-organization|self-organizing]] and [[autopoiesis|autopoietic]] (self-producing). Variations of this definition include [[Stuart Kauffman]]'s definition as an [[autonomous agent]] or a [[multi-agent system]] capable of reproducing itself or themselves, and of completing at least one [[thermodynamic cycle|thermodynamic work cycle]].<ref>2004, "Autonomous Agents", in John D. Barrow, P.C.W. Davies, and C.L. Harper Jr., eds., Science and Ultimate Reality: Quantum Theory, Cosmology, and Complexity, Cambridge University Press.</ref>
* Life is a self-sustained chemical system capable of undergoing Darwinian evolution.<ref>[[Gerald Joyce|Gerald Francis Joyce]], ''"The RNA World: Life Before DNA and Protein"''.</ref>
* Things with the capacity for metabolism and motion.<ref name=McKay/>

====Viruses====
[[Virus]]es are most often considered [[replicator]]s rather than forms of life. They have been described as "organisms at the edge of life,"<ref>{{Cite journal |last=Rybicki |first=EP |year=1990 |title=The classification of organisms at the edge of life, or problems with virus systematics |journal=S Aft J Sci |volume=86 |pages=182–186}}</ref> since they possess [[gene]]s, [[evolution|evolve]] by [[natural selection]],<ref name="pmid17914905">{{Cite journal |author=Holmes EC |title=Viral evolution in the genomic age |journal=PLoS Biol. |volume=5 |issue=10 |pages=e278 |date=October 2007 |pmid=17914905 |pmc=1994994 |doi=10.1371/journal.pbio.0050278 |url=http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0050278 |accessdate=2008-09-13}}</ref> and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not [[metabolism|metabolize]] and 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 |author=Koonin EV, Senkevich TG, Dolja VV |title= The ancient Virus World and evolution of cells |journal=Biol. Direct |volume=1 |page=29 |year=2006 |pmid=16984643 |pmc=1594570 |doi=10.1186/1745-6150-1-29 |url=http://www.biology-direct.com/content/1//29 |accessdate=2008-09-14}}</ref><ref>{{Cite web |url=http://www.mcb.uct.ac.za/tutorial/virorig.html#Virus%20Origins |title=Origins of Viruses |accessdate=2009-04-12 |last=Rybicki |first=Ed |date=November 1997}}</ref>

===Biophysics===
[[Biophysicist]]s have also commented on the nature and qualities of life forms—notably that they function on [[negentropy|negative entropy]].<ref>{{Cite book |last=Schrödinger |first=Erwin |title=What is Life? |publisher=Cambridge University Press |year=1944 |isbn=0-521-42708-8}}</ref><ref>{{Cite book |last=Margulis |first=Lynn |coauthor=Sagan, Dorion |title=What is Life? |publisher=University of California Press |year=1995 |isbn=0-520-22021-8}}</ref> In more detail, according to physicists such as [[John Desmond Bernal|John Bernal]], [[Erwin Schrödinger]], [[Eugene Wigner]], and [[John Scales Avery|John Avery]], life is a member of the class of [[phenomena]] which are open or continuous systems able to decrease their internal [[entropy]] at the expense of substances or [[thermodynamic free energy|free energy]] taken in from the [[environment (biophysical)|environment]] and subsequently rejected in a degraded form (see: [[entropy and life]]).<ref>{{Cite book |last=Lovelock |first=James |title=Gaia – a New Look at Life on Earth |publisher=Oxford University Press |year=2000 |isbn=0-19-286218-9}}</ref><ref>{{Cite book |last=Avery |first=John |title=Information Theory and Evolution |publisher=World Scientific |isbn=9812383999 |year=2003}}</ref><ref>{{Cite web |url=http://biocab.org/Exobiology.html#anchor_41 |title=BIOPHYSICS: DEFINITION OF LIFE AND BRIEF EXPLANATION OF EACH TERM |accessdate=2009-07-22 |first=Nahle, Nasif S. Exobiology |date= September 29, 2006 |publisher=Biology Cabinet}}</ref>

===Living systems theories===
Some scientists have proposed in the last few decades that a general [[living systems|living systems theory]] is required to explain the nature of life.<ref>{{Cite book |last=Woodruff |first=T. Sullivan |coauthor=John Baross |title=Planets and Life: The Emerging Science of Astrobiology |publisher=Cambridge University Press |date=October 8, 2007}} Cleland and Chyba wrote a chapter in Planets and Life: "In the absence of such a theory, we are in a position analogous to that of a 16th-century investigator trying to define 'water' in the absence of molecular theory." [...] "Without access to living things having a different historical origin, it is difficult and perhaps ultimately impossible to formulate an adequately general theory of the nature of living systems".</ref> Such a general theory, arising out of the [[ecology|ecological]] and [[biology|biological sciences]], attempts to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into component parts, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.<ref>{{Cite web |url=http://www.mollyyoungbrown.com/systems_article.htm |title=Patterns, Flows, and Interrelationship |accessdate=2009-06-27 |first=Molly Young Brown |year=2002}}</ref>

====Gaia hypothesis====
The idea that the Earth is alive is probably as old as humankind, but the first public expression of it as a fact of science was by a Scottish scientist, [[James Hutton]]. In 1785 he stated that the Earth was a superorganism and that its proper study should be [[physiology]]. Hutton is rightly remembered as the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century.<ref>''GAIA - A new look at life on Earth. [[James Lovelock]] 1979. pp. 10. Oxford University Press. ISBN 0-19-286030-5.</ref> The [[Gaia hypothesis]], originally proposed in the 1960s by scientist [[James Lovelock]],<ref name="Lovelock1965">{{Cite journal |author=Lovelock, J.E. |year=1965 |title=A physical basis for life detection experiments |journal=[[Nature (journal)|Nature]] |volume=207 |issue=7 |pages=568–570 |doi=10.1038/207568a0}}</ref><ref>[http://www.jameslovelock.org/page4.html Geophysiology]</ref> explores the idea that the life on Earth functions as a single organism which actually defines and maintains environmental conditions necessary for its survival.<ref>''GAIA - A new look at life on Earth. [[James Lovelock]]. 1979. Oxford University Press. ISBN 0-19-286030-5.</ref>

====Nonfractionability====
[[Robert Rosen]] (1991) built on the assumption that the explanatory powers of the mechanistic worldview cannot help understand the realm of living systems. One of several important clarifications he made was to define a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." From this and other starting concepts, he developed a "relational theory of systems" that attempts to explain the special properties of life. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines."<ref>''Life Itself: A Comprehensive Inquiry into the Nature, Origin, and Fabrication of Life''. Rosen, Robert. November, 1991. ISBN 978-0-231-07565-7</ref>

====Life as a property of ecosystems====
A systems view of life treats environmental [[flux]]es and biological fluxes together as a "reciprocity of influence",<ref>{{Cite web |url=http://www.calresco.org/fiscus/esl.htm |title=The Ecosystemic Life Hypothesis |accessdate=2009-08-28 |first=Daniel A. Fiscus |date=April 2002 |publisher=Bulletin of the Ecological Society of America}}</ref> and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it, life is a property of an [[ecosystem|ecological system]] rather than a single organism or species.<ref>Morowitz, Harold J. (1992) "Beginnings of Cellular Life: Metabolism Recapitulates Biogenesis". Yale University Press. ISBN 0-300-05483-1</ref> He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. [[Robert Ulanowicz]] (2009) also highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.<ref>''A Third Window: Natural Life Beyond Newton and Darwin'', Templeton Foundation Press (2009) ISBN 1-59947-154-X</ref>

==Origin==
{{Main|Origin of life}}
{{for|religious beliefs about the creation of life|Creation myth}}
Evidence suggests that [[life on Earth]] has existed for about 3.7 [[1000000000 (number)|billion]] years.<ref>"[http://www.ucmp.berkeley.edu/exhibits/historyoflife.php History of life through time]". University of California Museum of Paleontology.</ref> All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple [[organic molecule]]s via pre-cellular life to protocells and metabolism. Many models fall into the "[[gene]]s-first" category or the "[[metabolism]]-first" category, but a recent trend is the emergence of hybrid models that combine both categories.<ref>Coveney, Peter V.; Philip W. Fowler. "Modelling biological complexity: a physical scientist's perspective". ''Journal of the Royal Society Interface''. 2005. 2 (4) pp. 267–280. {{Doi|10.1098/rsif.2005.0045}}</ref>

There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses:
* The [[Miller-Urey experiment]], and the work of [[Sidney W. Fox|Sidney Fox]], suggest that conditions on the primitive Earth may have favored chemical reactions that synthesized some [[amino acid]]s and other [[organic compounds]] from inorganic precursors.
* [[Phospholipid]]s spontaneously form [[lipid bilayer]]s, the basic structure of a [[cell membrane]].

Life as we know it today synthesizes proteins, which are [[polymer]]s of amino acids using instructions encoded by cellular [[genes]]—which are polymers of [[deoxyribonucleic acid]] (DNA). [[Protein synthesis]] also entails intermediary [[ribonucleic acid]] (RNA) polymers. One possibility is that genes came first<ref>Senapathy, Periannan, Independent Birth of Organisms, Madison, WI. Genome Press, 1994.</ref> and then proteins. Another possibility is that proteins came first<ref>Eigen, Manfred, Steps Towards Life: A Perspective on Evolution (German edition, 1987), Oxford University Press, 1992. p 31.</ref> and then genes. However, because genes are required to make proteins, and proteins are required to make genes, the problem of considering which came first is like that of the [[chicken or the egg]]. Most scientists have adopted the hypothesis that because DNA and proteins function together so intimately, it's unlikely that they arose independently.<ref name="Barazesh, Solmaz 2009">Barazesh, Solmaz, How RNA Got Started: Scientists Look for the Origins of Life, Science News, May 13, 2009.</ref> Therefore, many scientists consider the possibility, apparently first suggested by [[Francis Crick]],<ref>Watson, James D., Prologue: Early Speculations and Facts about RNA Templates, p. xv-xxiii, The RNA World, R.F. Gesteland and J.F. Atkins, Eds., Cold Spring Harbor Laboratory Press, 1993.</ref> that the first life was based on the DNA-protein intermediary: [[RNA]].<ref name="Barazesh, Solmaz 2009"/> In fact, RNA has the DNA-like properties of information storage and replication and the [[catalysis|catalytic]] properties of some proteins. Crick and others actually favored the [[RNA world hypothesis|RNA-first hypothesis]]<ref>Gilbert, Walter, The RNA world, p 618 v 319, Nature, 1986.</ref> even before the catalytic properties of RNA had been demonstrated by [[Thomas Cech]].<ref>Cech, Thomas R., A model for the RNA-catalyzed replication of RNA, p 4360-4363 v 83, Proc. Nat. Acad. Sci., USA, 1986.</ref>

A significant issue with the RNA-first hypothesis is that experiments designed to synthesize RNA from simple precursors have not been nearly as successful as the Miller-Urey experiments that synthesized other organic molecules from inorganic precursors. One reason for the failure to create RNA in the laboratory is that RNA precursors are very stable and do not react with each other under ambient conditions. However, the successful synthesis of certain RNA molecules under conditions hypothesized to exist prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor [[phosphate]] present throughout the reaction.<ref>Powner, Matthew W., Béatrice Gerland and John D. Sutherland, Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions, Nature 459, 239-242 (14 May 2009).</ref> This study makes the RNA-first hypothesis more plausible to many scientists.<ref>Szostak, Jack W., Origins of life: Systems chemistry on early Earth, Nature 459, 171-172 (14 May 2009).</ref>

Recent experiments have demonstrated true [[Darwinian evolution]] of unique RNA enzymes ([[ribozymes]]) made up of two separate catalytic components that replicate each other ''in vitro''.<ref>Lincoln, Tracey A. and Gerald F. Joyce, Self-Sustained Replication of an RNA Enzyme, Science 27 February 2009: Vol. 323, No. 5918, pp. 1229–1232, DOI: 10.1126/science.1167856.</ref> In describing this work from his laboratory, [[Gerald Joyce]] stated: "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."<ref>Joyce, Gerald F., Evolution in an RNA World, Cold Spring Harb Symp Quant Biol sqb.2009.74.004; Published in Advance August 10, 2009, doi:10.1101/sqb.2009.74.004.</ref> Such experiments make the possibility of a primordial ''RNA World'' even more attractive to many scientists.

==Conditions for life==
The diversity of life on Earth today is a result of the dynamic interplay between [[Genetics#Genetic change|genetic opportunity]], metabolic capability, [[environment (biophysical)|environmental]] challenges,<ref name=astrobiology>{{Cite web |url=http://astrobiology.arc.nasa.gov/roadmap/g5.html |title=Understand the evolutionary mechanisms and environmental limits of life |accessdate=2009-07-13 |last= Rothschild |first=Lynn |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 |url=http://www.springerlink.com/content/n10p775113175l67/ |accessdate=2010-02-22}}</ref><ref>{{Cite book |last= Margulis |first=Lynn |title=The Symbiotic Planet: A New Look at Evolution |publisher=Orion Books Ltd. |year=2001 |location=London, England |isbn=0-75380-785-8}}</ref><ref>{{Cite book |author=Douglas J. Futuyma |coauthor=Janis Antonovics |title=Oxford surveys in evolutionary biology: Symbiosis in evolution |publisher=Oxford University Press |year=1992 |volume=8 |location=London, England |pages=347–374 |isbn= 0-19-507623-0}}</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 such microbial activities on a [[geologic time scale]], the physical-chemical environment on Earth has been changing, thereby determining 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 fundamental, global changes in the Earth's environment. The altered environment, in turn, posed novel evolutionary challenges to the organisms present, which ultimately resulted in the formation of our planet's major animal and plant species. Therefore this "co-evolution" between organisms and their environment is apparently an inherent feature of living systems.<ref name=astrobiology/>

===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]], [[temperature]], [[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 |accessdate=2009-07-14 |publisher=CMEX-NASA}}</ref> In most ecosystems the conditions vary during the day and often shift from one [[season]] to the next. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called "range of tolerance."<ref name=tolerance>{{Cite book |last=Chiras |first=Daniel C. |title=Environmental Science – Creating a Sustainable Future |year=2009}}</ref> Outside of that are the "zones of physiological stress," where the survival and reproduction are possible but not optimal. Outside of these zones are the "zones of intolerance," where life for that organism is implausible. It has been determined that organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.<ref name=tolerance/>

===Extremophiles===
[[File:Deinococcus radiodurans.jpg|thumb|''[[Deinococcus radiodurans]]'' can resist [[Radioresistance|radiation exposure]].]]
{{Main|Extremophile}}
To survive, some microorganisms can assume forms that enable them to withstand [[psychrophile|freezing]], [[xerophile|complete desiccation]], [[oligotroph|starvation]], high-levels of [[radioresistance|radiation exposure]], and other physical or chemical challenges. Furthermore, some microorganisms can survive exposure to such conditions for weeks, months, years, or even centuries.<ref name= astrobiology/> [[Extremophiles]] are microbial life forms that thrive outside the ranges life is commonly found in. They also excel at exploiting uncommon sources of energy. While all organisms are composed of nearly identical [[molecules]], evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. Characterization of the [[morphology (biology)|structure]] and metabolic diversity of microbial communities in such [[extreme environment]]s is ongoing. An understanding of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, will provide a critical foundation for the search for [[extraterrestrial life|life beyond Earth]].<ref name=astrobiology/>

===Chemical element requirements===
All life forms require certain core [[chemical element|chemical elements]] needed for [[biochemistry|biochemical]] functioning.  This list of core life elements usually includes carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the "Big Six" elemental [[nutrient|macronutrients]] for all organisms<ref name=wsj20101203>
[http://online.wsj.com/article/SB10001424052748703377504575650840897300342.html?mod=ITP_pageone_1#printMode New Link in Chain of Life], ''[[Wall Street Journal]]'', 2010-12-03, accessed 2010-12-05. ''"Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA."''</ref>
—often represented by the [[Acronym and initialism|acronym]] CHNOPS.  Together these make up nucleic acids, proteins and lipids, the bulk of living matter.  

However, alternative [[hypothetical types of biochemistry]] have been proposed which eliminate one or more of these elements, swap out an element for one not on the list, or change required [[chirality|chiralities]] or other chemical properties.  For example, the recently discovered [[GFAJ-1]] [[bacteria]] in [[Mono Lake]], [[California]] apparently is able to substitute [[arsenic]] for [[phosphorus]], which is [[toxic]] to most forms of life.<ref>{{Cite news |title=NASA reveals bacteria that can live on arsenic instead of phosphorus |publisher=The Guardian |date=2010-12-2 |accessdate=2010-12-2 |url= http://www.guardian.co.uk/science/2010/dec/02/nasa-bacteria-arsenic-phosphorus}}</ref><ref name="Science">{{cite journal|author=Felisa Wolfe-Simon, ''et al''.| year=2010 |doi=10.1126/science.1197258|journal=[[Science (journal)|Science]]|title=A bacterium that can grow by using arsenic instead of phosphorus}}</ref>

==Classification of life==
{{Main|Biological classification}}
{{Biological classification}}
Traditionally, people have divided organisms into the classes of [[plants]] and [[animals]], based mainly on their ability of movement. The first known attempt to classify organisms was conducted by the Greek philosopher [[Aristotle]] (384–322 BC). He classified all living organisms known at that time as either a plant or an animal. Aristotle distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of [[vertebrate]]s and [[invertebrate]]s respectively. He divided the blooded animals into five groups: viviparous quadrupeds ([[mammal]]s), [[bird]]s, oviparous quadrupeds ([[reptile]]s and [[amphibian]]s), [[fish]]es and [[Cetacea|whales]]. The bloodless animals were also divided into five groups: [[cephalopod]]s, [[crustacean]]s, insects (which also included the [[spider]]s, [[scorpion]]s, and [[centipede]]s, in addition to what we now define as [[insect]]s), shelled animals (such as most [[mollusc]]s and [[echinoderm]]s) and "[[zoophyte]]s." Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.<ref>{{Cite news |title=Aristotle -biography |publisher=University of California Museum of Paleontology |url=http://www.ucmp.berkeley.edu/history/aristotle.html |accessdate=2008-10-20}}</ref>

The exploration of the [[American continent]] revealed large numbers of new plants and animals that needed descriptions and classification. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification.

In the late 1740s, [[Carolus Linnaeus]] introduced his method, still used, to formulate the [[scientific name]] of every species.<ref>{{Cite journal |author=Knapp S, Lamas G, Lughadha EN, Novarino G |title= Stability or stasis in the names of organisms: the evolving codes of nomenclature |journal=Philosophical transactions of the Royal Society of London. Series B, Biological sciences |volume=359 |issue=1444 |pages=611–22 |date=April 2004 |pmid=15253348 |pmc=1693349 |doi=10.1098/rstb.2003.1445 |url=http://journals.royalsociety.org/openurl.asp?genre=article&issn=0962-8436&volume=359&issue=1444&spage=611}}</ref> Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and defining their meaning with an unprecedented precision. By consistently using his system, Linnaeus separated [[nomenclature]] from [[taxonomy]]. This convention for naming species is referred to as [[binomial nomenclature]].

The [[fungi]] were originally treated as plants. For a short period Linnaeus had placed them in the taxon [[Vermes]] in Animalia. He later placed them back in Plantae. [[Herbert Copeland|Copeland]] classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledged their special status.<ref name=Copeland1938>{{Cite journal |author=Copeland, H.F. |year=1938 |title=The Kingdoms of Organisms |journal=Quarterly Review of Biology |volume=13 |issue=4 |pages=383 |doi=10.1086/394568 |url=http://links.jstor.org/sici?sici=0033-5770(193812)13%3A4%3C383%3ATKOO%3E2.0.CO%3B2-K}}</ref> The problem was eventually solved by [[Robert Whittaker|Whittaker]], when he gave them their own kingdom in his [[Kingdom (biology)#Five kingdoms|five-kingdom system]]. As it turned out, the fungi are more closely related to animals than to plants.<ref>{{Cite journal |author=Whittaker RH |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=863 |pages=150–60 |date=January 1969 |pmid=5762760 |doi=10.1126/science.163.3863.150}}</ref>

As new discoveries enabled us to study [[cell (biology)|cells]] and [[microorganism]]s, 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|Haeckel]] in his kingdom [[protista]], later the group of [[prokaryote]]s were split off in the kingdom [[Monera]], eventually this kingdom would be divided in two separate groups, the [[Bacteria]] and the [[Archaea]], leading to the [[Kingdom (biology)#Six kingdoms|six-kingdom system]] and eventually to the current [[three-domain system]].<ref name="Woese1990"/> The classification of eukaryotes is still controversial, with protist taxonomy especially problematic.<ref name="Adl 05">{{Cite journal |author=Adl SM, Simpson AG, Farmer MA, ''et al.'' |title=The new higher level classification of eukaryotes with emphasis on the taxonomy of protists |journal=J. Eukaryot. Microbiol. |volume=52 |issue=5 |pages=399–451 |year=2005 |pmid=16248873 |doi=10.1111/j.1550-7408.2005.00053.x}}</ref>

As [[microbiology]], [[molecular biology]] and [[virology]] developed, non-cellular reproducing agents were discovered, such as [[virus]]es and [[viroid]]s. Sometimes these entities are considered to be alive but others argue that viruses are not living organisms since they lack characteristics such as [[cell membrane]], [[metabolism]] and do not grow or respond to their environments. Viruses can however be classed into "species" based on their biology and genetics but many aspects of such a classification remain controversial.<ref>{{Cite journal |author=Van Regenmortel MH |title=Virus species and virus identification: past and current controversies |journal=Infection, genetics and evolution: journal of molecular epidemiology and evolutionary genetics in infectious diseases |volume=7 |issue=1 |pages=133–44 |date=January 2007 |pmid=16713373 |doi=10.1016/j.meegid.2006.04.002}}</ref>

Since the 1960s a trend called [[cladistics]] has emerged, arranging taxa in an [[phylogenetic tree|evolutionary or phylogenetic tree]]. It is unclear, should this be implemented, how the different codes will coexist.<ref>{{Cite journal |author=Pennisi E |title=Taxonomy. Linnaeus's last stand? |journal=Science |location=New York, N.Y. |volume=291 |issue=5512 |pages=2304–7 |date=March 2001 |pmid=11269295 |url= http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=11269295 |doi=10.1126/science.291.5512.2304}}</ref>

{{Biological systems}}

==Extraterrestrial life==
{{Main|Extraterrestrial life|astrobiology}}
[[File:Panspermie.svg|thumb|[[Panspermia]] hypothesis]]
[[Earth]] is the only planet in the universe known to harbor life. The [[Drake equation]], which relates the number of extraterrestrial civilizations in our galaxy with which we might come in contact, has been used to discuss the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation. Depending on those values, the equation may either suggest that life arises frequently or infrequently.

The region around a [[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 will survive. Stars more massive than the Sun have a larger habitable zone, but will remain on the main sequence for a shorter time interval during which life can evolve. Small [[red dwarf]] stars have the opposite problem, compounded with 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 possess the optimal conditions for Earth-like life to develop. The location of the star within a galaxy may also have an impact on the likelihood of life forming.

[[Panspermia]], also called exogenesis, is a [[hypothesis]] proposing that life originated elsewhere in the universe and was subsequently transferred to Earth in the form of [[spore]]s perhaps via [[meteorite]]s, [[comet]]s or [[cosmic dust]]. However, this hypothesis does not help explain the ultimate origin of life.

cT5D=[ is a total noob at life and he can never stop being a noob, unless he goes on a quest to find the replacement noob. This said, he is in 89RGT and will come and beg for you to take his place. STAY AWAY FROM HIMj[onifkutlyi¿'&:T{V

==See also==
{{col-begin}}
{{col-2}}
* [[Alpha taxonomy]]
* [[Artificial life]]
* [[Biological immortality]]
* [[Biology]]—the study of life
* [[Carbon-based life]]
* [[Cellular life]]
* [[Conway's Game of Life]]
* [[Entropy and life]]
* [[Evolutionary history of life]]
* [[Extraterrestrial life]]
* [[Extremophile]]
* [[Gaia hypothesis]]
* [[Genetics]]
* [[Genetic engineering]]
* [[Hierarchy of life]]
{{col-2}}
* [[Kingdom (biology)]]
* [[Life on Mars]]
* [[Meaning of life]]
* [[Nature]]
* [[Non-cellular life]]
* [[Organic life]]
* [[Organism]]
* [[Origin of life]]
* [[Personal life]]
* [[Phylogenetics]]
* [[Prehistoric life]]
* [[Prion]]
* [[Quality of life]]
* [[Silicon-based life]]
* [[Synthetic life]]

{{col-end}}

==References==
{{Reflist|2}}

==Further reading==
* Kauffman, Stuart. [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html The Adjacent Possible: A Talk with Stuart Kauffman]
* {{Cite journal|author=Nealson KH, Conrad PG |title=Life: past, present and future |journal=Philos. Trans. R. Soc. Lond., B, Biol. Sci. |volume=354 |issue=1392 |pages=1923–39 |year=1999 |month=December |pmid=10670014 |pmc=1692713 |doi=10.1098/rstb.1999.0532 |url=http://journals.royalsociety.org/content/7r10hqn3rp1g1vag/}}
* Walker, Martin G. [http://rationalphilosophy.net/index.php/the-book ''LIFE! Why We Exist...And What We Must Do to Survive''] Dog Ear Publishing, 2006, ISBN 1-59858-243-7

==External links==
{{Commons category|Tree of life}}
{{Wikiquote}}
{{Wiktionary|life|living}}
{{wikispecies|Main Page|The Taxonomy of Life}}
* [http://species.wikimedia.org/wiki/Main_Page Wikispecies] - a free directory of life
* [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html "The Adjacent Possible: A Talk with Stuart Kauffman"]
* [http://plato.stanford.edu/entries/life/ Stanford Encyclopedia of Philosophy entry]
* [http://www.larger-than-life.org/modules.php?name=Content&pa=showpage&pid=2 Life under extreme conditions] An in depth look at how life can form under the most extreme conditions and circumstaces.

{{Nature nav}}
{{biological organisation}}
{{evolution}}
{{Taxonomic ranks}}

[[Category:Life| ]]
[[Category:Biology]]
[[Category:Biological systems]]

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