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:''For other uses, see [[Life (disambiguation)|Life]] and [[Living (disambiguation)|Living]]'' |
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'''Life''' is a multi-faceted concept. ''Life'' may refer to the ongoing process of which living things are a part; the period between the [[conception]] (or a point at which the entity can be considered to be an individualized being) and [[death]] of an [[organism]]; the condition of an entity that has been born (or reached the point in its existence at which it can be established to be alive) and has yet to die; and that which makes a living thing alive. |
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[[Image:Baby goats.jpg|thumb|right|300px|"Life is about not knowing, having to change, taking the moment and making the best of it, without knowing what's going to happen next. Delicious Ambiguity." - [[Gilda Radner]]]] |
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==Defining the concept of life== |
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How can one tell when an entity is a lifeform? It would be relatively straightforward to offer a practical set of guidelines if one's only concern were life on [[Earth]] as we know it (see [[biosphere]]), but as soon as one considers questions about life's [[Origin of life (disambiguation)|origin]]s on Earth, or the possibility of extraterrestrial life, or the concept of [[artificial life]], it becomes clear that the question is fundamentally difficult and comparable in many respects to the problem of defining [[intelligence (trait)|intelligence]]. Also, loosely speaking, some theories are grounded in the basic assumption that [[Meme|"ideas have a life of their own"]]. |
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===A conventional definition=== |
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{{unsolved|biology|How did life start? Is life a cosmic phenomenon? Are the conditions necessary for the origin of life narrow or broad? How did life originate and diversify in hundreds of millions of years? Why, after rapid diversification, do microorganisms remain unchanged for millions of years? Why have so many biological systems developed sexual reproduction? How do organisms recognize like species? How are the sizes of cells, organs, and bodies controlled?}} |
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In [[biology]], a lifeform has traditionally been considered to be a member of a population whose members can exhibit all the following phenomena at least once during their [[existence]]: |
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#[[Growth]] |
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#[[Metabolism|Metabolism]], consuming, transforming and storing [[energy]]/[[mass]]; growing by absorbing and reorganizing mass; excreting waste |
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#Motion, either moving itself, or having internal motion |
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#[[Reproduction|Reproduction]], the ability to create entities that are similar to, yet separate from, itself |
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#Response to [[stimulus|stimuli]] - the ability to measure properties of its surrounding [[natural environment|environment]], and act upon certain conditions. |
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===Exceptions to the conventional definition=== |
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These criteria are not without their uses, but their disparate nature makes them unsatisfactory from a number of perspectives; in fact, it is not difficult to find counterexamples and examples that require further elaboration. For example, according to the above definition, one could say: |
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*[[Mule]]s and people who are [[infertility|infertile]] cannot reproduce and thus would not qualify as lifeforms. Also worker bees and other organisms living in colonies would not qualify; only the queen and the drones (or the whole colony) can be considered 'alive'. |
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*[[Fire]] and [[Star]]s could be considered lifeforms. |
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*A [[virus (biology)|virus]] does not grow and cannot reproduce outside of a host cell and thus would not qualify as a lifeform. |
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Many individual organisms are incapable of reproduction and yet are still considered to be lifeforms; see [[mule]]s and [[ant]]s for examples. This is because the term "lifeform" applies on the level of entire [[species]] or of individual [[gene]]s. (For example, see ''[[kin selection]]'' for information about one way by which non-reproducing individuals can still enhance the spread of their genes and the survival of their species.) It is important to keep in mind the difference between a "lifeform" and "a being that is alive." One example of [[sterile|sterility]] does not render the rest of the species a non-lifeform, any more than one dead animal renders the rest of the species dead. |
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Note also that the two cases of fire and stars fitting the definition of life can be simply remedied by defining metabolism in a more biochemically exact way. ''[[Fundamentals of Biochemistry]]'' by [[Donald Voet]] and [[Judith Voet]] (ISBN 0471586501) defines metabolism as follows: "Metabolism is the overall process through which living systems acquire and utilize the free energy they need to carry out their various functions. They do so by coupling the exergonic reactions of nutrient oxidation to the endergonic processes required to maintain the living state, such as the performance of mechanical work, the active transport of molecules against concentration gradients, and the biosynthesis of complex molecules." This definition, in use by most biochemists, makes it clear that fire is not alive, because fire releases all the oxidative energy of its fuel as heat. |
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This could also be remedied by adding the requirement of locality, where there is an obvious feature that delineates the spatial extension of the living being, such as a [[cell membrane]] (although this would then discount [[fungus|fungi]], and [[grass]]es from being life forms). |
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A conceptual problem with saying that fire is life is that it collapses the distinction between "growth" and "reproduction." It is possible to think of a spreading flame as either growing or reproducing, but what would it mean to say that the same act is ''both'' growth and reproduction? (Although it is colloquial to speak of flames "getting bigger," there are no discernable "generations" of flames of which to speak.) |
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Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and proto-life, consisting of metabolizing cells without reproduction apparatus, can have existed. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered a lifeform. |
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===Other definitions=== |
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[[Biologist]]s who are content to focus on terrestrial [[organism]]s often note some additional signs of life, including these: |
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# Living organisms contain [[molecular]] components such as: [[carbohydrate]]s, [[lipid]]s, [[nucleic acid]]s, and [[protein]]s. |
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# Living organisms ''require'' both energy and matter in order to continue living. |
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# Living organisms are composed of at least one [[cell (biology)|cell]]. |
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# Living organisms maintain [[homeostasis]] for some period of time. |
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# [[Species]] of living organisms will [[evolution|evolve]]. |
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All life on Earth is based on the [[chemistry]] of [[carbon]] compounds. Some assert that this must be the case for all possible forms of life throughout the universe; others describe this position as '[[carbon chauvinism]]'. |
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The [[Systems Theory|systemic]] definition is that living things are self-organizing and [[autopoiesis|autopoietic]] (self-producing). These objects are not to be confused with [[Dissipative system|dissipative structures]] (e.g. fire). Variations of this definition include: |
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*[[Francisco Varela]] and [[Humberto Maturana]]'s definition of life (also widely used by [[Lynn Margulis]]) as an [[autopoiesis|autopoietic]] (self-producing), [[water]] based, [[lipid]]-[[protein]] bound, [[carbon]] metabolic, [[nucleic acid]] replicated, protein readout [[system]] |
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*"a system of inferior negative feedbacks subordinated to a superior positive [[feedback]]" ([http://www.mol.uj.edu.pl/~benio/cyber_def_life.pdf J. theor Biol. 2001]) |
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*Tom Kinch's definition of life as a highly organized auto-cannibalizing system naturally emerging from conditions common on planetary bodies, and consisting of a population of replicators capable of mutation, around each set of which a homeostatic metabolizing organism, which actively helps reproduce and/or protect the replicator(s), has evolved |
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*[[Stuart Kauffman]]'s definition of life as an [[agent|autonomous agent]] or a [[multi-agent system]] capable of reproducing itself or themselves, and of completing at least one [[thermodynamic work cycle]] |
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*[[Robert Pirsig]]'s definition of life, found in his book ''[[Lila: An Inquiry into Morals]]'', as that which maximizes its range of possible futures, in other words, that which makes decisions that result in the most future choices, or that which strives to keep its options open. |
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* A [[system]] converting [[entropy]] to [[negentropy]], using flow of [[energy]]. |
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<!-- (this approach seems to assume the organism is walking a [[decision tree]] --question: can [[decision making]] happen in a [[deterministic system]]? --> |
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Other definitions: |
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* That which seeks to continue its own existence (attributed to Clifford A. Schaffer). |
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===Descent with modification: a "useful" characteristic=== |
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A useful characteristic upon which to base a definition of life is that of [[common descent|descent]] with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to [[chance]]. Descent with modification is sufficient by itself to allow [[evolution]], assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called [[genetics]]. In all known life forms (assuming [[prion]]s are not counted as such), the genetic material is primarily [[DNA]] or the related molecule, [[RNA]]. Another exception might be the [[software]] code of certain forms of [[computer virus|viruses]] and programs created through [[genetic programming]], but whether [[computer]] programs can be alive even by this definition is still a matter of some contention. |
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==Origin of life== |
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''Main article:'' [[Origin of life]] |
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There is no truly "standard" model of the origin of life, but most currently accepted models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence: |
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#Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the [[Miller experiment|Urey-Miller experiment]]. |
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#[[Phospholipid]]s spontaneously form [[lipid bilayer]]s, the basic structure of a [[cell membrane]]. |
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#Procedures for producing random [[RNA]] molecules can produce [[ribozyme]]s, which are able to produce more of themselves under very specific conditions. |
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There are many different hypotheses regarding the path that might have been taken from simple organic molecules 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 do not fit into either of these categories. |
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==The possibility of extraterrestrial life== |
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''Main articles:'' [[Extraterrestrial life]], [[Xenobiology]] |
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[[As of 2005]], [[Earth]] is the only planet in the [[universe]] known by humans to support life. The question of whether life exists elsewhere in the universe remains open, but analyses such as the [[Drake equation]] have been used to estimate the probability of such life existing. There have been a number of claims of the discovery of life elsewhere in the universe, but none of these have yet survived scientific scrutiny. |
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Today, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible [[bacterium|bacteria]]l life on [[Mars (planet)|Mars]] (via the [[ALH84001]] meteorite). Searches for extraterrestrial life are currently focusing on planets and moons believed to possess liquid water, at present or in the past. Recent evidence from the NASA rovers [[MER-A|Spirit]] and [[MER-B|Opportunity]] supports the theory that Mars once had surface water. See ''[[Life on Mars]]'' for further discussion. |
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[[Jupiter (planet)|Jupiter]]'s moons are also considered good candidates for extraterrestrial life, especially [[Europa (moon)|Europa]], which seems to possess oceans of liquid water. |
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Other highly (and somewhat doubtful) speculative places for present or past life include atmosphere of [[Venus]], [[Titan (moon)|Titan]] cyrovolcanoes, or even [[Enceladus]]. |
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==See also== |
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* [[Animal]] |
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* [[Artificial life]] |
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* [[Bacteria]] |
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* [[Biology]] |
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* [[Fungi]] |
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* [[Kingdom_%28biology%29|Biological kingdom]] |
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* [[Biological life cycle]] |
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* [[Monera]] |
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* [[Odic force]] |
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* [[Origin of life (disambiguation)]] |
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* [[Plant]] |
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* [[Prehistoric life]] |
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* [[Protista]] |
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==References== |
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*Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html] |
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==External links== |
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{{define}} |
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*[http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html "The Adjacent Possible: A Talk with Stuart Kauffman"] |
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*[http://www.angelfire.com/linux/vjtorley/ Animals and other living things: their interests, mental capacities and moral entitlements] |
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*[http://tolweb.org/tree?group=life Tree of Life Web Project - Life on Earth] |
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*[http://plato.stanford.edu/entries/life/ Stanford Encyclopedia of Philosophy entry] |
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*[http://people.cornell.edu/pages/tg21/DHB.html The Deep Hot Biosphere Theory (Thomas Gold)] |
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[[Category:Life| ]] |
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[[Category:Biology]] |
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[[ca:Vida]] |
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[[da:Liv (biologi)]] |
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[[de:Leben (Biologie)]] |
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[[eo:Vivo (biologio)]] |
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[[es:Vida]] |
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[[fi:Elämä]] |
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[[fr:Vie]] |
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[[ko:생명]] |
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[[he:חיים]] |
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[[it:Vita (biologia)]] |
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[[la:Vita]] |
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[[ms:Benda hidup]] |
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[[nl:Leven]] |
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[[no:liv]] |
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[[pl:Życie]] |
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[[pt:Vida]] |
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[[ru:Жизнь]] |
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[[simple:Life]] |
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[[sk:Život]] |
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[[sl:Življenje]] |
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[[sv:liv]] |
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[[uk:Життя]] |
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[[zh:生命]] |
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