Life
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{{two other uses|life in general|life on Earth|Organism|other meanings of "life"|Life (disambiguation)}}
{{dablink|For different meanings of related words, see [[Lives]], [[Live]], [[Live!]], [[Living]], [[Alive]].}}
{{Taxobox | color = limegreen
| name = Life
| image = Waitakere Piha n.jpg
| image_width = 250px
| image_caption = '''Life''' on a rocky peak in the [[Waitakere Ranges]]
| unranked_classis = '''Life (''[[Biota (taxonomy)|Biota]]'')'''
|subdivision_ranks = [[Domain (biology)|Domain]]s and [[Kingdom (biology)|Kingdom]]s
| subdivision =
*[[Life on Earth]] (''Gaeabionta'')
**[[Nanobes]] [[non-life|<span title="Whether nanobes should be considered as life is disputed."><sup>?</sup></span>]]
**[[Nanobacterium]] [[non-life|<span title="Whether nanobacteria are alive is disputed."><sup>?</sup></span>]]
**[[Non-cellular life|Acytota]] [[Paraphyly|<span title="Acytota 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="Acytota 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>]][[non-life|<span title="Whether viruses and other similar forms should be considered as life is disputed."><sup>?</sup></span>]]
**[[Cellular life|Cytota]]
***[[Bacteria]] [[Paraphyly|<span title="Bacteria may be paraphyletic: Cavalier-Smith has recently proposed that Neomura evolved from Bacteria.">*</span>]]
***[[Neomura]]
****[[Archaea]]
****[[Eukaryota]]
*****[[Bikonta]]
******[[Apusozoa]]
******[[Rhizaria]]
******[[Excavata]]
******[[Archaeplastida]]
*******[[Rhodophyta]]
*******[[Glaucophyta]]
*******'''[[Plantae]]'''
******[[Heterokontophyta]]
******[[Haptophyta]]
******[[Cryptophyta]]
******[[Alveolata]]
*****[[Unikonta]]
******[[Amoebozoa]]
******[[Opisthokonta]]
*******[[Choanozoa]]
*******[[Fungi]]
*******[[Animalia]]
*[[Extraterrestrial life]] (hypothetical)
}}
'''Life''' is a condition that distinguishes [[organisms]] from non-living objects, such as [[non-life]], and [[Death|dead]] organisms, being manifested by growth through [[metabolism]] and [[reproduction]]. Some living things can [[communicate]] and many can [[adapt]] to their environment through changes originating internally. A [[physics|physical]] characteristic of life is that it feeds on [[Negentropy|negative entropy]].<ref>{{cite book | last = Schrödinger | first = Erwin | title = What is Life? | publisher = Cambridge University Press | year = 1944 | id = ISBN 0-521-42708-8}}</ref><ref>{{cite book | last = Margulis | first = Lynn | coauthors = Sagan, Dorion | title = What is Life? | publisher = University of California Press | year = 1995 | id = 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 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 | id = ISBN 0-19-286218-9}}</ref><ref>{{cite book | last = Avery | first = John | title = Information Theory and Evolution | publisher = World Scientific | year = 2003 | id = ISBN 9812383999}}</ref>
A diverse array of living organisms can be found in the [[biosphere]] on Earth. 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. They undergo [[metabolism]], possess a capacity to grow, respond to [[stimuli]], [[reproduce]] and, through [[natural selection]], adapt to their environment in successive generations.
An entity with the above properties is considered to be a ''living'' [[organism]], that is an organism that is alive hence can be called a life form. However, not every definition of life considers all of these properties to be essential. For example, the capacity for descent with modification is often taken as the only essential property of life. This definition notably includes [[virus]]es, which do not qualify under narrower definitions as they are [[acellular]] and do not metabolize. <!--Unsourced statements, and wet alife go to unsourced article. Software to study processes of life is not life or artificial life, theoretical life is NOT alive: Broader definitions of life may also include theoretical [[alternative biochemistry|non-carbon-based life]] and other [[alternative biology]]. Some forms of [[artificial life]], however, especially [[wet artificial life]], might alternatively be classified as real life.{{fact}} -->
==Definitions==
There is no universal definition of life; there are a variety of definitions proposed by different scientists. To define life in unequivocal terms is still a challenge for scientists.<ref>[http://www.astrobio.net/news/article226 Defining Life :: Astrobiology Magazine - earth science - evolution distribution Origin of life universe - life beyond :: Astrobiology is study of earth science evolution distribution Origin of life in universe terrestrial<!-- Bot generated title -->]</ref><ref>[http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html Defining Life, Explaining Emergence<!-- Bot generated title -->]</ref>
'''Conventional definition''': Often scientists say that life is a characteristic of organisms that exhibit the following phenomena:
#'''[[Homeostasis]]''': Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
#'''Organization''': Being composed of one or more [[cell (biology)|cell]]s, which are the basic units of life.
#'''Metabolism''': Consumption of [[energy]] by converting nonliving material into cellular components ([[anabolism]]) and decomposing organic matter ([[catabolism]]). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
#'''[[Cell growth|Growth]]''': Maintenance of a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
#'''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 stimuli''': A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun or an animal chasing its prey.
#'''Reproduction''': The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either [[asexual reproduction|asexually]], from a single parent organism, or [[sexual reproduction|sexually]], from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
[[Image:Hoh rain forest trees.jpg|right|thumb|250px|[[Plant]] life.]]
[[Image:Herds Maasi Mara (cropped and straightened).jpg|right|thumb|250px|Herds of zebra and impala gathering on the [[Masai Mara]] plain]]
[[Image:Nwhi - French Frigate Shoals reef - many fish.jpg|right|thumb|250px|Marine life around a [[coral reef]].]]
However, others cite several limitations of this definition.{{Fact|date=April 2008}}<!-- Removed the reference to a forum as such are not considered valid to Wikipedia --> Thus, many members of several species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence, sterile or hybrid organisms such as [[mule]]s, [[liger]]s, and [[eunuch]]s are alive although they are not capable of self-reproduction. However, (a) The species as a whole does reproduce, (b) There are no cases of species where 100% of the individuals reproduce, and (c) specialized non-reproducing individuals of the species may still partially propagate their DNA or other master pattern through mechanisms such as [[kin selection]].
Viruses and aberrant [[prion]] proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. Also, the [[Rickettsia]] and [[Chlamydia (bacterium)|Chlamydia]] are examples of [[bacteria]] that cannot independently fulfill many vital biochemical processes, and depend on entry, growth, and replication within the [[cytoplasm]] of [[eukaryotic]] host cells. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.
The [[Systems Theory|systemic]] definition of life 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 [[Stuart Kauffman]]'s definition of life 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]].
'''Proposed definitions of life include:'''
#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.<ref>Witzany, G. (2007). The Logos of the Bios 2. Bio-Communication. Helsinki, Umweb.</ref>
#Life (a living individual) is defined as a network of inferior negative feedbacks (regulatory mechanisms) subordinated to a superior positive feedback (potential of expansion, reproduction)<ref>[http://www.ncbi.nlm.nih.gov/pubmed/11312589 Korzeniewski, Bernard (2001). ''Cybernetic formulation of the definition of life.'' Journal of Theoretical Biology. 2001 Apr 7;209(3):275-86]</ref>
#Life is a [[characteristic]] of [[self-organizing]], self-recycling [[system]]s consisting of [[population]]s of [[replicator]]s that are capable of [[mutation]], around most of which [[homeostatic]], [[metabolizing]] organisms evolve.
#Type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate ''almost perfectly'' by using matter and energy available in their environment to which they may adapt. In this definition "almost perfectly" relates to mutations happening during replication of organisms that may have adaptive benefits.
#Life is a potentially self-perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselves produced by the open system.
==Origin of life==
{{Main|Origin of life}}
[[Image:Grand prismatic spring.jpg|thumb|right|250px|An aerial photo of microbial mats around the [[Grand Prismatic Spring]] of [[Yellowstone National Park]].]]
Although it cannot be pinpointed exactly, 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<!-- Bot generated title -->]</ref>
There is no truly "standard" model for the origin of life, but most currently accepted scientific models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:
#Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the [[Miller-Urey experiment]], and in the work of [[Sidney W. Fox|Sidney Fox]].
#[[Phospholipid]]s spontaneously form [[lipid bilayer]]s, the basic structure of a [[cell membrane]].
#Procedures for producing random [[RNA]] molecules can produce [[ribozyme]]s, which are able to produce more of themselves under very specific conditions.
#The [[Panspermia]] hypothesis proposes that life originated elsewhere in the universe and was subsequently transferred to Earth perhaps via [[meteorite]]s, [[comet]]s or [[cosmic dust]].
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 do not fit into either of these categories.<ref>[http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&doi=10.1098/rsif.2005.0045 Royal Society Publishing - Home<!-- Bot generated title -->]</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, as per personal observations, was conducted by the Greek philosopher [[Aristotle]].
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. His observations on the anatomy of octopus, cuttlefish, crustaceans, and many other marine invertebrates are remarkably accurate, and could only have been made from first-hand experience with dissection.<ref>http://www.ucmp.berkeley.edu/history/aristotle.html, references for this site are located [http://www.ucmp.berkeley.edu/history/ancientrefs.html here]</ref>
The exploration of parts of the [[New World]] produced large numbers of new plants and animals that needed descriptions and classification. The old systems made it difficult to study and locate all these new specimens within a collection and often the same plants or animals were given different names because the number of specimens were too large to memorize. A system was needed that could group these specimens together so they could be found, the binomial system was developed based on [[Morphology (biology)|morphology]] with groups having similar appearances. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification.
[[Carolus Linnaeus]] is best known for his introduction of the method still used to formulate the [[scientific name]] of every species. Before Linnaeus, long many-worded names (composed of a generic name and a ''differentia specifica'') had been used, but as these names gave a description of the species, they were not fixed. In his ''Philosophia Botanica'' (1751) Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetorics, introducing new descriptive terms and defining their meaning with an unprecedented precision. In the late 1740s Linnaeus began to use a parallel system of naming species with ''nomina trivialia.'' ''Nomen triviale'', a trivial name, was a single- or two-word epithet placed on the margin of the page next to the many-worded "scientific" name. The only rules Linnaeus applied to them was that the trivial names should be short, unique within a given genus, and that they should not be changed. Linnaeus consistently applied ''nomina trivialia'' to the species of plants in ''[[Species Plantarum]]'' (1st edn. 1753) and to the species of animals in the 10th edition of ''[[Systema Naturae]]'' (1758). By consistently using these specific epithets, Linnaeus separated [[nomenclature]] from [[taxonomy]]. Even though the parallel use of ''nomina trivialia'' and many-worded descriptive names continued until late in the eighteenth century, it was gradually replaced by the practice of using shorter proper names combined of the generic name and the trivial name of the species. In the nineteenth century, this new practice was codified in the first Rules and Laws of Nomenclature, and the 1st edn. of ''[[Species Plantarum]]'' and the 10th edn. of ''[[Systema Naturae]]'' were chosen as starting points for the [[International Code of Botanical Nomenclature|Botanical]] and [[International Code of Zoological Nomenclature|Zoological Nomenclature]] respectively. This convention for naming species is referred to as [[binomial nomenclature]]. Today, nomenclature is regulated by [[Nomenclature Codes]], which allows names divided into ranks; separately [[rank (botany)|for botany]] and [[rank (zoology)|for zoology]]. Whereas Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of [[common descent]].
The [[Fungi]] have long been a problematic group in the biological classification: Originally, they were treated as plants. For a short period Linnaeus had placed them in the taxon [[Vermes]] in Animalia because he was misinformed: the [[hypha]]e were said to have been [[worm]]s. He later placed them back in Plantae. [[Herbert Copeland|Copeland]] classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging their special status. 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.
As new discoveries enabled us to study [[cell (biology)|cells]] and [[microorganism]]s, new groups of life where 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 of 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 [[three-domain system]]. The 'remaining' protists would later be divided into smaller groups in [[clade]]s in relation to more complex organisms. [[Thomas Cavalier-Smith]], who has published extensively on the classification of protists, has recently proposed that the [[Neomura]], the clade which groups together the [[Archaea]] and [[Eukarya]], would have evolved from [[Bacteria]], more precisely from [[Actinobacteria]].
As [[microbiology]], [[molecular biology]] and [[virology]] developed, non-cellular reproducing agents were discovered, sometimes these are considered to be alive and are treated in the domain of [[non-cellular life]] named Acytota or Aphanobionta, which are [[virus]].
And thus all the primary [[taxonomy|taxonomical]] [[rank]]s were established: [[Domain (biology)|Domain]], [[Kingdom (biology)|Kingdom]], [[Phylum]], [[Class (biology)|Class]], [[Order (biology)|Order]], [[Family (biology)|Family]], [[Genus]], [[Species]]
Since the 1960s a trend called [[cladistics]] has emerged, arranging taxa in an [[phylogenetic tree|evolutionary or phylogenetic tree]]. If a [[taxon]] includes all the descendants of some ancestral form, it is called [[Monophyly|monophyletic]], as opposed to [[Paraphyly|paraphyletic]], groups based on traits which have evolved separately and where the [[most recent common ancestor]] is not included are called [[Polyphyly|polyphyletic]].
A new formal code of nomenclature, the [[PhyloCode]], to be renamed "International Code of [[Phylogenetic nomenclature|Phylogenetic Nomenclature]]" (ICPN), is currently under development, intended to deal with clades, which do not have set ranks, unlike conventional [[Linnaean taxonomy]]. It is unclear, should this be implemented, how the different codes will coexist.
{{Biological systems}}
==Extraterrestrial life==
{{Main|Extraterrestrial life|astrobiology}}
[[Earth]] is the only planet in the [[universe]] ''known'' to harbour life. The [[Drake equation]] has been used to estimate the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation (although strictly speaking Drake equation estimates relate the number of extraterrestrial civilizations in our galaxy with which we might come in contact - not probability of life elsewhere). Depending on those values, the equation may either suggest that life arises frequently or infrequently. Drake himself estimated the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time as equal to one.
Relating to the origin of life on Earth, [[panspermia]] and exogenesis are theories proposing that life originated elsewhere in the universe and was subsequently transferred to Earth perhaps via [[meteorite]]s, [[comet]]s or [[cosmic dust]]. For example, there is the [[meteorite]] [[ALH84001]]. However those theories do not help explain the origin of this extraterrestrial life.
==See also==
<div style="-moz-column-count:2; column-count:2;">
* [[Artificial life]]
* [[Kingdom (biology)|Biological kingdom]]
* [[Biology]], the scientific study of life
* [[Carbon-based life]]
* [[Cellular automaton]], a discrete model of an infinite, regular grid of ''cells''
* [[Cellular life]]
* [[Conway's Game of Life]], simple mathematical 'cellular automaton' that mimicks the dynamics of an ecosystem.
* [[Death]], the termination of life
* [[Ecological literacy]]
* [[Entropy and life]]
* [[Extraterrestrial life]]
* [[Extremophile]], organisms that live in so called 'extreme' conditions e.g. [[hydrothermal vents]]
* [[Gaia hypothesis]]
* [[Meaning of life]]
* [[Nature]], in the original meaning, it is strongly associated with life.
* [[Non-cellular life]]
* [[Non-life]]
* [[Organic life]]
* [[Organism]]
* [[Origin of life]]
* [[Personal life]]
* [[Phylogenetics]], is the study of evolutionary relatedness among [[species]]
* [[Prehistoric life]], life from before the human history started on Earth
* [[Quality of life]]
* [[Synthetic life]]
* [[Alpha taxonomy|Taxonomy]], the science of describing, categorising and naming organisms
* [[genetics]]
* [[genetic engineering]]
</div>
==References==
{{reflist}}
==Further reading==
*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]
* Walker, Martin G. ''LIFE! Why We Exist...And What We Must Do to Survive'' ([http://rationalphilosophy.net/the-book Book Page]) ([http://rationalphilosophy.net Web Site]), Dog Ear Publishing, 2006, ISBN 1-59858-243-7
==External links==
{{commonscat|Tree of life}}
{{wikiquote}}
{{wiktionarypar2|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.
{{Nature nav}}
{{Evolution}}
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