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[[File:The Earth seen from Apollo 17.jpg|thumb|right|300px|[[Earth]] is the only planet in the [[Universe]] from which life is known; the cradle and home of [[Homo sapiens|humanity]], and of all known forms of life.]]
[[File:Ruwenpflanzen.jpg|thumb|[[Plants]] in [[Ruwenzori Park]], [[Uganda]].]] 
'''Life''' is a [[biological]] concept regarding the characteristics, state, or mode that separates a living thing from [[dead]] matter.
The word itself may refer to a living being or ongoing processes of which living things are a part of. It may also refer to the period during which something is functional (as between birth and death), the condition of an entity that has been born but yet has to die or that which makes a living thing alive.

The study of life is called [[biology]], and people who study life are called biologists. A lifespan is the [[average]] length of life in a species. Most life on [[Earth]] is powered by solar energy, the only known exceptions being the chemosynthetic bacteria living around the hydrothermal vents on the ocean floor. All life on Earth is based on the [[chemistry]] of [[carbon]] compounds, specifically involving long-chain molecules such as [[protein]]s and [[nucleic acid]]. With [[water]], which is essential, the long molecules are wrapped inside [[Cell membrane|membranes]] to form [[cell]]s. This may or may not be true of all possible forms of life in the [[Universe]]: it is true of all life on Earth today.

==Summary==
{{Life timeline}}
Living organisms can be explained as [[Systems theory|open systems]]. They are always changing, because they exchange [[material]]s and [[information]] with their [[environment]]. They undergo [[metabolism]], maintain [[homeostasis]], possess a capacity to [[cell growth|grow]], respond to [[stimulus|stimuli]] and [[Reproduction|reproduce]].

Through [[natural selection]], they adapt to their environment in successive generations. More complex living organisms can communicate through various means.<ref name=Koshland>{{Cite journal |title=The Seven Pillars of Life |journal=Science |date=22 March 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=25 May 2009 |pmid=11910092}}</ref><ref name=Chambers>{{cite encyclopedia |encyclopedia=Chambers 21st Century Dictionary |edition=online |year=1999 |title=organism}}</ref> Many life forms can be found on [[Earth]]. The properties common to these organisms—[[plant]]s, [[animal]]s, [[fungus|fungi]], [[protist]]s, [[archaea]], and [[bacteria]]—are a carbon and water-based [[cell (biology)|cellular]] form with complex [[organization]] and [[heritable]] [[genetics|genetic]] information.

At present, the [[Earth]] is the only planet humans have detailed information about. The question of whether life exists elsewhere in the Universe is open. There have been a number of claims of life elsewhere in the Universe. None of these have been confirmed so far. The best evidence of life outside of Earth is are nucleic acids that have been found in certain types of meteorites.<ref>{{cite journal | url = http://astrobiology.gsfc.nasa.gov/analytical/PDF/Martinsetal2008.pdf |quotes= |author=Zita Martins |authorlink= |coauthors=Oliver Botta, Marilyn L. Fogel, Mark A. Sephton, Daniel P. Glavin, Jonathan S. Watson, Jason P. Dworkin, Alan W. Schwartz, Pascale Ehrenfreund |year=2008 |title=Extraterrestrial nucleobases in the Murchison meteorite |journal=Earth and Planetary Science Letters |volume=270 |issue= |pages=130-136 |id= |accessdate=2015-03-10}}</ref>

==Definitions of life==
One explanation of life is called the [[cell theory]]. The cell theory has three basic points: all living things are made up of cells. The cell is the smallest living thing that can do all the things needed for life. All cells must come from pre-existing cells.

Something is often said to be alive if it:
* grows,
* takes in [[food]], uses the [[food]] for [[energy]], and passes [[waste]] products (see [[metabolism]]),
* [[Motion (physics)|moves]]: it must either move itself, or have movement inside itself,
* [[Reproduction|reproduces]], either [[sexual reproduction|sexually]] (with another living thing) or [[asexual reproduction|asexually]], by creating [[wikt:copy|copies]] of itself,
* [[wikt:reaction|reacts]] to its surroundings,
* [[wikt:function|functions]]

However, not all living things fit every point on this list.
* [[mule|Mules]] cannot reproduce, and neither can worker ants.
* [[virus (biology)|Viruses]] and spores are not actively alive (metabolising) until the conditions are right.

They do, however, fit the biochemical definitions: they are made of the same kind of chemicals.

The [[thermodynamics|thermodynamic]] definition of life is any system which can keep its [[Thermodynamic entropy|entropy]] levels below maximum (usually through [[adaptation]] and [[mutation|mutations]]).

===A modern approach===
A modern definition was given by [[Humberto Maturana]] and [[Francisco Varela]] in 1980,<ref>Maturana H. & Varela F. 1980. ''Autopoiesis and cognition: the realization of the living''. Boston: Reidel.</ref> to which they gave the name ''[[autopoiesis]]'':
#The production of their own components
#The correct assembly of these components
#Continuous repair and maintenance of their own existence.
Roth commented that "In short, organisms are self-reproducing and self-maintaining, or 'autopoietic', systems".<ref>Roth G. 2013. ''The long evolution of brains and minds''. Heidelberg: Springer. p41, 48</ref> This approach makes use of [[molecular biology]] ideas and [[systems science]] ideas.

==Gallery of images of life==
<center>
<gallery>
Image:Linde von linn.jpg|A [[tree]] is an example of a [[plant]]
Image:Port Ghalib march 2006-0108.jpg|[[Fish]] are examples of [[Marine biology|marine]] life
Image:Adult citrus root weevil, Diaprepes abbreviatus.jpg|An Adult citrus root weevil is an example of an [[insect]]
Image:Salmonella typhimurium.png|''[[Salmonella]] typhimurium'' is an example of [[bacteria]]
Image:Chromista collage 2.jpg|[[Chromalveolate]]s are a group of [[protista]]
Image:Amanita muscaria tyndrum.jpg|''[[Amanita muscaria]]'' (Fly agaric) is an example of [[fungi]]
Image:Fungoid Frog.JPG|The Fungold [[frog]] is an example of an [[amphibian]]
Image:Cyanocitta-cristata-004.jpg| A [[Blue Jay]] is an example of a [[bird]]
Image:Djola-2006.jpg|[[Human]]s are an example of people
</gallery>
</center>
{{LifeOnEarth}}

==Related pages==
* [[Artificial life]]
* [[Biology]]
* [[Birth]]
* [[Death]]
* [[Earliest known life forms]]
* [[Evolution]]
* [[Tree of life (biology)|Tree of life]]

==References==
{{Reflist}}

[[Category:Life]]