View source for Life

{{other uses}}
{{automatic taxobox
| name = Life
| fossil_range = {{Long fossil range|3770|0|earliest=4280}} [[Archean]] – [[Holocene|present]] (possible [[Hadean]] origin)
| image = Stegosaurus ungulatus.jpg
| image_caption = [[Stegosaurus]] in museum
| taxon = Life
| authority = 
| subdivision_ranks = [[Domain (biology)|Domains]] and [[Kingdom (biology)|Supergroups]]
| subdivision = Life on Earth:
* [[Non-cellular life]]
** [[Virus]]es{{refn |group="note" |Viruses are strongly believed not to descend from a common ancestor, with each [[Realm (virology)|realm]] corresponding to separate instances of a virus coming into existence.<ref name=exec >{{cite journal |author=International Committee on Taxonomy of Viruses Executive Committee |date=May 2020 |title=The New Scope of Virus Taxonomy: Partitioning the Virosphere Into 15 Hierarchical Ranks |journal=Nature Microbiology |volume=5 |issue=5 |pages=668–674 |doi=10.1038/s41564-020-0709-x |pmc=7186216 |pmid=32341570| issn = 2058-5276 }}</ref>}}
** [[Viroid]]s
* [[Cellular life]]
** Domain [[Bacteria]]
** Domain [[Archaea]]
** Domain [[Eukaryota]]
*** [[Archaeplastida]]
**** [[Plantae]]
*** [[SAR supergroup|SAR]]
*** [[Excavata]]
*** [[Amoebozoa]]
*** [[Opisthokont]]a
**** [[Holomycota]]
***** [[Fungi]]
**** [[Holozoa]]
***** [[Animalia]]
}}

'''Life''' is a concept in [[biology]]. It is about what separates a living thing from [[dead]] matter.

Most life on [[Earth]] is powered by solar energy: the only known exceptions are the chemo-synthetic bacteria living around the [[hydrothermal vent]]s on the ocean floor. All life on Earth is based on the [[chemistry]] of [[carbon]] compounds, involving long-chain [[molecules]] such as [[protein]]s and [[nucleic acid]]. With [[water]], which all life needs, the long molecules are wrapped inside [[Cell membrane|membranes]] as [[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 things, or 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 by 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 |access-date=25 May 2009 |pmid=11910092 |s2cid=153363768 |archive-date=28 February 2009 |archive-url=https://web.archive.org/web/20090228202918/http://sciencemag.org/cgi/content/full/295/5563/2215 |url-status=live }}</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.

The systems that make up life have many levels of organization. From smallest to biggest, they are: molecule, cell, [[tissue]] (group of cells with a common purpose), [[Organ (anatomy)|organ]] (part of the body with a purpose), [[organ system]] (group of organs that work together), organism, [[population]] (group of organisms of the same species), [[Community (ecology)|community]] (all of the organisms that [[wikt:interact|interact]] in an area), [[ecosystem]] (all of the organisms in an area and the non-living surroundings), and [[biosphere]] (all parts of the Earth that have life).{{R|Starr}}{{Rp|4-5}}

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 |last=Martins |first=Zita |others=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=1–2 |pages=130–136 |doi=10.1016/j.epsl.2008.03.026 |arxiv=0806.2286 |bibcode=2008E&PSL.270..130M |s2cid=14309508 |access-date=2015-03-10 |archive-date=2011-08-10 |archive-url=https://web.archive.org/web/20110810095031/http://astrobiology.gsfc.nasa.gov/analytical/PDF/Martinsetal2008.pdf |url-status=dead }}</ref>

==Definitions==
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]]s 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]]s).

===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.

== What life needs ==
=== Chemistry ===
{{See also|Biochemistry}}
Life on Earth is made from [[organic compound]]s—molecules that contain carbon. Four types of long-chain molecules ([[macromolecule]]s) are important: [[carbohydrate]]s, [[lipid]]s, [[protein]]s, and [[nucleic acid]]s.
* Simple carbohydrates (sugars) are used for [[energy]], or as a building block. Complex carbohydrates, like starch and cellulose, can keep energy for a long time. They are also used to make a strong structure, like a [[plant stem]].
* Lipids can be [[Thermal insulation|insulation]] to keep a living thing warm, such as fat on a [[penguin]], or to stop water from passing in or out, such as waterproof [[feather]]s. Two layers of phospholid (a kind of lipid) make up all [[cell membrane]]s. Some kinds of lipids are [[hormone]]s, which send messages from one cell to another.
* Proteins, long chains of [[amino acid]]s, have many purposes. They fold into complex shapes because their amino acids interact. Proteins are involved in many chemical reactions, to make them go faster.
* Nucleic acids, including [[DNA]] and [[RNA]], are long chains of [[nucleotide]]s. There are only four kinds of nucleotides in each chain, but they are the instructions for life, like a language. Each three nucleotides tell the cell to make one amino acid. One part of a nucleic acid is the [[wikt:code|code]] for one protein molecule.<ref name="Starr">{{Cite book|url=https://www.worldcat.org/oclc/57966041|title=Biology : concepts and applications|last=Starr|first=Cecie|date=2006|publisher=Thomson, Brooks/Cole|others=Christine A. Evers, Lisa Starr|isbn=0-534-46223-5|edition=6th|location=Belmont, CA|oclc=57966041}}</ref>{{rp|34-48}}

Almost all living things need the [[chemical element]]s carbon, [[hydrogen]], [[oxygen]], [[nitrogen]], [[sulfur]], and [[phosphorus]], to build these macromolecules.<ref name="wsj20101203">{{cite news |first1=Robert Lee |last1=Hotz |title=New link in chain of life |work=[[Wall Street Journal]] |date=3 December 2010 |publisher=Dow Jones & Company, Inc |url=https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |quote=Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA. |url-status=live |archive-url=https://web.archive.org/web/20170817163835/https://www.wsj.com/articles/SB10001424052748703377504575650840897300342?mod=ITP_pageone_1#printMode |archive-date=17 August 2017 |df=dmy-all }}</ref> Living things also need small amounts of other elements, called ''trace elements''.{{r|Starr}}{{rp|18-19}} Water is a very important part of all living things. For example, humans are about two-thirds water. Water is a [[solvent]] that lets molecules mix and react with other molecules.<ref>{{Cite web|date=2007-11-30|title=NASA - Water: The Molecule of Life An Interview with Philip Ball|url=https://www.nasa.gov/vision/universe/solarsystem/Water:_Molecule_of_Life.html|url-status=live|access-date=2021-03-29|website=www.nasa.gov|language=en |archive-url=https://web.archive.org/web/20210314180612/https://www.nasa.gov/vision/universe/solarsystem/Water:_Molecule_of_Life.html |archive-date=2021-03-14}}</ref>

=== Energy sources ===
All living things need energy to survive, move, grow, and reproduce. Some can get energy from the [[environment]] without help from other living things: these are called producers, or [[autotroph]]s. Plants, algae, and some bacteria, a group of producers called [[photoautotroph]]s, use the sun's light for energy. When producers use light to make and store organic compounds, this is called [[photosynthesis]].{{r|Starr}}{{rp|92-93}} Some other producers, called [[chemoautotroph]]s, get energy from chemicals that come out of the ocean floor in [[hydrothermal vent]]s.{{r|Starr}}{{rp|292}} Other living things get their energy from [[organic compound]]s: these are called consumers, or [[heterotroph]]s. Animals, fungi, most bacteria, and most protists are consumers. Consumers can eat other living things or dead material.{{r|Starr}}{{rp|92-93}}

Both producers and consumers need to break down organic compounds to free energy. The best way to do this is [[aerobic respiration]], which frees the most energy, but living things can only do aerobic respiration if they have oxygen (O2). They can also break down these compounds without oxygen, using [[anaerobic respiration]] or [[fermentation]].{{r|Starr}}{{rp|108-120}}

== Cells ==
{{Main|Cell (biology)}}

All living things have cells. Every cell has a cell membrane on the outside, and a jelly-like material that fills the inside, called [[cytoplasm]]. The membrane is important because it separates the chemicals inside and outside. Some molecules can pass through the membrane, but others cannot. Living cells have [[gene]]s, made of DNA. Genes say to the cell what to do, like a language. One DNA molecule, with many genes, is called a [[chromosome]]. Cells can [[Cell division|copy themselves]] to make two new cells.

There are two main kinds of cells: [[prokaryote|prokaryotic]] and [[eukaryote|eukaryotic]]. Prokaryotic cells have only a few parts. Their DNA is the shape of a circle, inside the cytoplasm, and they have no membranes inside the cell. Eukaryotic cells are more complex, and they have a [[cell nucleus]]. The DNA is inside the nucleus, and a membrane is around the nucleus. Eukaryotic cells also have other parts, called [[organelle]]s. Some of these other organelles also have membranes.<ref>{{Cite web|date=2014|title=What Is a Cell?|url=https://www.nature.com/scitable/topicpage/what-is-a-cell-14023083/|url-status=live|archive-url=https://web.archive.org/web/20201103225302/https://www.nature.com/scitable/topicpage/what-is-a-cell-14023083/|archive-date=2020-11-03|access-date=2021-03-28|website=Scitable by Nature Education}}</ref>

== Types of life ==
{{main|Taxonomy}}
Taxonomy is how lifeforms are put into groups. The smaller groups are more closely related, but the larger classes are more distantly related. The levels, or ranks, of taxonomy are [[Domain (biology)|domain]], [[Kingdom (biology)|kingdom]], [[phylum]], [[Class (biology)|class]], [[Order (biology)|order]], [[Family (biology)|family]], [[genus]], and [[species]]. There are many ideas for the meaning of [[species]].{{r|Wilcox}} One idea, called the [[biological species concept]], is as follows. A species is a group of living things that can mate with each other, and whose children can make their own children.{{R|Starr}}{{Rp|272}}

Taxonomy aims to group together living things with a common [[ancestor]]. This can now be done by comparing their [[DNA]]. Originally, it was done by [[comparative anatomy|comparing their anatomy]].<ref name="Wilcox">{{Cite web|last=Wilcox|first=Christie|title=What's in a name? Taxonomy problems vex biologists|url=https://www.quantamagazine.org/phyla-and-other-flawed-taxonomic-categories-vex-biologists-20190624/|access-date=2021-03-29|website=Quanta Magazine|date=24 June 2019|language=en|archive-date=2023-02-16|archive-url=https://web.archive.org/web/20230216145226/https://www.quantamagazine.org/phyla-and-other-flawed-taxonomic-categories-vex-biologists-20190624/|url-status=live}}</ref><ref>{{Cite journal|last=Luketa|first=Stefan|date=2012|title=New views on the megaclassification of life|journal=Protistology|volume=7|issue=4|pages=218–237|issn=1680-0826}}</ref>

The three domains of life are [[Bacteria]], [[Archaea]], and [[Eukarya]].<ref name="Freeman">{{Cite book|url=https://www.worldcat.org/oclc/472790415|title=Biological Science|last=Freeman|first=Scott|publisher=Pearson Education, Inc.|year=2011|isbn=978-0-321-59796-0|edition=4th|location=San Francisco, CA|oclc=472790415}}</ref>{{rp|6-7}} Bacteria and archaea are prokaryotic and have [[Unicellular organism|only one cell]]. Bacteria range in size from 0.15 cubic [[micrometre]]s (''[[Mycoplasma]]'') to 200,000,000 cubic micrometres (''[[Thiomargarita namibiensis]]''). Bacteria have shapes which are useful in classification, such as round, long and thin, and [[spiral]]. Some bacteria cause [[diseases]]. Bacteria in our [[intestines]] are part of our [[gut flora]]. They break down some of our food. Both bacteria and archaea may live where larger forms of life cannot. Bacteria have a molecule called [[peptidoglycan]] in their [[cell wall]], but [[archaea]] do not. Archaea have a molecule called [[isoprene]] in their cell membrane, but bacteria do not.{{R|Freeman}}{{rp|496-516}}

Eukarya are living things with eukaryotic cells, and they can have one cell or [[Multicellular organism|many cells]]. Most eukaryotes use [[sexual reproduction]] to make new copies of themselves. In sexual reproduction, two sex cells, one from each parent, join to make a new living thing.{{r|Starr}}{{Rp|138-139}}

[[Plant]]s are eukaryotes that use the Sun's light for energy. They include [[algae]], which live in water, and land plants. All land plants have two forms during their [[life cycle]], called [[alternation of generations]]. One form is [[diploid]], where the cells have two copies of their chromosomes, and the other form is [[haploid]], where the cells have one copy of their chromosomes. In land plants, both diploid and haploid forms have many cells. Two kinds of land plants are [[vascular plant]]s and [[bryophyte]]s. Vascular plants have long tissues that stretch from end to end of the plant. These tissues carry water and food. Most plants have [[root]]s and [[leaves]].{{R|Freeman}}{{Rp|546-577}}

[[Animal]]s are eukaryotes with many cells, which have no rigid cell walls. All animals are consumers: they survive by eating other [[organic]] material. Almost all animals have [[neuron]]s, a signalling system. They usually have [[muscle]]s, which make the body move. Many animals have a head and legs. Most animals are either male or female. They need a mate of the opposite [[sex]] to make offspring. Sex cells from the male and female [[Fertilization|can meet]] inside or outside the body.{{R|Freeman}}{{Rp|601-617}}

[[Fungi]] are eukaryotes which may have one cell, like [[yeast]]s, or many cells, like [[mushroom]]s. They are [[saprophyte]]s. Fungi break down living or dead material, so they are [[decomposer]]s. Only fungi, and a few bacteria, can break down [[lignin]] and [[cellulose]], two parts of [[wood]]. Some fungi are [[mycorrhiza]]. They live under ground and give nutrients to plants, like nitrogen and phosphorus.{{R|Freeman}}{{Rp|579}} Eukaryotes that are not plants, animals, or fungi are called [[protist]]s. Most protists live in water.{{R|Freeman}}{{Rp|519-520}}

== Evolution ==
{{main|Evolution}}

Over thousands or millions of years, living things can change, through the process of evolution. One kind of evolution is when a species changes over time, such as giraffes growing longer necks. Most of the time, the species becomes better suited to its environment, a process called [[adaptation]]. Evolution can also cause one group of living things to split into two groups. This is called [[speciation]] if it makes a new species. An example is mockingbirds on the Galapagos Islands—one species of mockingbird lives on each island, but all the species split from a shared ancestor species. Groups that are bigger than species can also split from a shared ancestor—for example, reptiles and mammals. A group of living things and their shared ancestor is called a [[clade]].<ref name="Mayr">Mayr, Ernst. 2001. ''What evolution is''. Weidenfeld & Nicolson, London. {{ISBN|0-465-04426-3}}</ref>

Living things can evolve to be quite different from their ancestors. As a result, parts of the body can also change. The same bone structure became the hands of humans, the hooves of horses, and the wings of birds. Different body parts that evolved from the same thing are called [[Homology|homologous]].<ref name="Mayr" />

[[Extinction]] is when all members of a species die. About 99.9% of all species that have ever lived are extinct. Extinction can happen at any time, but it is more common in certain time periods called [[extinction event]]s. The most recent was [[K/T extinction event|65 million years ago]], when the [[dinosaur]]s{{Efn|All dinosaurs went extinct except [[bird]]s. Birds are sometimes considered dinosaurs.}} went extinct.<ref>{{Cite journal|last1=Newman|first1=M. E. J.|last2=Palmer|first2=R. G.|date=1999-08-06|title=Models of Extinction: A Review|url=https://archive.org/details/arxiv-adap-org9908002|arxiv=adap-org/9908002|bibcode=1999adap.org..8002N}}</ref>

=== Origin of life ===
{{Main|Origin of life}}
By comparing [[fossil]]s and DNA, we know that all life on Earth today had a shared ancestor, called the [[last universal common ancestor]] (LUCA). Other living things may have been alive at the same time as the LUCA, but they died out. A study from 2018 suggests that the LUCA is about 4.5 billion (4,500,000,000) years old, nearly as old as the Earth.<ref>{{Cite web|title=A timescale for the origin and evolution of all of life on Earth|url=https://phys.org/news/2018-08-timescale-evolution-life-earth.html|url-status=live|archive-url=https://web.archive.org/web/20210303082055/https://phys.org/news/2018-08-timescale-evolution-life-earth.html|archive-date=2021-03-03|access-date=2021-04-02|website=phys.org|publisher=[[University of Bristol]]|language=en}}</ref> The oldest fossil evidence of life is about 3.5 billion years old.<ref>{{Cite web|title=Oldest fossils ever found show life on Earth began before 3.5 billion years ago|url=https://news.wisc.edu/oldest-fossils-found-show-life-began-before-3-5-billion-years-ago/|access-date=2021-05-01|website=news.wisc.edu|language=en-US|archive-date=2021-05-31|archive-url=https://web.archive.org/web/20210531022655/https://news.wisc.edu/oldest-fossils-found-show-life-began-before-3-5-billion-years-ago/|url-status=live}}</ref>

How did non-living material become alive? This is a difficult question. The first step must have been the creation of organic compounds. In 1953, the [[Miller–Urey experiment]] made inorganic compounds into organic compounds, such as amino acids, using heat and energy.<ref name="AmericanScientist">{{Cite web|last1=Trefil|first1=James|last2=Morowitz|first2=Harold J.|last3=Smith|first3=Eric|date=2017-02-06|title=The Origin of Life|url=https://www.americanscientist.org/article/the-origin-of-life|url-status=live|access-date=2021-04-02|website=American Scientist|language=en|archive-date=2021-04-01|archive-url=https://web.archive.org/web/20210401170852/https://www.americanscientist.org/article/the-origin-of-life}}</ref>

Life needs a source of energy for chemical reactions. On the early Earth, the atmosphere did not have oxygen. [[Oxidation]] using the [[Krebs cycle]], which is common today, was not possible. The Krebs cycle may have acted backwards, doing [[Reduction (chemistry)|reduction]] instead of oxidation, and the cycle may have made larger molecules. To make life, molecules needed to make copies of themselves. DNA and RNA make copies of themselves, but only if there is a [[catalyst]]—a compound which speeds up the chemical reaction. One guess is that RNA itself served as a catalyst. At some time, the molecules were surrounded by membranes, which made cells.{{r|AmericanScientist}}

==Gallery of images of life==
<center>
<gallery>
File:Linde von linn.jpg|alt=Linden tree by road|A [[tree]] is an example of a [[plant]].
File:Port Ghalib march 2006-0108.jpg|alt=School of fish in coral reef|[[Fish]] are examples of [[Marine biology|marine]] life.
File:Adult citrus root weevil, Diaprepes abbreviatus.jpg|alt=Weevil on green leaf with a bite taken out|An adult citrus root weevil is an example of an [[insect]].
File:Salmonella typhimurium.png|alt=Microscopic image of rod-shaped bacteria|''[[Salmonella]] typhimurium'' is an example of [[bacteria]].
File:Chromista collage 2.jpg|alt=Microscopic images of single cells|[[Chromalveolate]]s are a group of [[protista]].
File:Amanita muscaria tyndrum.jpg|alt=Large red and white mushroom|''[[Amanita muscaria]]'' (Fly agaric) is an example of [[fungi]].
File:Fungoid Frog.JPG|alt=Brown, black, and white frog on soil|The fungold [[frog]] is an example of an [[amphibian]].
File:Cyanocitta-cristata-004.jpg|alt=Blue jay sitting on tree branch covered in snow|A [[blue jay]] is an example of a [[bird]].
File:Djola-2006.jpg|alt=A group of humans of different ages|People are an example of [[mammal]]s.
</gallery>
</center>

{{LifeOnEarth}}

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

==Notes==
{{reflist|group=note}}

==References==
{{Notelist}}{{Reflist}}

{{Biology nav}}

[[Category:Life]]