Life

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For other uses, see Life (disambiguation).
For "Life" in the personal sense, see Personal life and Everyday life.

Life

Temporal range: 4280–0Ma

Had'n

Archean

Proterozoic
Pha.

Plants in the Rwenzori Mountains, Uganda

Scientific classification

Domains and Supergroups

Life on Earth:

 Non-cellular life[note 1] [note 2]

 Viruses[note 3]
 Viroids
 Cellular life
 Domain Bacteria
 Domain Archaea
 Domain Eukarya
 Archaeplastida
 SAR
 Excavata
 Amoebozoa
 Opisthokonta

This article is one of a series on:

Life in the Universe

Astrobiology

Habitability in the Solar System

 Habitability of Venus
 Life on Earth
 Habitability of Mars
 Habitability of Enceladus
 Habitability of Europa
 Habitability of Titan

Life outside the Solar System

 Circumstellar habitable zone

 Exoplanetology
 Planetary habitability
 SETI

 v
 t
  e

Life is a characteristic that distinguishes physical entities that have biological processes, such
as signaling and self-sustaining processes, from those that do not, either because such functions
have ceased (they have died), or because they never had such functions and are classified
as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea,
and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids,
or potential synthetic life as "living". Biology is the science concerned with the study of life.
There is currently no consensus regarding the definition of life. One popular definition is
that organisms are open systems that maintain homeostasis, are composed of cells, have a life
cycle, undergo metabolism, can grow, adapt to their environment, respond
to stimuli, reproduce and evolve. However, several other definitions have been proposed, and
there are some borderline cases of life, such as viruses or viroids.
Abiogenesis attempts to describe the natural process of life arising from non-living matter, such
as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-
living to living entities was not a single event, but a gradual process of increasing complexity. Life
on Earth first appeared as early as 4.28 billion years ago, soon after ocean formation 4.41 billion
years ago, and not long after the formation of the Earth 4.54 billion years ago.[1][2][3][4] The earliest
known life forms are microfossils of bacteria.[5][6] Earth's current life may have descended from
an RNA world, although RNA-based life may not have been the first. The mechanism by
which life began on Earth is unknown, though many hypotheses have been formulated and are
often based on the Miller–Urey experiment.
Since its primordial beginnings, life on Earth has changed its environment on a geologic time
scale, but it has also adapted to survive in most ecosystems and conditions. Some
microorganisms, called extremophiles, thrive in physically or geochemically extreme
environments that are detrimental to most other life on Earth. The cell is considered the structural
and functional unit of life.[7] There are two kinds of cells, prokaryotic and eukaryotic, both of which
consist of cytoplasmenclosed within a membrane and contain many biomolecules such
as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the
parent cell divides into two or more daughter cells.
In the past, there have been many attempts to define what is meant by "life" through obsolete
concepts such as odic force, hylomorphism, spontaneous generationand vitalism, that have now
been disproved by biological discoveries. Aristotle was the first person to classify organisms.
Later, Carl Linnaeus introduced his system of binomial nomenclature for the classification
of species. Eventually new groups and categories of life were discovered, such as cells and
microorganisms, forcing dramatic revisions of the structure of relationships between living
organisms. Though currently only known on Earth, life need not be restricted to it, and many
scientists speculate in the existence of extraterrestrial life. Artificial life is a computer simulation
or man-made reconstruction of any aspect of life, which is often used to examine systems related
to natural life.
Death is the permanent termination of all biological functions which sustain an organism, and as
such, is the end of its life. Extinction is the term describing the dying out of a group or taxon,
usually a species. Fossils are the preserved remains or traces of organisms.

Contents

 1Definitions
o 1.1Biology
 1.1.1Alternative definitions
 1.1.2Viruses
o 1.2Biophysics
o 1.3Living systems theories
  1.3.1Gaia hypothesis
 1.3.2Nonfractionability
 1.3.3Life as a property of ecosystems
 1.3.4Complex systems biology
 1.3.5Darwinian dynamic
 1.3.6Operator theory
 2History of study
o 2.1Materialism
o 2.2Hylomorphism
o 2.3Spontaneous generation
o 2.4Vitalism
 3Origin
 4Environmental conditions
o 4.1Biosphere
o 4.2Range of tolerance
o 4.3Extremophiles
o 4.4Chemical elements
 4.4.1DNA
 5Classification
o 5.1Antiquity
o 5.2Linnaean
o 5.3Cladistic
 6Cells
 7Extraterrestrial
 8Artificial
 9Death
o 9.1Extinction
o 9.2Fossils
 10See also
 11Notes
 12References
 13Further reading
 14External links

Definitions
The definition of life has long been a challenge for scientists and philosophers, with many varied
definitions put forward.[8][9][10] This is partially because life is a process, not a substance.[11][12][13] This
is complicated by a lack of knowledge of the characteristics of living entities, if any, that may
have developed outside of Earth.[14][15] Philosophical definitions of life have also been put forward,
with similar difficulties on how to distinguish living things from the non-living.[16] Legal definitions
of life have also been described and debated, though these generally focus on the decision to
declare a human dead, and the legal ramifications of this decision.[17]
Biology
See also: Organism
The characteristics of life

Since there is no unequivocal definition of life, most current definitions in biology are descriptive.
Life is considered a characteristic of something that preserves, furthers or reinforces its existence
in the given environment. This characteristic exhibits all or most of the following
traits:[10][18][19][20][21][22][23]

1. Homeostasis: regulation of the internal environment to

maintain a constant state; for example, sweating to reduce
temperature
2. Organization: being structurally composed of one or
more cells – the basic units of life
3. Metabolism: transformation of energy by converting
chemicals and energy 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.
4. 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.
5. Adaptation: the ability to change over time in response to
the environment. This ability is fundamental to the process
of evolution and is determined by the organism's heredity,
diet, and external factors.
6. Response to 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 chemotaxis.
7. Reproduction: the ability to produce new individual
organisms, either asexually from a single parent organism
or sexually from two parent organisms.
These complex processes, called physiological functions, have underlying physical and chemical
bases, as well as signaling and control mechanisms that are essential to maintaining life.
Alternative definitions
See also: Entropy and life
From a physics perspective, living beings are thermodynamic systems with an organized
molecular structure that can reproduce itself and evolve as survival
dictates.[24][25] Thermodynamically, life has been described as an open system which makes use
of gradients in its surroundings to create imperfect copies of itself.[26] Hence, life is a self-
sustained chemical system capable of undergoing Darwinian evolution.[27][28] A major strength of
this definition is that it distinguishes life by the evolutionary process rather than its chemical
composition.[29]
Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One
systemic definition of life is that living things are self-organizing and 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 work cycle.[30]This definition is extended by the apparition of novel functions
over time.[31]
Viruses
Main article: Virus

Adenovirus as seen under an electron microscope

Whether or not viruses should be considered as alive is controversial. They are most often
considered as just replicators rather than forms of life.[32] They have been described as
"organisms at the edge of life"[33] because they possess genes, evolve by natural
selection,[34][35] and replicate by creating multiple copies of themselves through self-assembly.
However, viruses do not metabolize and they 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.[36][37][38]
Biophysics
To reflect the minimum phenomena required, other biological definitions of life have been
proposed,[39] with many of these being based upon chemical systems. Biophysicists have
commented that living things function on negative entropy.[40][41] In other words, living processes
can be viewed as a delay of the spontaneous diffusion or dispersion of the internal energy of
biological molecules towards more potential microstates.[8] In more detail, according to physicists
such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of
the class of phenomena that are open or continuous systems able to decrease their
internal entropy at the expense of substances or free energy taken in from the environment and
subsequently rejected in a degraded form.[42][43]
Living systems theories
Living systems are open self-organizing living things that interact with their environment. These
systems are maintained by flows of information, energy, and matter.
Some scientists have proposed in the last few decades that a general living systems theory is
required to explain the nature of life.[44] Such a general theory would arise out of the ecological
and biological sciences and attempt to map general principles for how all living systems work.
Instead of examining phenomena by attempting to break things down into components, a general
living systems theory explores phenomena in terms of dynamic patterns of the relationships of
organisms with their environment.[45]
Gaia hypothesis
Main article: Gaia hypothesis
The idea that the Earth is alive is found in philosophy and religion, but the first scientific
discussion of it was by the Scottish scientist James Hutton. In 1785, he stated that the Earth was
a superorganism and that its proper study should be physiology. Hutton is considered the father
of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th
century.[46]:10 The Gaia hypothesis, proposed in the 1960s by scientist James
Lovelock,[47][48] suggests that life on Earth functions as a single organism that defines and
maintains environmental conditions necessary for its survival.[46] This hypothesis served as one of
the foundations of the modern Earth system science.
Nonfractionability
The first attempt at a general living systems theory for explaining the nature of life was in 1978,
by American biologist James Grier Miller.[49] Robert Rosen (1991) built on this by defining 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."[50]
Life as a property of ecosystems
A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity
of influence,"[51] 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 ecological system rather than a single organism or species.[52] He argues
that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert
Ulanowicz (2009) highlights mutualism as the key to understand the systemic, order-generating
behavior of life and ecosystems.[53]
Complex systems biology
Main article: Complex systems biology
See also: Mathematical biology
Complex systems biology (CSB) is a field of science that studies the emergence of complexity in
functional organisms from the viewpoint of dynamic systems theory.[54] The latter is also often
called systems biology and aims to understand the most fundamental aspects of life. A closely
related approach to CSB and systems biology called relational biology is concerned mainly with
understanding life processes in terms of the most important relations, and categories of such
relations among the essential functional components of organisms; for multicellular organisms,
this has been defined as "categorical biology", or a model representation of organisms as
a category theory of biological relations, as well as an algebraic topology of the functional
organization of living organisms in terms of their dynamic, complex networks of metabolic,
genetic, and epigenetic processes and signaling pathways.[55][56] Alternative but closely related
approaches focus on the interdependance of constraints, where constraints can be either
molecular, such as enzymes, or macroscopic, such as the geometry of a bone or of the vascular
system.[57]
Darwinian dynamic
It has also been argued that the evolution of order in living systems and certain physical systems
obeys a common fundamental principle termed the Darwinian dynamic.[58][59] The Darwinian
dynamic was formulated by first considering how macroscopic order is generated in a simple
non-biological system far from thermodynamic equilibrium, and then extending consideration to
short, replicating RNA molecules. The underlying order-generating process was concluded to be
basically similar for both types of systems.[58]
Operator theory
Another systemic definition called the operator theory proposes that "life is a general term for the
presence of the typical closures found in organisms; the typical closures are a membrane and an
autocatalytic set in the cell"[60] and that an organism is any system with an organisation that
complies with an operator type that is at least as complex as the cell.[61][62][63][64] Life can also be
modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a
superior positive feedback formed by the potential of expansion and reproduction.[65]

History of study
Materialism
Main article: Materialism

Plant growth in the Hoh Rainforest

Herds of zebra and impala gathering on the Maasai Mara plain

An aerial photo of microbial mats around the Grand Prismatic Spring of Yellowstone National Park

Some of the earliest theories of life were materialist, holding that all that exists is matter, and that
life is merely a complex form or arrangement of matter. Empedocles(430 BC) argued that
everything in the universe is made up of a combination of 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.[66]
Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche).
Like other ancient writers, he was attempting to explain what makes something a living thing. His
explanation was that fiery atoms make a soul in exactly the same way atoms and void account
for any other thing. He elaborates on fire because of the apparent connection between life and
heat, and because fire moves.[67]
Plato's world of eternal and unchanging Forms, imperfectly represented in matter by a
divine Artisan, contrasts sharply with the various mechanistic Weltanschauungen, of
which atomism was, by the fourth century at least, the most prominent ... This debate persisted
throughout the ancient world. Atomistic mechanism got a shot in the arm from Epicurus ... while
the Stoics adopted a divine teleology ... The choice seems simple: either show how a structured,
regular world could arise out of undirected processes, or inject intelligence into the system.[68]

— R.J. Hankinson, Cause and Explanation in Ancient Greek Thought

The mechanistic materialism that originated in ancient Greece was revived and revised by the
French philosopher René Descartes, who held that animals and humans were assemblages of
parts that together functioned as a machine. In the 19th century, the advances in cell theory in
biological science encouraged this view. The evolutionary theory of Charles Darwin (1859) is a
mechanistic explanation for the origin of species by means of natural selection.[69]
Hylomorphism
Main article: Hylomorphism

The structure of the souls of plants, animals, and humans, according to Aristotle

Hylomorphism is a theory first expressed by the Greek philosopher Aristotle (322 BC). The
application of hylomorphism to biology was important to Aristotle, and biology is extensively
covered in his extant writings. In this view, everything in the material universe has both matter
and form, and 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.[70] Each higher soul has all of the attributes of the lower
ones. Aristotle believed that while matter can exist without form, form cannot exist without matter,
and that therefore the soul cannot exist without the body.[71]
This account is consistent with teleological explanations of life, which account for phenomena in
terms of purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained
by its purpose of camouflage. The direction of causality (from the future to the past) is in
contradiction with the scientific evidence for natural selection, which explains the consequence in
terms of a prior cause. Biological features are explained not by looking at future optimal results,
but by looking at the past evolutionary history of a species, which led to the natural selection of
the features in question.[72]
Spontaneous generation
Main article: Spontaneous generation
Spontaneous generation was the belief that living organisms can form without descent from
similar organisms. Typically, the idea was that certain forms such as fleas could arise from
inanimate matter such as dust or the supposed seasonal generation of mice and insects from
mud or garbage.[73]
The theory of spontaneous generation was proposed by Aristotle,[74] who compiled and expanded
the work of prior natural philosophers and the various ancient explanations of the appearance of
organisms; it held sway for two millennia. It was decisively dispelled by the experiments of Louis
Pasteur in 1859, who expanded upon the investigations of predecessors such as Francesco
Redi.[75][76] Disproof of the traditional ideas of spontaneous generation is no longer controversial
among biologists.[77][78][79]
Vitalism
Main article: Vitalism
Vitalism is the belief that the life-principle is non-material. This originated with Georg Ernst
Stahl (17th century), and remained popular until the middle of the 19th century. It appealed to
philosophers such as Henri Bergson, Friedrich Nietzsche, and Wilhelm Dilthey,[80] anatomists
like Marie François Xavier Bichat, and chemists like Justus von Liebig.[81] Vitalism included the
idea that there was a fundamental difference between 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.[82] This Wöhler synthesis is
considered the starting point of modern organic chemistry. It is of historical significance because
for the first time an organic compound was produced in inorganicreactions.[81]
During the 1850s, Hermann von Helmholtz, anticipated by Julius Robert von Mayer,
demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital
forces" necessary to move a muscle.[83] These results led to the abandonment of scientific
interest in vitalistic theories, although the belief lingered on in pseudoscientific theories such
as homeopathy, which interprets diseases and sickness as caused by disturbances in a
hypothetical vital force or life force.[84]

Origin
Life timeline
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Axis scale: million years
Also see: Human timeline and Nature timeline

Main article: Abiogenesis

The age of the Earth is about 4.54 billion years.[85][86][87] Evidence suggests that life on Earth has
existed for at least 3.5 billion years,[88][89][90][91][92][93][94][95][96] with the oldest physical traces of life dating
back 3.7 billion years;[97][98][99] however, some theories, such as the Late Heavy Bombardment
theory, suggest that life on Earth may have started even earlier, as early as 4.1–4.4 billion years
ago,[88][89][90][91][92] and the chemistry leading to life may have begun shortly after the Big Bang, 13.8
billion years ago, during an epoch when the universe was only 10–17 million years old.[100][101][102]
More than 99% of all species of life forms, amounting to over five billion species,[103] that ever
lived on Earth are estimated to be extinct.[104][105]
Although the number of Earth's catalogued species of lifeforms is between 1.2 million and 2
million,[106][107] the total number of species in the planet is uncertain. Estimates range from 8 million
to 100 million,[106][107] with a more narrow range between 10 and 14 million,[106] but it may be as high
as 1 trillion (with only one-thousandth of one percent of the species described) according to
studies realized in May 2016.[108][109] The total number of related DNA base pairs on Earth is
estimated at 5.0 x 1037 and weighs 50 billion tonnes.[110] In comparison, the total mass of
the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).[111] In July
2016, scientists reported identifying a set of 355 genes from the Last Universal Common
Ancestor (LUCA) of all organisms living on Earth.[112]
All known life forms share fundamental molecular mechanisms, reflecting their common descent;
based on these observations, hypotheses on the origin of life attempt to find a mechanism
explaining the formation of a universal common ancestor, from simple organic molecules via pre-
cellular life to protocells and metabolism. Models have been divided into "genes-first" and
"metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine
both categories.[113]
There is no current scientific consensus as to how life originated. However, most accepted
scientific models build on the Miller–Urey experiment and the work of Sidney Fox, which show
that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and
other organic compounds from inorganic precursors,[114] and phospholipids spontaneously
form lipid bilayers, the basic structure of a cell membrane.
Living organisms synthesize proteins, which are polymers of amino acids using instructions
encoded by deoxyribonucleic acid (DNA). Protein synthesisentails intermediary ribonucleic
acid (RNA) polymers. One possibility for how life began is that genes originated first, followed by
proteins;[115] the alternative being that proteins came first and then genes.[116]
However, because genes and proteins are both required to produce the other, the problem of
considering which came first is like that of the chicken or the egg. Most scientists have adopted
the hypothesis that because of this, it is unlikely that genes and proteins arose independently.[117]
Therefore, a possibility, first suggested by Francis Crick,[118] is that the first life was based
on RNA,[117] which has the DNA-like properties of information storage and the catalytic properties
of some proteins. This is called the RNA world hypothesis, and it is supported by the observation
that many of the most critical components of cells (those that evolve the slowest) are composed
mostly or entirely of RNA. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either
nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet
been demonstrated when the hypothesis was first proposed,[119] but they were confirmed
by Thomas Cech in 1986.[120]
One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic
precursors is more difficult than for other organic molecules. One reason for this is that RNA
precursors are very stable and react with each other very slowly under ambient conditions, and it
has also been proposed that living organisms consisted of other molecules before
RNA.[121] However, the successful synthesis of certain RNA molecules under the conditions that
existed prior to life on Earth has been achieved by adding alternative precursors in a specified
order with the precursor phosphate present throughout the reaction.[122]This study makes the RNA
world hypothesis more plausible.[123]
Geological findings in 2013 showed that reactive phosphorus species (like phosphite) were in
abundance in the ocean before 3.5 Ga, and that Schreibersite easily reacts with
aqueous glycerol to generate phosphite and glycerol 3-phosphate.[124] It is hypothesized
that Schreibersite-containing meteorites from the Late Heavy Bombardment could have provided
early reduced phosphorus, which could react with prebiotic organic molecules to
form phosphorylated biomolecules, like RNA.[124]
In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA
enzymes (ribozymes) in vitro.[125] The work was performed in the laboratory of Gerald Joyce, who
stated "This is the first example, outside of biology, of evolutionary adaptation in a molecular
genetic system."[126]
Prebiotic compounds may have originated extraterrestrially. NASA findings in 2011, based on
studies with meteorites found on Earth, suggest DNA and RNA components
(adenine, guanine and related organic molecules) may be formed in outer space.[127][128][129][130]
In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic
compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory
under outer spaceconditions, using starting chemicals, such as pyrimidine, found in meteorites.
Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in
the universe, may have been formed in red giants or in interstellar dust and gas clouds,
according to the scientists.[131]
According to the panspermia hypothesis, microscopic life—distributed
by meteoroids, asteroids and other small Solar System bodies—may exist throughout the
universe.[132]

Environmental conditions

Cyanobacteria dramatically changed the composition of life forms on Earth by leading to the near-extinction
of oxygen-intolerant organisms.

The diversity of life on Earth is a result of the dynamic interplay between genetic opportunity,
metabolic capability, environmental challenges,[133] and symbiosis.[134][135][136] For most of its
existence, Earth's habitable environment has been dominated by microorganisms and subjected
to their metabolism and evolution. As a consequence of these microbial activities, the physical-
chemical environment on Earth has been changing on a geologic time scale, thereby affecting
the path of evolution of subsequent life.[133] For example, the release of
molecular oxygen by cyanobacteria as a by-product of photosynthesis induced global changes in
the Earth's environment. Because oxygen was toxic to most life on Earth at the time, this posed
novel evolutionary challenges, and ultimately resulted in the formation of Earth's major animal
and plant species. This interplay between organisms and their environment is an inherent feature
of living systems.[133]
Biosphere
Main article: Biosphere
The biosphere is the global sum of all ecosystems. It can also be termed as the zone of life
on Earth, a closed system (apart from solar and cosmic radiation and heat from the interior of the
Earth), and largely self-regulating.[137] By the most general biophysiological definition, the
biosphere is the global ecological system integrating all living beings and their relationships,
including their interaction with the elements of the lithosphere, geosphere, hydrosphere,
and atmosphere.
Life forms live in every part of the Earth's biosphere, including soil, hot springs, inside rocks at
least 19 km (12 mi) deep underground, the deepest parts of the ocean, and at least 64 km
(40 mi) high in the atmosphere.[138][139][140] Under certain test conditions, life forms have been
observed to thrive in the near-weightlessness of space[141][142] and to survive in the vacuum of
outer space.[143][144] Life forms appear to thrive in the Mariana Trench, the deepest spot in the
Earth's oceans.[145][146]Other researchers reported related studies that life forms thrive inside rocks
up to 580 m (1,900 ft; 0.36 mi) below the sea floor under 2,590 m (8,500 ft; 1.61 mi) of ocean off
the coast of the northwestern United States,[145][147] as well as 2,400 m (7,900 ft; 1.5 mi) beneath
the seabed off Japan.[148] In August 2014, scientists confirmed the existence of life forms living
800 m (2,600 ft; 0.50 mi) below the ice of Antarctica.[149][150] According to one researcher, "You can
find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they
are."[145]
The biosphere is postulated to have evolved, beginning with a process of biopoesis (life created
naturally from non-living matter, such as simple organic compounds) or biogenesis (life created
from living matter), at least some 3.5 billion years ago.[151][152] The earliest evidence for life on
Earth includes biogenic graphite found in 3.7 billion-year-old metasedimentary
rocks from Western Greenland[97] and microbial mat fossils found in 3.48 billion-year-
old sandstone from Western Australia.[98][99] More recently, in 2015, "remains of biotic life" were
found in 4.1 billion-year-old rocks in Western Australia.[89][90] In 2017, putative
fossilized microorganisms (or microfossils) were announced to have been discovered
in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, Canada that were as old as
4.28 billion years, the oldest record of life on earth, suggesting "an almost instantaneous
emergence of life" after ocean formation 4.4 billion years ago, and not long after the formation of
the Earth 4.54 billion years ago.[1][2][3][4] According to biologist Stephen Blair Hedges, "If life arose
relatively quickly on Earth ... then it could be common in the universe."[89]
In a general sense, biospheres are any closed, self-regulating systems containing ecosystems.
This includes artificial biospheres such as Biosphere 2 and BIOS-3, and potentially ones on other
planets or moons.[153]
Range of tolerance

Deinococcus radiodurans is an extremophile that can resist extremes of cold, dehydration, vacuum, acid,
and radiation exposure.

The inert components of an ecosystem are the physical and chemical factors necessary for life—
energy (sunlight or chemical energy), water, heat, atmosphere, gravity, nutrients,
and ultraviolet solar radiation protection.[154] In most ecosystems, the conditions vary during the
day and from one season to the next. To live in most ecosystems, then, organisms must be able
to survive a range of conditions, called the "range of tolerance."[155] Outside that are the "zones of
physiological stress," where the survival and reproduction are possible but not optimal. Beyond
these zones are the "zones of intolerance," where survival and reproduction of that organism is
unlikely or impossible. Organisms that have a wide range of tolerance are more widely
distributed than organisms with a narrow range of tolerance.[155]
Extremophiles
Further information: Extremophile
To survive, selected microorganisms can assume forms that enable them to
withstand freezing, complete desiccation, starvation, high levels of radiation exposure, and other
physical or chemical challenges. These microorganisms may survive exposure to such
conditions for weeks, months, years, or even centuries.[133] Extremophiles are microbial life
forms that thrive outside the ranges where life is commonly found.[156] They 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 structure and metabolic diversity of microbial communities in
such extreme environments is ongoing.[157]
Microbial life forms thrive even in the Mariana Trench, the deepest spot in the Earth's
oceans.[145][146] Microbes also thrive inside rocks up to 1,900 feet (580 m) below the sea floor
under 8,500 feet (2,600 m) of ocean.[145][147]
Investigation of the tenacity and versatility of life on Earth,[156] as well as an understanding of the
molecular systems that some organisms utilize to survive such extremes, is important for the
search for life beyond Earth.[133] For example, lichen could survive for a month in a simulated
Martian environment.[158][159]
Chemical elements
All life forms require certain core chemical elements needed for biochemical functioning. These
include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the
elemental macronutrients for all organisms[160]—often represented by the acronym CHNOPS.
Together these make up nucleic acids, proteins and lipids, the bulk of living matter. Five of these
six elements comprise the chemical components of DNA, the exception being sulfur. The latter is
a component of the amino acids cysteine and methionine. The most biologically abundant of
these elements is carbon, which has the desirable attribute of forming multiple, stable covalent
bonds. This allows carbon-based (organic) molecules to form an immense variety of chemical
arrangements.[161] Alternative hypothetical types of biochemistry have been proposed that
eliminate one or more of these elements, swap out an element for one not on the list, or change
required chiralities or other chemical properties.[162][163]
DNA
Main article: DNA
Deoxyribonucleic acid is a molecule that carries most of the genetic instructions used in the
growth, development, functioning and reproduction of all known living organisms and many
viruses. DNA and RNAare nucleic acids; alongside proteins and complex carbohydrates, they
are one of the three major types of macromolecule that are essential for all known forms of life.
Most DNA molecules consist of two biopolymer strands coiled around each other to form
a double helix. The two DNA strands are known as polynucleotides since they are composed
of simpler units called nucleotides.[164] Each nucleotide is composed of a nitrogen-
containing nucleobase—either cytosine (C), guanine (G), adenine (A), or thymine (T)—as well as
a sugar called deoxyribose and a phosphate group. The nucleotides are joined to one another in
a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next,
resulting in an alternating sugar-phosphate backbone. According to base pairing rules (A with T,
and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide
strands to make double-stranded DNA. The total amount of related DNA base pairs on Earth is
estimated at 5.0 x 1037, and weighs 50 billion tonnes.[110] In comparison, the total mass of
the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).[111]
DNA stores biological information. The DNA backbone is resistant to cleavage, and both strands
of the double-stranded structure store the same biological information. Biological information is
replicated as the two strands are separated. A significant portion of DNA (more than 98% for
humans) is non-coding, meaning that these sections do not serve as patterns for protein
sequences.
The two strands of DNA run in opposite directions to each other and are therefore anti-parallel.
Attached to each sugar is one of four types of nucleobases (informally, bases). It is
the sequence of these four nucleobases along the backbone that encodes biological information.
Under the genetic code, RNA strands are translated to specify the sequence of amino
acids within proteins. These RNA strands are initially created using DNA strands as a template in
a process called transcription.
Within cells, DNA is organized into long structures called chromosomes. During cell
division these chromosomes are duplicated in the process of DNA replication, providing each cell
its own complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists)
store most of their DNA inside the cell nucleus and some of their DNA in organelles, such
as mitochondria or chloroplasts.[165] In contrast, prokaryotes (bacteria and archaea) store their
DNA only in the cytoplasm. Within the chromosomes, chromatin proteins such
as histones compact and organize DNA. These compact structures guide the interactions
between DNA and other proteins, helping control which parts of the DNA are transcribed.
DNA was first isolated by Friedrich Miescher in 1869.[166] Its molecular structure was identified
by James Watson and Francis Crick in 1953, whose model-building efforts were guided by X-ray
diffraction data acquired by Rosalind Franklin.[167]

Classification
Main article: Biological classification

The hierarchy of biological classification's eight major taxonomic ranks. Life is divided into domains, which
are subdivided into further groups. Intermediate minor rankings are not shown.

Antiquity
The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle
(384–322 BC), who classified all living organisms known at that time as either a plant or an
animal, based mainly on their ability to move. He also distinguished animals with blood from
animals without blood (or at least without red blood), which can be compared with the concepts
of vertebrates and invertebrates respectively, and divided the blooded animals into five groups:
viviparous quadrupeds (mammals), oviparous quadrupeds (reptiles and amphibians), birds,
fishes and whales. The bloodless animals were also divided into five
groups: cephalopods, crustaceans, insects (which included the spiders, scorpions,
and centipedes, in addition to what we define as insects today), shelled animals (such as
most molluscs and echinoderms), and "zoophytes" (animals that resemble plants). 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.[168]
Linnaean
The exploration of the Americas 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, Carl Linnaeus introduced his system of binomial nomenclature for the
classification of species. Linnaeus attempted to improve the composition and reduce the length
of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new
descriptive terms and precisely defining their meaning.[169] The Linnaean classification has eight
levels: domains, kingdoms, phyla, class, order, family, genus, and species.
The fungi were originally treated as plants. For a short period Linnaeus had classified them in the
taxon Vermes in Animalia, but later placed them back in Plantae. Copelandclassified the Fungi in
his Protoctista, thus partially avoiding the problem but acknowledging their special status.[170] The
problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-
kingdom system. Evolutionary history shows that the fungi are more closely related to animals
than to plants.[171]
As new discoveries enabled detailed study of cells and microorganisms, 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 protophyta/thallophyta as plants,
but were united by Haeckel in the kingdom Protista; later, the prokaryotes were split off in the
kingdom Monera, which would eventually be divided into two separate groups, the Bacteria and
the Archaea. This led to the six-kingdom systemand eventually to the current three-domain
system, which is based on evolutionary relationships.[172] However, the classification of
eukaryotes, especially of protists, is still controversial.[173]
As microbiology, molecular biology and virology developed, non-cellular reproducing agents were
discovered, such as viruses and viroids. Whether these are considered alive has been a matter
of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability
to grow or respond to their environments. Viruses can still be classed into "species" based on
their biology and genetics, but many aspects of such a classification remain controversial.[174]
In May 2016, scientists reported that 1 trillion species are estimated to be on Earth currently with
only one-thousandth of one percent described.[108]
The original Linnaean system has been modified over time as follows:
Woese et Cavalier- Cavalier-
Linnaeus Haeckel Chatton Copeland Whittaker
al. Smith Smith
1735[175] 1866[176] 1925[177] 1938[178] 1969[179]
1990[172] 1998[180] 2015[181]
2 3 4 5 2 empires, 6 2 empires, 7
2 empires 3 domains
kingdoms kingdoms kingdoms kingdoms kingdoms kingdoms
Bacteria Bacteria
Prokaryota Monera Monera Bacteria
(not Archaea Archaea
Protista
treated) Protozoa Protozoa
Protoctista Protista
Chromista Chromista
Eukaryota Plantae Eucarya Plantae Plantae
Vegetabilia Plantae Plantae
Fungi Fungi Fungi
Animalia Animalia Animalia Animalia Animalia Animalia
Main article: Kingdom (biology) § Summary
Cladistic
In the 1960s cladistics emerged: a system arranging taxa based on clades in an evolutionary or
phylogenetic tree.[182]

Cells
Main article: Cell (biology)
Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells
by division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf
Virchow and others during the early nineteenth century, and subsequently became widely
accepted.[183] The activity of an organism depends on the total activity of its cells, with energy
flow occurring within and between them.[184] Cells contain hereditary information that is carried
forward as a genetic code during cell division.[185]
There are two primary types of cells. Prokaryotes lack a nucleus and other membrane-
bound organelles, although they have circular DNA and ribosomes. Bacteria and Archaea are
two domains of prokaryotes. The other primary type of cells are the eukaryotes, which have
distinct nuclei bound by a nuclear membrane and membrane-bound organelles,
including mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum,
and vacuoles. In addition, they possess organized chromosomes that store genetic material. All
species of large complex organisms are eukaryotes, including animals, plants and fungi, though
most species of eukaryote are protist microorganisms.[186] The conventional model is that
eukaryotes evolved from prokaryotes, with the main organelles of the eukaryotes forming
through endosymbiosis between bacteria and the progenitor eukaryotic cell.[187]
The molecular mechanisms of cell biology are based on proteins. Most of these are synthesized
by the ribosomes through an enzyme-catalyzed process called protein biosynthesis. A sequence
of amino acids is assembled and joined together based upon gene expression of the cell's
nucleic acid.[188] In eukaryotic cells, these proteins may then be transported and processed
through the Golgi apparatus in preparation for dispatch to their destination.[189]
Cells reproduce through a process of cell division in which the parent cell divides into two or
more daughter cells. For prokaryotes, cell division occurs through a process of fission in which
the DNA is replicated, then the two copies are attached to parts of the cell membrane.
In eukaryotes, a more complex process of mitosis is followed. However, the end result is the
same; the resulting cell copies are identical to each other and to the original cell (except
for mutations), and both are capable of further division following an interphase period.[190]
Multicellular organisms may have first evolved through the formation of colonies of identical cells.
These cells can form group organisms through cell adhesion. The individual members of a
colony are capable of surviving on their own, whereas the members of a true multi-cellular
organism have developed specializations, making them dependent on the remainder of the
organism for survival. Such organisms are formed clonally or from a single germ cell that is
capable of forming the various specialized cells that form the adult organism. This specialization
allows multicellular organisms to exploit resources more efficiently than single cells.[191] In January
2016, scientists reported that, about 800 million years ago, a minor genetic change in a
single molecule, called GK-PID, may have allowed organisms to go from a single cell
organism to one of many cells.[192]
Cells have evolved methods to perceive and respond to their microenvironment, thereby
enhancing their adaptability. Cell signaling coordinates cellular activities, and hence governs the
basic functions of multicellular organisms. Signaling between cells can occur through direct cell
contact using juxtacrine signalling, or indirectly through the exchange of agents as in
the endocrine system. In more complex organisms, coordination of activities can occur through a
dedicated nervous system.[193]

Extraterrestrial
Main articles: Extraterrestrial life, Astrobiology, and Astroecology
Though life is confirmed only on Earth, many think that extraterrestrial life is not only plausible,
but probable or inevitable.[194][195] Other planets and moons in the Solar System and
other planetary systems are being examined for evidence of having once supported simple life,
and projects such as SETI are trying to detect radio transmissions from possible alien
civilizations. Other locations within the Solar Systemthat may host microbial life include the
subsurface of Mars, the upper atmosphere of Venus,[196] and subsurface oceans on some of
the moons of the giant planets.[197][198] Beyond the Solar System, the region around another 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 survives. Stars more massive than the Sun have a larger habitable
zone, but remain on the Sun-like "main sequence" of stellar evolution for a shorter time interval.
Small red dwarfs have the opposite problem, with a smaller habitable zone that is subject to
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 have a greater likelihood for Earth-like life to
develop.[199] The location of the star within a galaxy may also affect the likelihood of life forming.
Stars in regions with a greater abundance of heavier elements that can form planets, in
combination with a low rate of potentially habitat-damaging supernova events, are predicted to
have a higher probability of hosting planets with complex life.[200] The variables of the Drake
equation are used to discuss the conditions in planetary systems where civilization is most likely
to exist.[201] Use of the equation to predict the amount of extraterrestrial life, however, is difficult;
because many of the variables are unknown, the equation functions as more of a mirror to what
its user already thinks. As a result, the number of civilizations in the galaxy can be estimated as
low as 9.1 x 10−11 or as high as 156 million; for the calculations, see Drake equation.

Artificial
Main articles: Artificial life and Synthetic biology
Artificial life is the simulation of any aspect of life, as through computers, robotics,
or biochemistry.[202] The study of artificial life imitates traditional biology by recreating some
aspects of biological phenomena. Scientists study the logic of living systems by creating artificial
environments—seeking to understand the complex information processing that defines such
systems.[184] While life is, by definition, alive, artificial life is generally referred to as data confined
to a digital environment and existence.
Synthetic biology is a new area of biotechnology that combines science and biological
engineering. The common goal is the design and construction of new biological functions and
systems not found in nature. Synthetic biology includes the broad redefinition and expansion
of biotechnology, with the ultimate goals of being able to design and build engineered biological
systems that process information, manipulate chemicals, fabricate materials and structures,
produce energy, provide food, and maintain and enhance human health and the environment.[203]

Death
Main article: Death

Animal corpses, like this African buffalo, are recycled by the ecosystem, providing energy and nutrients for
living creatures

Death is the permanent termination of all vital functions or life processes in an organism or
cell.[204][205] It can occur as a result of an accident, medical conditions, biological
interaction, malnutrition, poisoning, senescence, or suicide. After death, the remains of an
organism re-enter the biogeochemical cycle. Organisms may be consumed by a predator or
a scavenger and leftover organic material may then be further decomposed by detritivores,
organisms that recycle detritus, returning it to the environment for reuse in the food chain.
One of the challenges in defining death is in distinguishing it from life. Death would seem to refer
to either the moment life ends, or when the state that follows life begins.[205] However, determining
when death has occurred is difficult, as cessation of life functions is often not simultaneous
across organ systems.[206] Such determination therefore requires drawing conceptual lines
between life and death. This is problematic, however, because there is little consensus over how
to define life. The nature of death has for millennia been a central concern of the world's religious
traditions and of philosophical inquiry. Many religions maintain faith in either a kind
of afterlife or reincarnation for the soul, or resurrection of the body at a later date.
Extinction
Main article: Extinction
Extinction is the process by which a group of taxa or species dies out, reducing
biodiversity.[207] The moment of extinction is generally considered the death of the last individual
of that species. Because a species' potential range may be very large, determining this moment
is difficult, and is usually done retrospectively after a period of apparent absence. Species
become extinct when they are no longer able to survive in changing habitat or against superior
competition. In Earth's history, over 99% of all the species that have ever lived are
extinct;[208][103][104][105] however, mass extinctions may have accelerated evolution by providing
opportunities for new groups of organisms to diversify.[209]
Fossils
Main article: Fossils
Fossils are the preserved remains or traces of animals, plants, and other organisms from the
remote past. The totality of fossils, both discovered and undiscovered, and their placement in
fossil-containing rockformations and sedimentary layers (strata) is known as the fossil record. A
preserved specimen is called a fossil if it is older than the arbitrary date of 10,000 years
ago.[210] Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the
oldest from the Archaean Eon, up to 3.4 billion years old.[211][212]

See also
 Biology, the study of life
 Astrobiology
 Biosignature
 Evolutionary history of life
 Lists of organisms by population
 Phylogenetics
 Viable System Theory

Notes
1. ^ The "evolution" of viruses and other similar forms is still
uncertain. Therefore, this classification may
be paraphyletic because cellular life might have evolved from non-
cellular life, or polyphyletic because the most recent common
ancestor might not be included.
2. ^ Infectious protein molecules prions are not considered living
organisms, but can be described as "organism-comparable
organic structures".
3. ^ Certain specific organism-comparable organic structures may be
considered subviral agents, including virus-dependent
entities: satellites and defective interfering particles, both of which
require another virus for their replication.

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Further reading
 Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart
Kauffman
 Seeding the Universe With Life Legacy Books, Washington
D.C., 2000, ISBN 0-476-00330-X
 Walker, Martin G. [permanent dead link] LIFE! Why We Exist ...
And What We Must Do to Survive Dog Ear Publishing,
2006, ISBN 1-59858-243-7

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