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Temporal range: Archeanpresent (possible Hadean origin)
Diverse forms of life on a coral reef
Scientific classification Edit this classification
Domains and Supergroups

Life on Earth:

Life is a quality that distinguishes

organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Viruses in particular make definition difficult as they replicate only in host cells. Life exists all over the earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles

Life has been studied since ancient times, with theories such as

universal common ancestor. This evolved into all the species that exist now, by way of many extinct species, some of which have left traces as fossils. Attempts to classify living things, too, began with Aristotle. Modern classification began with Carl Linnaeus's system of binomial nomenclature
in the 1740s.

Living things are composed of biochemical molecules, formed mainly from a few core chemical elements. All living things contain two types of large molecule, proteins and nucleic acids, the latter usually both DNA and RNA: these carry the information needed by each species, including the instructions to make each type of protein. The proteins, in turn, serve as the machinery which carries out the many chemical processes of life. The cell is the structural and functional unit of life. Smaller organisms, including prokaryotes (bacteria and archaea), consist of small single cells. Larger organisms, mainly eukaryotes, can consist of single cells or may be multicellular with more complex structure. Life is confirmed only on Earth but extraterrestrial life is thought probable. Artificial life is being simulated and explored by scientists and engineers.



The definition of life has long been a challenge for scientists and philosophers.[2][3][4] This is partially because life is a process, not a substance.[5][6][7] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside Earth.[8][9] Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living.[10] Legal definitions of life have been debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[11] At least 123 definitions of life have been compiled.[12]


Since there is no consensus for a 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 implies all or most of the following traits:[4][13][14][15][16][17]

  1. Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature
  2. Organisation: being structurally composed of one or more cells
     – the basic units of life
  3. Metabolism: transformation of energy, used to convert chemicals into cellular components (anabolism) and to decompose organic matter (catabolism). Living things require energy for homeostasis and other activities.
  4. Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size and structure.
  5. Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat.[18][19][20]
  6. Response to
    multicellular organisms, or the motion of 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.


From a

exobiology, based on a suggestion by Carl Sagan.[24][25] This definition, however, has been widely criticized because according to it, a single sexually reproducing individual is not alive as it is incapable of evolving on its own.[26] The reason for this potential flaw is that "NASA's definition" refers to life as a phenomenon, not a living individual, which makes it incomplete.[27] Alternative definitions based on the notion of life as a phenomenon and a living individual have been proposed as continuum of a self-maintainable information, and a distinct element of this continuum, respectively. A major strength of this approach is that it defines life in terms of mathematics and physics, avoiding biological vocabulary.[27]

Living systems

Others take a

living systems theory 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 include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself, and of completing at least one thermodynamic work cycle.[28] This definition is extended by the evolution of novel functions over time.[29]


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

Death is the termination of all vital functions or life processes in an organism or cell.[30][31] 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.[31] However, determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems.[32] Such determination, therefore, requires drawing conceptual lines between life and death. This is problematic 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.[33]

"At the edge of life": viruses

as seen under an electron microscope

Whether or not viruses should be considered as alive is controversial.

organic molecules.[40][41]

History of study


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.[42] Democritus (460 BC) was an atomist; he thought that the essential characteristic of life was having a soul (psyche), and that the soul, like everything else, was composed of fiery atoms. He elaborated on fire because of the apparent connection between life and heat, and because fire moves.[43]

forms, reflected imperfectly in matter; forms provided direction or intelligence, explaining the regularities observed in the world.[44] The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes (1596–1650), who held that animals and humans were assemblages of parts that together functioned as a machine. This idea was developed further by Julien Offray de La Mettrie (1709–1750) in his book L'Homme Machine.[45] 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.[46] At the beginning of the 20th century Stéphane Leduc (1853–1939) promoted the idea that biological processes could be understood in terms of physics and chemistry, and that their growth resembled that of inorganic crystals immersed in solutions of sodium silicate. His ideas, set out in his book La biologie synthétique[47] was widely dismissed during his lifetime, but has incurred a resurgence of interest in the work of Russell, Barge and colleagues.[48]


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

Hylomorphism is a theory first expressed by the Greek philosopher

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.[49] 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.[50]

This account is consistent with

evolutionary history of a species, which led to the natural selection of the features in question.[51]

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.[52]

The theory of spontaneous generation was proposed by Aristotle,[53] who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it was considered the best explanation 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.[54][55] Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.[56][57][58]


Vitalism is the belief that there is a non-material life-principle. This originated with

organic material can only be derived from living things. This was disproved in 1828, when Friedrich Wöhler prepared urea from inorganic materials.[61] 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 inorganic reactions.[60]

During the 1850s

homoeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.[64]


Origin of life


organic molecules via pre-cellular life to protocells and metabolism.[73] In 2016, a set of 355 genes from the last universal common ancestor was tentatively identified.[74]

The biosphere is postulated to have developed, from the origin of life onwards, at least some 3.5 billion years ago.

formation of the Earth 4.54 billion years ago.[76]


heritable characteristics of biological populations over successive generations. It results in the appearance of new species and often the disappearance of old ones.[77][78] Evolution occurs when evolutionary processes such as natural selection (including sexual selection) and genetic drift act on genetic variation, resulting in certain characteristics increasing or decreasing in frequency within a population over successive generations.[79] The process of evolution has given rise to biodiversity at every level of biological organisation.[80][81]


Fossils are the preserved remains or

billion years old.[83][84]


Extinction is the process by which a

Mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.[90]

Environmental conditions

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

environmental challenges,[91] and symbiosis.[92][93][94] 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.[91] 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.[91]



microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are."[104]

Range of tolerance

The inert components of an ecosystem are the physical and chemical factors necessary for life—energy (sunlight or

atmosphere, gravity, nutrients, and ultraviolet solar radiation protection.[110] 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."[111] 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.[111]


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

To survive, some microorganisms have evolved 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 long periods.[91][112] They excel at exploiting uncommon sources of energy. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing.[113]



The first classification of organisms was made by the Greek philosopher Aristotle (384–322 BC), who grouped living things as either plants or animals, based mainly on their ability to move. He distinguished animals with blood from animals without blood, which can be compared with the concepts of

molluscs and echinoderms), and "zoophytes" (animals that resemble plants). This remained the ultimate authority for many centuries after his death.[114]


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.[115]

The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon

Evolutionary history shows that the fungi are more closely related to animals than to plants.[117]

As advances in

six-kingdom system and eventually to the current three-domain system, which is based on evolutionary relationships.[118] However, the classification of eukaryotes, especially of protists, is still controversial.[119]

As microbiology developed, viruses, which are non-cellular, were discovered. 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 have been classed into "species" based on their genetics, but many aspects of such a classification remain controversial.[120]

The original Linnaean system has been modified many times, for example as follows:

Woese et al.
1998,[126] 2015[127]
2 kingdoms 3 kingdoms 2 empires 4 kingdoms 5 kingdoms 3 domains 2 empires,
6/7 kingdoms
(not treated) Protista
Monera Monera Bacteria Bacteria
Archaea Archaea (2015)
Protista Eucarya Protozoa
Plantae Plantae Plantae Plantae
Fungi Fungi
Animalia Animalia Animalia Animalia Animalia

The attempt to organise the Eukaryotes into a small number of kingdoms has been challenged. The Protozoa do not form a clade or natural grouping,[128] and nor do the Chromista (Chromalveolata).[129]


Chemical elements

All life forms require certain core chemical elements for their biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the elemental macronutrients for all organisms.[130] 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 abundant of these elements in organisms is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based (organic) molecules to form the immense variety of chemical arrangements described in organic chemistry.[131] 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.[132][133]


Deoxyribonucleic acid or

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. This has the key property that each strand contains all the information needed to recreate the other strand, enabling the information to be preserved during reproduction and cell division.[135] Within cells, DNA is organised 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. Eukaryotes store most of their DNA inside the cell nucleus.[136]


Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division.[137][138] Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted.[139] The activity of an organism depends on the total activity of its cells, with energy flow occurring within and between them. Cells contain hereditary information that is carried forward as a genetic code during cell division.[140]

There are two primary types of cells, reflecting their evolutionary origins.

endosymbiosis between bacteria and the progenitor eukaryotic cell.[142]

The molecular mechanisms of cell biology are based on proteins. Most of these are synthesised by the ribosomes through an enzyme-catalyzed process called protein biosynthesis. A sequence of amino acids is assembled and joined based upon gene expression of the cell's nucleic acid.[143] In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.[144]

Cells reproduce through a process of

mutations), and both are capable of further division following an interphase period.[145]

Multicellular structure

GK-PID, may have allowed organisms to go from a single cell organism to one of many cells.[147]

Cells have evolved methods to perceive and respond to their microenvironment, thereby enhancing their adaptability.

Cell signalling 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.[148]


Though life is confirmed only on Earth, many think that

Investigation of the tenacity and versatility of life on Earth,

Beyond the Solar System, the region around another

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.[156] 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.[157] The variables of the Drake equation are used to discuss the conditions in planetary systems where civilisation is most likely to exist, within wide bounds of uncertainty.[158] A "Confidence of Life Detection" scale (CoLD) for reporting evidence of life beyond Earth has been proposed.[159][160]


Artificial life is the simulation of any aspect of life, as through computers, robotics, or biochemistry.[161] 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.[162]

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