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Temporal range: 3770–0 Ma 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 matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not. It is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. All life over time eventually reaches a state of death and none is immortal. 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 Empedocles's materialism asserting that it was composed of four eternal elements, and Aristotle's hylomorphism asserting that living things have souls and embody both form and matter. Life originated at least 3.5 billion years ago, resulting in a 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 only known to exist 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 stimuli: such as the contraction of a unicellular organism away from external chemicals, the complex reactions involving all the senses of 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 physics perspective, an organism is a thermodynamic system with an organised molecular structure that can reproduce itself and evolve as survival dictates.[21][22] Thermodynamically, life has been described as an open system which makes use of gradients in its surroundings to create imperfect copies of itself.[23] Another way of putting this is to define life as "a self-sustained chemical system capable of undergoing Darwinian evolution", a definition adopted by a NASA committee attempting to define life for the purposes of 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]


Adenoviruses as seen under an electron microscope

Whether or not viruses should be considered as alive is controversial.[34][35] They are most often considered as just gene coding replicators rather than forms of life.[36] They have been described as "organisms at the edge of life"[37] because they possess genes, evolve by natural selection,[38][39] and replicate by making multiple copies of themselves through self-assembly. However, viruses do not metabolise 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.[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] Plato, in contrast, held that the world was organized by permanent 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 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.[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 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.[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 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,[59] anatomists like Xavier Bichat, and chemists like Justus von Liebig.[60] 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.[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 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.[62] These results led to the abandonment of scientific interest in vitalistic theories, especially after Eduard Buchner's demonstration that alcoholic fermentation could occur in cell-free extracts of yeast.[63] Nonetheless, belief still exists in pseudoscientific theories such as homoeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.[64]


Origin of life

The age of Earth is about 4.54 billion years.[65] Life on Earth has existed for at least 3.5 billion years,[66][67][68][69] with the oldest physical traces of life dating back 3.7 billion years.[70][71] Estimates from molecular clocks, as summarized in the TimeTree public database, place the origin of life around 4.0 billion years ago.[72] Hypotheses on the origin of life attempt to explain the formation of a universal common ancestor from simple 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.[75] The earliest evidence for life on Earth includes biogenic graphite found in 3.7 billion-year-old metasedimentary rocks from Western Greenland[70] and microbial mat fossils found in 3.48 billion-year-old sandstone from Western Australia.[71] More recently, in 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia.[66] In 2017, putative fossilised 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.[76]


Evolution is the change in 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 traces of organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in layers (strata) of sedimentary rock 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.[82] 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.[83][84]


Extinction is the process by which a species dies out.[85] The moment of extinction is 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. Over 99% of all the species that have ever lived are now extinct.[86][87][88][89] Mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.[90]

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,[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]


Deinococcus geothermalis, a bacterium that thrives in geothermal springs and deep ocean subsurfaces[95]

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.[96] Organisms exist in every part of the 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.[97][98][99] For example, spores of Aspergillus niger have been detected in the mesosphere at an altitude of 48 to 77 km.[100] Under test conditions, life forms have been observed to thrive in the near-weightlessness of space[101][102] and to survive in the vacuum of space.[103][104] Life forms thrive in the deep Mariana Trench,[105] and 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,[106][107] and 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan.[108] In 2014, life forms were found living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica.[109][110] Expeditions of the International Ocean Discovery Program found unicellular life in 120 °C sediment 1.2 km below seafloor in the Nankai Trough subduction zone.[111] According to one researcher, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are."[106]

Range of tolerance

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.[112] 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".[113] 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.[113]


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 extremophile microorganisms may survive exposure to such conditions for long periods.[91][114] They excel at exploiting uncommon sources of energy. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing.[115]



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 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 divided into five groups: cephalopods, crustaceans, insects (which included the spiders, scorpions, and centipedes), shelled animals (such as most molluscs and echinoderms), and "zoophytes" (animals that resemble plants). This theory remained dominant for more than a thousand years.[116]


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.[117]

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. Herbert Copeland classified the Fungi in his Protoctista, including them with single-celled organisms and thus partially avoiding the problem but acknowledging their special status.[118] 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.[119]

As advances in microscopy 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 Ernst 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 system and eventually to the current three-domain system, which is based on evolutionary relationships.[120] However, the classification of eukaryotes, especially of protists, is still controversial.[121]

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.[122]

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

Woese et al.
1998,[128] 2015[129]
2 kingdoms 3 kingdoms 2 empires 4 kingdoms 5 kingdoms 3 domains 2 empires,
6/7 kingdoms
(not treated) Protista Prokaryota Monera Monera Bacteria Bacteria
Archaea Archaea (2015)
Eukaryota Protoctista Protista Eucarya "Protozoa"
Vegetabilia 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,[130] and nor do the Chromista (Chromalveolata).[131]


The ability to sequence large numbers of complete genomes has allowed biologists to take a metagenomic view of the phylogeny of the whole tree of life. This has led to the realisation that the majority of living things are bacteria, and that all have a common origin.[120][132]


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.[133] 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.[134] 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.[135][136]


Deoxyribonucleic acid or DNA 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 RNA are 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.[137] 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. 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.[138] 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.[139]


Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by division.[140][141] Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted.[142] 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.[143]

There are two primary types of cells, reflecting their evolutionary origins. Prokaryote cells 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 is the eukaryote cell, which has a distinct nucleus bound by a nuclear membrane and membrane-bound organelles, including mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, and vacuoles. In addition, their DNA is organised into chromosomes. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though with a wide diversity of protist microorganisms.[144] 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.[145]

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.[146] In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination.[147]

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 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.[148]

Multicellular structure

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 specialisations, 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 specialised cells that form the adult organism. This specialisation allows multicellular organisms to exploit resources more efficiently than single cells.[149] About 800 million years ago, a minor genetic change in a single molecule, the enzyme GK-PID, may have allowed organisms to go from a single cell organism to one of many cells.[150]

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.[151]


Though life is confirmed only on Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable.[152][153] 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 civilisations. Other locations within the Solar System that may host microbial life include the subsurface of Mars, the upper atmosphere of Venus,[154] and subsurface oceans on some of the moons of the giant planets.[155][156]

Investigation of the tenacity and versatility of life on Earth,[114] as well as an understanding of the molecular systems that some organisms utilise to survive such extremes, is important for the search for extraterrestrial life.[91] For example, lichen could survive for a month in a simulated Martian environment.[157][158]

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.[159] 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.[160] 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.[161] A "Confidence of Life Detection" scale (CoLD) for reporting evidence of life beyond Earth has been proposed.[162][163]


Artificial life is the simulation of any aspect of life, as through computers, robotics, or biochemistry.[164] 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.[165]

See also


  1. ^ Viruses are strongly believed not to descend from a common ancestor, with each realm corresponding to separate instances of a virus coming into existence.[1]


  1. ^ International Committee on Taxonomy of Viruses Executive Committee (May 2020). "The New Scope of Virus Taxonomy: Partitioning the Virosphere Into 15 Hierarchical Ranks". Nature Microbiology. 5 (5): 668–674. doi:10.1038/s41564-020-0709-x. PMC 7186216. PMID 32341570.
  2. ^ Tsokolov, Serhiy A. (May 2009). "Why Is the Definition of Life So Elusive? Epistemological Considerations". Astrobiology. 9 (4): 401–412. Bibcode:2009AsBio...9..401T. doi:10.1089/ast.2007.0201. PMID 19519215.
  3. ^ Emmeche, Claus (1997). "Defining Life, Explaining Emergence". Niels Bohr Institute. Archived from the original on 14 March 2012. Retrieved 25 May 2012.
  4. ^ a b McKay, Chris P. (14 September 2004). "What Is Life—and How Do We Search for It in Other Worlds?". PLOS Biology. 2 (9): 302. doi:10.1371/journal.pbio.0020302. PMC 516796. PMID 15367939.
  5. ^ Mautner, Michael N. (1997). "Directed panspermia. 3. Strategies and motivation for seeding star-forming clouds" (PDF). Journal of the British Interplanetary Society. 50: 93–102. Bibcode:1997JBIS...50...93M. Archived (PDF) from the original on 2 November 2012.
  6. ^ Mautner, Michael N. (2000). Seeding the Universe with Life: Securing Our Cosmological Future (PDF). Michael Mautner. ISBN 978-0-476-00330-9. Archived (PDF) from the original on 2 November 2012.
  7. ^ McKay, Chris (18 September 2014). "What is life? It's a Tricky, Often Confusing Question". Astrobiology Magazine.
  8. ^ Nealson, K.H.; Conrad, P.G. (December 1999). "Life: past, present and future". Philosophical Transactions of the Royal Society of London B. 354 (1392): 1923–1939. doi:10.1098/rstb.1999.0532. PMC 1692713. PMID 10670014. Archived from the original on 3 January 2016.
  9. ^ Mautner, Michael N. (2009). "Life-centered ethics, and the human future in space" (PDF). Bioethics. 23 (8): 433–440. doi:10.1111/j.1467-8519.2008.00688.x. PMID 19077128. S2CID 25203457. Archived (PDF) from the original on 2 November 2012.
  10. ^ Jeuken M (1975). "The biological and philosophical defitions of life". Acta Biotheoretica. 24 (1–2): 14–21. doi:10.1007/BF01556737. PMID 811024. S2CID 44573374.
  11. ^ Capron AM (1978). "Legal definition of death". Annals of the New York Academy of Sciences. 315 (1): 349–362. Bibcode:1978NYASA.315..349C. doi:10.1111/j.1749-6632.1978.tb50352.x. PMID 284746. S2CID 36535062.
  12. ^ Trifonov, Edward N. (17 March 2011). "Vocabulary of Definitions of Life Suggests a Definition". Journal of Biomolecular Structure and Dynamics. 29 (2): 259–266. doi:10.1080/073911011010524992. PMID 21875147.
  13. ^ Koshland, Daniel E. Jr. (22 March 2002). "The Seven Pillars of Life". Science. 295 (5563): 2215–2216. doi:10.1126/science.1068489. PMID 11910092.
  14. ^ "life". The American Heritage Dictionary of the English Language (4th ed.). Houghton Mifflin. 2006. ISBN 978-0-618-70173-5.
  15. ^ "Life". Merriam-Webster Dictionary. Archived from the original on 13 December 2021. Retrieved 25 July 2022.
  16. ^ "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Archived from the original on 16 April 2014. Retrieved 6 June 2013.
  17. ^ Trifonov, Edward N. (2012). "Definition of Life: Navigation through Uncertainties". Journal of Biomolecular Structure & Dynamics. 29 (4): 647–650. doi:10.1080/073911012010525017. PMID 22208269. S2CID 8616562.
  18. ^ Dobzhansky, Theodosius (1968). "On Some Fundamental Concepts of Darwinian Biology". Evolutionary Biology. Boston, MA: Springer US. pp. 1–34. doi:10.1007/978-1-4684-8094-8_1. ISBN 978-1-4684-8096-2. Archived from the original on 30 July 2022. Retrieved 23 July 2022.
  19. ^ Wang, Guanyu (2014). Analysis of complex diseases : a mathematical perspective. CRC Press. ISBN 978-1-4665-7223-2. OCLC 868928102. Archived from the original on 30 July 2022. Retrieved 23 July 2022.
  20. ^ Sejian, Veerasamy; Gaughan, John; Baumgard, Lance; Prasad, C. S., eds. (2015). Climate change impact on livestock : adaptation and mitigation. Springer. ISBN 978-81-322-2265-1. OCLC 906025831. Archived from the original on 30 July 2022. Retrieved 23 July 2022.
  21. ^ Luttermoser, Donald G. "ASTR-1020: Astronomy II Course Lecture Notes Section XII" (PDF). East Tennessee State University. Archived from the original (PDF) on 22 March 2012. Retrieved 28 August 2011.
  22. ^ Luttermoser, Donald G. (Spring 2008). "Physics 2028: Great Ideas in Science: The Exobiology Module" (PDF). East Tennessee State University. Archived from the original (PDF) on 22 March 2012. Retrieved 28 August 2011.
  23. ^ Lammer, H.; Bredehöft, J.H.; Coustenis, A.; et al. (2009). "What makes a planet habitable?" (PDF). The Astronomy and Astrophysics Review. 17 (2): 181–249. Bibcode:2009A&ARv..17..181L. doi:10.1007/s00159-009-0019-z. S2CID 123220355. Archived from the original (PDF) on 2 June 2016. Retrieved 3 May 2016. Life as we know it has been described as a (thermodynamically) open system (Prigogine et al. 1972), which makes use of gradients in its surroundings to create imperfect copies of itself.
  24. ^ Benner, Steven A. (December 2010). "Defining Life". Astrobiology. 10 (10): 1021–1030. Bibcode:2010AsBio..10.1021B. doi:10.1089/ast.2010.0524. PMC 3005285. PMID 21162682.
  25. ^ Joyce, Gerald F. (1995). "The RNA World: Life before DNA and Protein". Extraterrestrials. Cambridge University Press. pp. 139–151. doi:10.1017/CBO9780511564970.017. hdl:2060/19980211165. ISBN 978-0-511-56497-0. S2CID 83282463.
  26. ^ Benner, Steven A. (December 2010). "Defining Life". Astrobiology. 10 (10): 1021–1030. Bibcode:2010AsBio..10.1021B. doi:10.1089/ast.2010.0524. PMC 3005285. PMID 21162682.
  27. ^ a b Piast, Radosław W. (June 2019). "Shannon's information, Bernal's biopoiesis and Bernoulli distribution as pillars for building a definition of life". Journal of Theoretical Biology. 470: 101–107. Bibcode:2019JThBi.470..101P. doi:10.1016/j.jtbi.2019.03.009. PMID 30876803. S2CID 80625250.
  28. ^ Kaufmann, Stuart (2004). "Autonomous agents". In Barrow, John D.; Davies, P.C.W.; Harper, Jr., C.L. (eds.). Science and Ultimate Reality. pp. 654–666. doi:10.1017/CBO9780511814990.032. ISBN 978-0-521-83113-0. Archived from the original on 5 November 2023. Retrieved 10 August 2023.
  29. ^ Longo, Giuseppe; Montévil, Maël; Kauffman, Stuart (1 January 2012). "No entailing laws, but enablement in the evolution of the biosphere". Proceedings of the 14th annual conference companion on Genetic and evolutionary computation. GECCO '12. pp. 1379–1392. arXiv:1201.2069. Bibcode:2012arXiv1201.2069L. CiteSeerX doi:10.1145/2330784.2330946. ISBN 978-1-4503-1178-6. S2CID 15609415. Archived from the original on 11 May 2017.
  30. ^ Definition of death. Archived from the original on 3 November 2009.
  31. ^ a b "Definition of death". Encyclopedia of Death and Dying. Advameg, Inc. Archived from the original on 3 February 2007. Retrieved 25 May 2012.
  32. ^ Henig, Robin Marantz (April 2016). "Crossing Over: How Science Is Redefining Life and Death". National Geographic. Archived from the original on 1 November 2017. Retrieved 23 October 2017.
  33. ^ "How the Major Religions View the Afterlife". Archived from the original on 4 February 2022. Retrieved 4 February 2022.
  34. ^ "Virus". Archived from the original on 11 May 2022. Retrieved 25 July 2022.
  35. ^ "Are Viruses Alive?". Yellowstone Thermal Viruses. Archived from the original on 14 June 2022. Retrieved 25 July 2022.
  36. ^ Koonin, E.V.; Starokadomskyy, P. (7 March 2016). "Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question". Studies in the History and Philosophy of Biology and Biomedical Science. 59: 125–134. doi:10.1016/j.shpsc.2016.02.016. PMC 5406846. PMID 26965225.
  37. ^ Rybicki, EP (1990). "The classification of organisms at the edge of life, or problems with virus systematics". S Afr J Sci. 86: 182–186. Archived from the original on 21 September 2021. Retrieved 5 November 2023.
  38. ^ Holmes, E.C. (October 2007). "Viral evolution in the genomic age". PLOS Biol. 5 (10): e278. doi:10.1371/journal.pbio.0050278. PMC 1994994. PMID 17914905.
  39. ^ Forterre, Patrick (3 March 2010). "Defining Life: The Virus Viewpoint". Orig Life Evol Biosph. 40 (2): 151–160. Bibcode:2010OLEB...40..151F. doi:10.1007/s11084-010-9194-1. PMC 2837877. PMID 20198436.
  40. ^ Koonin, E.V.; Senkevich, T.G.; Dolja, V.V. (2006). "The ancient Virus World and evolution of cells". Biology Direct. 1: 29. doi:10.1186/1745-6150-1-29. PMC 1594570. PMID 16984643.
  41. ^ Rybicki, Ed (November 1997). "Origins of Viruses". Archived from the original on 9 May 2009. Retrieved 12 April 2009.
  42. ^ Parry, Richard (4 March 2005). "Empedocles". Stanford Encyclopedia of Philosophy. Archived from the original on 13 May 2012. Retrieved 25 May 2012.
  43. ^ Parry, Richard (25 August 2010). "Democritus". Stanford Encyclopedia of Philosophy. Archived from the original on 30 August 2006. Retrieved 25 May 2012.
  44. ^ Hankinson, R.J. (1997). Cause and Explanation in Ancient Greek Thought. Oxford University Press. p. 125. ISBN 978-0-19-924656-4. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  45. ^ de la Mettrie, J.J.O. (1748). L'Homme Machine [Man a machine]. Leyden: Elie Luzac.
  46. ^ Thagard, Paul (2012). The Cognitive Science of Science: Explanation, Discovery, and Conceptual Change. MIT Press. pp. 204–205. ISBN 978-0-262-01728-2. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  47. ^ Leduc, Stéphane (1912). La Biologie Synthétique [Synthetic Biology]. Paris: Poinat.
  48. ^ Russell, Michael J.; Barge, Laura M.; Bhartia, Rohit; et al. (2014). "The Drive to Life on Wet and Icy Worlds". Astrobiology. 14 (4): 308–343. Bibcode:2014AsBio..14..308R. doi:10.1089/ast.2013.1110. PMC 3995032. PMID 24697642.
  49. ^ Aristotle. On the Soul. Book II.
  50. ^ Marietta, Don (1998). Introduction to ancient philosophy. M.E. Sharpe. p. 104. ISBN 978-0-7656-0216-9. Archived from the original on 13 April 2023. Retrieved 25 August 2020.
  51. ^ Stewart-Williams, Steve (2010). Darwin, God and the meaning of life: how evolutionary theory undermines everything you thought you knew of life. Cambridge University Press. pp. 193–194. ISBN 978-0-521-76278-6. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  52. ^ Stillingfleet, Edward (1697). Origines Sacrae. Cambridge University Press.
  53. ^ André Brack (1998). "Introduction" (PDF). In André Brack (ed.). The Molecular Origins of Life. Cambridge University Press. p. 1. ISBN 978-0-521-56475-5. Retrieved 7 January 2009.
  54. ^ Levine, Russell; Evers, Chris. "The Slow Death of Spontaneous Generation (1668–1859)". North Carolina State University. National Health Museum. Archived from the original on 9 October 2015. Retrieved 6 February 2016.
  55. ^ Tyndall, John (1905). Fragments of Science. Vol. 2. New York: P.F. Collier. Chapters IV, XII, and XIII.
  56. ^ Bernal, J.D. (1967) [Reprinted work by A.I. Oparin originally published 1924; Moscow: The Moscow Worker]. The Origin of Life. The Weidenfeld and Nicolson Natural History. Translation of Oparin by Ann Synge. London: Weidenfeld & Nicolson. LCCN 67098482.
  57. ^ Zubay, Geoffrey (2000). Origins of Life: On Earth and in the Cosmos (2nd ed.). Academic Press. ISBN 978-0-12-781910-5.
  58. ^ Smith, John Maynard; Szathmary, Eors (1997). The Major Transitions in Evolution. Oxford Oxfordshire: Oxford University Press. ISBN 978-0-19-850294-4.
  59. ^ Schwartz, Sanford (2009). C.S. Lewis on the Final Frontier: Science and the Supernatural in the Space Trilogy. Oxford University Press. p. 56. ISBN 978-0-19-988839-9. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  60. ^ a b Wilkinson, Ian (1998). "History of Clinical Chemistry – Wöhler & the Birth of Clinical Chemistry" (PDF). The Journal of the International Federation of Clinical Chemistry and Laboratory Medicine. 13 (4). Archived from the original (PDF) on 5 January 2016. Retrieved 27 December 2015.
  61. ^ Friedrich Wöhler (1828). "Ueber künstliche Bildung des Harnstoffs". Annalen der Physik und Chemie. 88 (2): 253–256. Bibcode:1828AnP....88..253W. doi:10.1002/andp.18280880206. Archived from the original on 10 January 2012.
  62. ^ Rabinbach, Anson (1992). The Human Motor: Energy, Fatigue, and the Origins of Modernity. University of California Press. pp. 124–125. ISBN 978-0-520-07827-7. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  63. ^ Cornish-Bowden Athel, ed. (1997). New Beer in an Old Bottle. Eduard Buchner and the Growth of Biochemical Knowledge. Valencia, Spain: Universitat de València. ISBN 978-8437-033280.
  64. ^ "NCAHF Position Paper on Homeopathy". National Council Against Health Fraud. February 1994. Archived from the original on 25 December 2018. Retrieved 12 June 2012.
  65. ^ Dalrymple, G. Brent (2001). "The age of the Earth in the twentieth century: a problem (mostly) solved". Special Publications, Geological Society of London. 190 (1): 205–221. Bibcode:2001GSLSP.190..205D. doi:10.1144/GSL.SP.2001.190.01.14. S2CID 130092094.
  66. ^ a b Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon" (PDF). PNAS. 112 (47): 14518–14521. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. PMC 4664351. PMID 26483481. Archived (PDF) from the original on 6 November 2015.
  67. ^ Schopf, J.W. (June 2006). "Fossil evidence of Archaean life". Philos. Trans. R. Soc. Lond. B Biol. Sci. 361 (1470): 869–885. doi:10.1098/rstb.2006.1834. PMC 1578735. PMID 16754604.
  68. ^ Hamilton Raven, Peter; Brooks Johnson, George (2002). Biology. McGraw-Hill Education. p. 68. ISBN 978-0-07-112261-0. Retrieved 7 July 2013.
  69. ^ Milsom, Clare; Rigby, Sue (2009). Fossils at a Glance (2nd ed.). John Wiley & Sons. p. 134. ISBN 978-1-4051-9336-8. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  70. ^ a b Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; et al. (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. 7 (1): 25–28. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025.
  71. ^ a b Noffke, Nora; Christian, Daniel; Wacey, David; et al. (8 November 2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–1124. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812.
  72. ^ Hedges, S. B. Hedges (2009). "Life". In S. B. Hedges; S. Kumar (eds.). The Timetree of Life. Oxford University Press. pp. 89–98. ISBN 978-0-1995-3503-3.
  73. ^ "Habitability and Biology: What are the Properties of Life?". Phoenix Mars Mission. The University of Arizona. Archived from the original on 17 April 2014. Retrieved 6 June 2013.
  74. ^ Wade, Nicholas (25 July 2016). "Meet Luca, the Ancestor of All Living Things". The New York Times. Archived from the original on 28 July 2016. Retrieved 25 July 2016.
  75. ^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall. ISBN 978-0-13-250882-7. Archived from the original on 2 November 2014. Retrieved 15 June 2016.
  76. ^ Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; et al. (1 March 2017). "Evidence for early life in Earth's oldest hydrothermal vent precipitates". Nature. 543 (7643): 60–64. Bibcode:2017Natur.543...60D. doi:10.1038/nature21377. PMID 28252057. Archived from the original on 8 September 2017. Retrieved 2 March 2017.
  77. ^ Hall, Brian K.; Hallgrímsson, Benedikt (2008). Strickberger's Evolution (4th ed.). Sudbury, Massachusetts: Jones and Bartlett Publishers. pp. 4–6. ISBN 978-0-7637-0066-9. LCCN 2007008981. OCLC 85814089.
  78. ^ "Evolution Resources". Washington, DC: National Academies of Sciences, Engineering, and Medicine. 2016. Archived from the original on 3 June 2016.
  79. ^ Scott-Phillips, Thomas C.; Laland, Kevin N.; Shuker, David M.; et al. (May 2014). "The Niche Construction Perspective: A Critical Appraisal". Evolution. 68 (5): 1231–1243. doi:10.1111/evo.12332. PMC 4261998. PMID 24325256. Evolutionary processes are generally thought of as processes by which these changes occur. Four such processes are widely recognized: natural selection (in the broad sense, to include sexual selection), genetic drift, mutation, and migration (Fisher 1930; Haldane 1932). The latter two generate variation; the first two sort it.
  80. ^ Hall & Hallgrímsson 2008, pp. 3–5
  81. ^ Voet, Donald; Voet, Judith G.; Pratt, Charlotte W. (2016). Fundamentals of Biochemistry: Life at the Molecular Level (Fifth ed.). Hoboken, New Jersey: John Wiley & Sons. Chapter 1: Introduction to the Chemistry of Life, pp. 1–22. ISBN 978-1-118-91840-1. LCCN 2016002847. OCLC 939245154.
  82. ^ "Frequently Asked Questions". San Diego Natural History Museum. Archived from the original on 10 May 2012. Retrieved 25 May 2012.
  83. ^ Vastag, Brian (21 August 2011). "Oldest 'microfossils' raise hopes for life on Mars". The Washington Post. Archived from the original on 19 October 2011. Retrieved 21 August 2011.
  84. ^ Wade, Nicholas (21 August 2011). "Geological Team Lays Claim to Oldest Known Fossils". The New York Times. Archived from the original on 1 May 2013. Retrieved 21 August 2011.
  85. ^ Extinction – definition. Archived from the original on 26 September 2009.
  86. ^ "What is an extinction?". Late Triassic. Bristol University. Archived from the original on 1 September 2012. Retrieved 27 June 2012.
  87. ^ McKinney, Michael L. (1996). "How do rare species avoid extinction? A paleontological view". In Kunin, W.E.; Gaston, Kevin (eds.). The Biology of Rarity: Causes and consequences of rare—common differences. Springer. ISBN 978-0-412-63380-5. Archived from the original on 3 February 2023. Retrieved 26 May 2015.
  88. ^ Stearns, Beverly Peterson; Stearns, Stephen C. (2000). Watching, from the Edge of Extinction. Yale University Press. p. x. ISBN 978-0-300-08469-6. Archived from the original on 5 November 2023. Retrieved 30 May 2017.
  89. ^ Novacek, Michael J. (8 November 2014). "Prehistory's Brilliant Future". The New York Times. Archived from the original on 29 December 2014. Retrieved 25 December 2014.
  90. ^ Van Valkenburgh, B. (1999). "Major patterns in the history of carnivorous mammals". Annual Review of Earth and Planetary Sciences. 27: 463–493. Bibcode:1999AREPS..27..463V. doi:10.1146/ Archived from the original on 29 February 2020. Retrieved 29 June 2019.
  91. ^ a b c d e Rothschild, Lynn (September 2003). "Understand the evolutionary mechanisms and environmental limits of life". NASA. Archived from the original on 29 March 2012. Retrieved 13 July 2009.
  92. ^ King, G.A.M. (April 1977). "Symbiosis and the origin of life". Origins of Life and Evolution of Biospheres. 8 (1): 39–53. Bibcode:1977OrLi....8...39K. doi:10.1007/BF00930938. PMID 896191. S2CID 23615028.
  93. ^ Margulis, Lynn (2001). The Symbiotic Planet: A New Look at Evolution. London: Orion Books. ISBN 978-0-7538-0785-9.
  94. ^ Futuyma, D.J.; Janis Antonovics (1992). Oxford surveys in evolutionary biology: Symbiosis in evolution. Vol. 8. London, England: Oxford University Press. pp. 347–374. ISBN 978-0-19-507623-3.
  95. ^ Liedert, Christina; Peltola, Minna; Bernhardt, Jörg; et al. (15 March 2012). "Physiology of Resistant Deinococcus geothermalis Bacterium Aerobically Cultivated in Low-Manganese Medium". Journal of Bacteriology. 194 (6): 1552–1561. doi:10.1128/JB.06429-11. PMC 3294853. PMID 22228732.
  96. ^ "Biosphere". The Columbia Encyclopedia (6th ed.). Columbia University Press. 2004. Archived from the original on 27 October 2011.
  97. ^ University of Georgia (25 August 1998). "First-Ever Scientific Estimate Of Total Bacteria On Earth Shows Far Greater Numbers Than Ever Known Before". Science Daily. Archived from the original on 10 November 2014. Retrieved 10 November 2014.
  98. ^ Hadhazy, Adam (12 January 2015). "Life Might Thrive a Dozen Miles Beneath Earth's Surface". Astrobiology Magazine. Archived from the original on 12 March 2017. Retrieved 11 March 2017.
  99. ^ Fox-Skelly, Jasmin (24 November 2015). "The Strange Beasts That Live in Solid Rock Deep Underground". BBC. Archived from the original on 25 November 2016. Retrieved 11 March 2017.
  100. ^ Imshenetsky, AA; Lysenko, SV; Kazakov, GA (June 1978). "Upper boundary of the biosphere". Applied and Environmental Microbiology. 35 (1): 1–5. Bibcode:1978ApEnM..35....1I. doi:10.1128/aem.35.1.1-5.1978. PMC 242768. PMID 623455.
  101. ^ Dvorsky, George (13 September 2017). "Alarming Study Indicates Why Certain Bacteria Are More Resistant to Drugs in Space". Gizmodo. Archived from the original on 14 September 2017. Retrieved 14 September 2017.
  102. ^ Caspermeyer, Joe (23 September 2007). "Space flight shown to alter ability of bacteria to cause disease". Arizona State University. Archived from the original on 14 September 2017. Retrieved 14 September 2017.
  103. ^ Dose, K.; Bieger-Dose, A.; Dillmann, R.; et al. (1995). "ERA-experiment "space biochemistry"". Advances in Space Research. 16 (8): 119–129. Bibcode:1995AdSpR..16h.119D. doi:10.1016/0273-1177(95)00280-R. PMID 11542696.
  104. ^ Horneck G.; Eschweiler, U.; Reitz, G.; et al. (1995). "Biological responses to space: results of the experiment "Exobiological Unit" of ERA on EURECA I". Adv. Space Res. 16 (8): 105–118. Bibcode:1995AdSpR..16h.105H. doi:10.1016/0273-1177(95)00279-N. PMID 11542695.
  105. ^ Glud, Ronnie; Wenzhöfer, Frank; Middelboe, Mathias; et al. (17 March 2013). "High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth". Nature Geoscience. 6 (4): 284–288. Bibcode:2013NatGe...6..284G. doi:10.1038/ngeo1773.
  106. ^ a b Choi, Charles Q. (17 March 2013). "Microbes Thrive in Deepest Spot on Earth". LiveScience. Archived from the original on 2 April 2013. Retrieved 17 March 2013.
  107. ^ Oskin, Becky (14 March 2013). "Intraterrestrials: Life Thrives in Ocean Floor". LiveScience. Archived from the original on 2 April 2013. Retrieved 17 March 2013.
  108. ^ Morelle, Rebecca (15 December 2014). "Microbes discovered by deepest marine drill analysed". BBC News. Archived from the original on 16 December 2014. Retrieved 15 December 2014.
  109. ^ Fox, Douglas (20 August 2014). "Lakes under the ice: Antarctica's secret garden". Nature. 512 (7514): 244–246. Bibcode:2014Natur.512..244F. doi:10.1038/512244a. PMID 25143097.
  110. ^ Mack, Eric (20 August 2014). "Life Confirmed Under Antarctic Ice; Is Space Next?". Forbes. Archived from the original on 22 August 2014. Retrieved 21 August 2014.
  111. ^ Heuer, Verena B.; Inagaki, Fumio; Morono, Yuki; et al. (4 December 2020). "Temperature limits to deep subseafloor life in the Nankai Trough subduction zone". Science. 370 (6521): 1230–1234. Bibcode:2020Sci...370.1230H. doi:10.1126/science.abd7934. hdl:2164/15700. PMID 33273103. S2CID 227257205. Archived from the original on 26 September 2022. Retrieved 5 November 2023.
  112. ^ "Essential requirements for life". CMEX-NASA. Archived from the original on 17 August 2009. Retrieved 14 July 2009.
  113. ^ a b Chiras, Daniel C. (2001). Environmental Science – Creating a Sustainable Future (6th ed.). Sudbury, MA : Jones and Bartlett. ISBN 978-0-7637-1316-4.
  114. ^ a b Chang, Kenneth (12 September 2016). "Visions of Life on Mars in Earth's Depths". The New York Times. Archived from the original on 12 September 2016. Retrieved 12 September 2016.
  115. ^ Rampelotto, Pabulo Henrique (2010). "Resistance of microorganisms to extreme environmental conditions and its contribution to astrobiology". Sustainability. 2 (6): 1602–1623. Bibcode:2010Sust....2.1602R. doi:10.3390/su2061602.
  116. ^ "Aristotle". University of California Museum of Paleontology. Archived from the original on 20 November 2016. Retrieved 15 November 2016.
  117. ^ Knapp, Sandra; Lamas, Gerardo; Lughadha, Eimear Nic; et al. (April 2004). "Stability or stasis in the names of organisms: the evolving codes of nomenclature". Philosophical Transactions of the Royal Society of London B. 359 (1444): 611–622. doi:10.1098/rstb.2003.1445. PMC 1693349. PMID 15253348.
  118. ^ Copeland, Herbert F. (1938). "The Kingdoms of Organisms". Quarterly Review of Biology. 13 (4): 383. doi:10.1086/394568. S2CID 84634277.
  119. ^ Whittaker, R.H. (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science. 163 (3863): 150–160. Bibcode:1969Sci...163..150W. CiteSeerX doi:10.1126/science.163.3863.150. PMID 5762760.
  120. ^ a b c d Woese, C.; Kandler, O.; Wheelis, M. (1990). "Towards a natural system of organisms:proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America. 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
  121. ^ Adl, S.M.; Simpson, A.G.; Farmer, M.A. (2005). "The new higher level classification of eukaryotes with emphasis on the taxonomy of protists". Journal of Eukaryotic Microbiology. 52 (5): 399–451. doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873. S2CID 8060916.
  122. ^ Van Regenmortel, M.H. (January 2007). "Virus species and virus identification: past and current controversies". Infection, Genetics and Evolution. 7 (1): 133–144. Bibcode:2007InfGE...7..133V. doi:10.1016/j.meegid.2006.04.002. PMID 16713373. S2CID 86179057.
  123. ^ Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
  124. ^ Haeckel, E. (1866). Generelle Morphologie der Organismen. Reimer, Berlin.
  125. ^ Chatton, É. (1925). "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires". Annales des Sciences Naturelles - Zoologie et Biologie Animale. 10-VII: 1–84.
  126. ^ Copeland, H. (1938). "The kingdoms of organisms". Quarterly Review of Biology. 13 (4): 383–420. doi:10.1086/394568. S2CID 84634277.
  127. ^ Whittaker, R. H. (January 1969). "New concepts of kingdoms of organisms". Science. 163 (3863): 150–60. Bibcode:1969Sci...163..150W. doi:10.1126/science.163.3863.150. PMID 5762760.
  128. ^ Cavalier-Smith, T. (1998). "A revised six-kingdom system of life". Biological Reviews. 73 (3): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012. S2CID 6557779.
  129. ^ Ruggiero, Michael A.; Gordon, Dennis P.; Orrell, Thomas M.; et al. (2015). "A higher level classification of all living organisms". PLOS ONE. 10 (4): e0119248. Bibcode:2015PLoSO..1019248R. doi:10.1371/journal.pone.0119248. PMC 4418965. PMID 25923521.
  130. ^ Simpson, Alastair G.B.; Roger, Andrew J. (2004). "The real 'kingdoms' of eukaryotes". Current Biology. 14 (17): R693–R696. Bibcode:2004CBio...14.R693S. doi:10.1016/j.cub.2004.08.038. PMID 15341755. S2CID 207051421.
  131. ^ Harper, J.T.; Waanders, E.; Keeling, P.J. (2005). "On the monophyly of chromalveolates using a six-protein phylogeny of eukaryotes". International Journal of Systematic and Evolutionary Microbiology. 55 (Pt 1): 487–496. doi:10.1099/ijs.0.63216-0. PMID 15653923.
  132. ^ a b Hug, Laura A.; Baker, Brett J.; Anantharaman, Karthik; et al. (11 April 2016). "A new view of the tree of life". Nature Microbiology. 1 (5). 16048. doi:10.1038/nmicrobiol.2016.48. PMID 27572647.
  133. ^ Hotz, Robert Lee (3 December 2010). "New link in chain of life". The Wall Street Journal. Archived from the original on 17 August 2017. Until now, however, they were all thought to share the same biochemistry, based on the Big Six, to build proteins, fats and DNA.
  134. ^ Lipkus, Alan H.; Yuan, Qiong; Lucas, Karen A.; et al. (2008). "Structural Diversity of Organic Chemistry. A Scaffold Analysis of the CAS Registry". The Journal of Organic Chemistry. 73 (12). American Chemical Society (ACS): 4443–4451. doi:10.1021/jo8001276. PMID 18505297.
  135. ^ Committee on the Limits of Organic Life in Planetary Systems; Committee on the Origins and Evolution of Life; National Research Council (2007). The Limits of Organic Life in Planetary Systems. National Academy of Sciences. ISBN 978-0-309-66906-1. Archived from the original on 10 May 2012. Retrieved 3 June 2012.
  136. ^ Benner, Steven A.; Ricardo, Alonso; Carrigan, Matthew A. (December 2004). "Is there a common chemical model for life in the universe?" (PDF). Current Opinion in Chemical Biology. 8 (6): 672–689. doi:10.1016/j.cbpa.2004.10.003. PMID 15556414. Archived from the original (PDF) on 16 October 2012. Retrieved 3 June 2012.
  137. ^ Purcell, Adam (5 February 2016). "DNA". Basic Biology. Archived from the original on 5 January 2017. Retrieved 15 November 2016.
  138. ^ Nuwer, Rachel (18 July 2015). "Counting All the DNA on Earth". The New York Times. New York. Archived from the original on 18 July 2015. Retrieved 18 July 2015.
  139. ^ Russell, Peter (2001). iGenetics. New York: Benjamin Cummings. ISBN 978-0-8053-4553-7.
  140. ^ "2.2: The Basic Structural and Functional Unit of Life: The Cell". LibreTexts. 2 June 2019. Archived from the original on 29 March 2020. Retrieved 29 March 2020.
  141. ^ Bose, Debopriya (14 May 2019). "Six Main Cell Functions". Leaf Group Ltd./Leaf Group Media. Archived from the original on 29 March 2020. Retrieved 29 March 2020.
  142. ^ Sapp, Jan (2003). Genesis: The Evolution of Biology. Oxford University Press. pp. 75–78. ISBN 978-0-19-515619-5.
  143. ^ Lintilhac, P.M. (January 1999). "Thinking of biology: toward a theory of cellularity—speculations on the nature of the living cell" (PDF). BioScience. 49 (1): 59–68. doi:10.2307/1313494. JSTOR 1313494. PMID 11543344. Archived from the original (PDF) on 6 April 2013. Retrieved 2 June 2012.
  144. ^ Whitman, W.; Coleman, D.; Wiebe, W. (1998). "Prokaryotes: The unseen majority". Proceedings of the National Academy of Sciences of the United States of America. 95 (12): 6578–6583. Bibcode:1998PNAS...95.6578W. doi:10.1073/pnas.95.12.6578. PMC 33863. PMID 9618454.
  145. ^ Pace, Norman R. (18 May 2006). "Concept Time for a change" (PDF). Nature. 441 (7091): 289. Bibcode:2006Natur.441..289P. doi:10.1038/441289a. PMID 16710401. S2CID 4431143. Archived from the original (PDF) on 16 October 2012. Retrieved 2 June 2012.
  146. ^ "Scientific background". The Nobel Prize in Chemistry 2009. Royal Swedish Academy of Sciences. Archived from the original on 2 April 2012. Retrieved 10 June 2012.
  147. ^ Nakano, A.; Luini, A. (2010). "Passage through the Golgi". Current Opinion in Cell Biology. 22 (4): 471–478. doi:10.1016/ PMID 20605430.
  148. ^ Panno, Joseph (2004). The Cell. Facts on File science library. Infobase Publishing. pp. 60–70. ISBN 978-0-8160-6736-7. Archived from the original on 13 April 2023. Retrieved 10 August 2023.
  149. ^ Alberts, Bruce; Bray, Dennis; Lewis, Julian; et al. (1994). "From Single Cells to Multicellular Organisms". Molecular Biology of the Cell (3rd ed.). New York: Garland Science. ISBN 978-0-8153-1620-6. Retrieved 12 June 2012.
  150. ^ Zimmer, Carl (7 January 2016). "Genetic Flip Helped Organisms Go From One Cell to Many". The New York Times. Archived from the original on 7 January 2016. Retrieved 7 January 2016.
  151. ^ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; et al. (2002). "General Principles of Cell Communication". Molecular Biology of the Cell. New York: Garland Science. ISBN 978-0-8153-3218-3. Archived from the original on 4 September 2015. Retrieved 12 June 2012.
  152. ^ Race, Margaret S.; Randolph, Richard O. (2002). "The need for operating guidelines and a decision making framework applicable to the discovery of non-intelligent extraterrestrial life". Advances in Space Research. 30 (6): 1583–1591. Bibcode:2002AdSpR..30.1583R. CiteSeerX doi:10.1016/S0273-1177(02)00478-7. There is growing scientific confidence that the discovery of extraterrestrial life in some form is nearly inevitable
  153. ^ Cantor, Matt (15 February 2009). "Alien Life 'Inevitable': Astronomer". Newser. Archived from the original on 23 May 2013. Retrieved 3 May 2013. Scientists now believe there could be as many habitable planets in the cosmos as there are stars, and that makes life's existence elsewhere "inevitable" over billions of years, says one.
  154. ^ Schulze-Makuch, Dirk; Dohm, James M.; Fairén, Alberto G.; et al. (December 2005). "Venus, Mars, and the Ices on Mercury and the Moon: Astrobiological Implications and Proposed Mission Designs". Astrobiology. 5 (6): 778–795. Bibcode:2005AsBio...5..778S. doi:10.1089/ast.2005.5.778. PMID 16379531. S2CID 13539394.
  155. ^ Woo, Marcus (27 January 2015). "Why We're Looking for Alien Life on Moons, Not Just Planets". Wired. Archived from the original on 27 January 2015. Retrieved 27 January 2015.
  156. ^ Strain, Daniel (14 December 2009). "Icy moons of Saturn and Jupiter may have conditions needed for life". The University of Santa Cruz. Archived from the original on 31 December 2012. Retrieved 4 July 2012.
  157. ^ Baldwin, Emily (26 April 2012). "Lichen survives harsh Mars environment". Skymania News. Archived from the original on 28 May 2012. Retrieved 27 April 2012.
  158. ^ de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation potential of extremophiles to Martian surface conditions and its implication for the habitability of Mars" (PDF). EGU General Assembly Conference Abstracts. 14: 2113. Bibcode:2012EGUGA..14.2113D. Archived from the original (PDF) on 4 May 2012. Retrieved 27 April 2012.
  159. ^ Selis, Frank (2006). "Habitability: the point of view of an astronomer". In Gargaud, Muriel; Martin, Hervé; Claeys, Philippe (eds.). Lectures in Astrobiology. Vol. 2. Springer. pp. 210–214. ISBN 978-3-540-33692-1. Archived from the original on 5 November 2023. Retrieved 10 August 2023.
  160. ^ Lineweaver, Charles H.; Fenner, Yeshe; Gibson, Brad K. (January 2004). "The Galactic Habitable Zone and the age distribution of complex life in the Milky Way". Science. 303 (5654): 59–62. arXiv:astro-ph/0401024. Bibcode:2004Sci...303...59L. doi:10.1126/science.1092322. PMID 14704421. S2CID 18140737. Archived from the original on 31 May 2020. Retrieved 30 August 2018.
  161. ^ Vakoch, Douglas A.; Harrison, Albert A. (2011). Civilizations beyond Earth: extraterrestrial life and society. Berghahn Series. Berghahn Books. pp. 37–41. ISBN 978-0-85745-211-5. Archived from the original on 13 April 2023. Retrieved 25 August 2020.
  162. ^ Green, James; Hoehler, Tori; Neveu, Marc; et al. (27 October 2021). "Call for a framework for reporting evidence for life beyond Earth". Nature. 598 (7882): 575–579. arXiv:2107.10975. Bibcode:2021Natur.598..575G. doi:10.1038/s41586-021-03804-9. PMID 34707302. S2CID 236318566. Archived from the original on 1 November 2021. Retrieved 1 November 2021.
  163. ^ Fuge, Lauren (30 October 2021). "NASA proposes playbook for communicating the discovery of alien life – Sensationalising aliens is so 20th century, according to NASA scientists". Cosmos. Archived from the original on 31 October 2021. Retrieved 1 November 2021.
  164. ^ "Artificial life". Archived from the original on 16 November 2016. Retrieved 15 November 2016.
  165. ^ Chopra, Paras; Akhil Kamma. "Engineering life through Synthetic Biology". In Silico Biology. 6. Archived from the original on 5 August 2008. Retrieved 9 June 2008.
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