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Life is a quality that distinguishes
matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...
that has biological processes, such as signaling and
self-sustaining Self-sustainability and self-sufficiency are overlapping states of being in which a person or organization needs little or no help from, or interaction with, others. Self-sufficiency entails the self being enough (to fulfill needs), and a self-s ...
processes, from that which does not, and is defined by the capacity for
growth Growth may refer to: Biology * Auxology, the study of all aspects of human physical growth * Bacterial growth * Cell growth * Growth hormone, a peptide hormone that stimulates growth * Human development (biology) * Plant growth * Secondary growth ...
, reaction to
stimuli A stimulus is something that causes a physiological response. It may refer to: * Stimulation ** Stimulus (physiology), something external that influences an activity ** Stimulus (psychology), a concept in behaviorism and perception * Stimulus (eco ...
,
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
, energy transformation, and reproduction. Various forms of life exist, such as
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s,
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
,
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s, archaea, and
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
.
Biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
is the
science Science is a systematic endeavor that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earli ...
that studies life. The
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
is the unit of heredity, whereas the
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...
is the structural and functional unit of life. There are two kinds of cells,
prokaryotic A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
and
eukaryotic Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the ...
, both of which consist of
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
enclosed within a
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. ...
and contain many
biomolecules A biomolecule or biological molecule is a loosely used term for molecules present in organisms that are essential to one or more typically biological processes, such as cell division, morphogenesis, or development. Biomolecules include lar ...
such as proteins and nucleic acids. Cells reproduce through a process of
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
, in which the parent cell divides into two or more daughter cells and passes its genes onto a new generation, sometimes producing genetic variation.
Organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s, or the individual entities of life, are generally thought to be open systems that maintain
homeostasis In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
, are composed of cells, have a
life cycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring * Life-cycle hypothesis ...
, undergo metabolism, can grow,
adapt ADAPT (formerly American Disabled for Attendant Programs Today) is a United States grassroots disability rights organization with chapters in 30 states and Washington, D.C. They use nonviolent direct action in order to bring about disability just ...
to their environment, respond to stimuli, reproduce and evolve over multiple generations. Other definitions sometimes include non-cellular life forms such as
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
es and
viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
s, but they are usually excluded because they do not function on their own; rather, they exploit the biological processes of hosts.
Abiogenesis In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
, also known as the Origin of Life, is the natural process of life arising from non-living matter, such as simple
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
. Since its primordial beginnings, life on Earth has changed its environment on a
geologic time scale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...
, but it has also adapted to survive in most
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
s and conditions. New lifeforms have evolved from common ancestors through hereditary variation and
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, and today, estimates of the number of distinct
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
range anywhere from 3 million to over 100 million.
Death Death is the irreversible cessation of all biological functions that sustain an organism. For organisms with a brain, death can also be defined as the irreversible cessation of functioning of the whole brain, including brainstem, and brain ...
is the permanent termination of all biological processes which sustain an organism, and as such, is the end of its life.
Extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
is the term describing the dying-out of a group or
taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
, usually a species.
Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s are the preserved remains or
traces Traces may refer to: Literature * ''Traces'' (book), a 1998 short-story collection by Stephen Baxter * ''Traces'' series, a series of novels by Malcolm Rose Music Albums * ''Traces'' (Classics IV album) or the title song (see below), 1969 * ''Tra ...
of organisms.


Definitions

The definition of life has long been a challenge for scientists and philosophers. This is partially because life is a process, not a substance. This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside of Earth. Philosophical definitions of life have also been put forward, with similar difficulties on how to distinguish living things from the non-living. Legal definitions of life have also been described and debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision. As many as 123 definitions of life have been compiled. One definition seems to be favoured by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
: "a self-sustaining chemical system capable of Darwinian evolution". More simply, life is "matter that can reproduce itself and evolve as survival dictates".


Biology

Since there is no unequivocal definition of life, most current definitions in biology are descriptive. Life is considered a characteristic of something that preserves, furthers or reinforces its existence in the given environment. This characteristic exhibits all or most of the following traits: #
Homeostasis In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature #
Organisation An organization or organisation (Commonwealth English; see spelling differences), is an entity—such as a company, an institution, or an association—comprising one or more people and having a particular purpose. The word is derived from ...
: being structurally composed of one or more cells – the basic units of life #
Metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
: transformation of energy by converting chemicals and energy into cellular components ( anabolism) and decomposing organic matter ( catabolism). Living things require
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
to maintain internal organisation (homeostasis) and to produce the other phenomena associated with life. #
Growth Growth may refer to: Biology * Auxology, the study of all aspects of human physical growth * Bacterial growth * Cell growth * Growth hormone, a peptide hormone that stimulates growth * Human development (biology) * Plant growth * Secondary growth ...
: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. # Adaptation: the evolutionary process whereby an organism becomes better able to live in its
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
or habitats. # Response to
stimuli A stimulus is something that causes a physiological response. It may refer to: * Stimulation ** Stimulus (physiology), something external that influences an activity ** Stimulus (psychology), a concept in behaviorism and perception * Stimulus (eco ...
: a response can take many forms, from the contraction of a
unicellular organism A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms a ...
to external chemicals, to complex reactions involving all the senses of
multicellular organisms A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni ...
. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun ( phototropism), and chemotaxis. # Reproduction: the ability to produce new individual organisms, either asexually from a single parent organism or sexually from two parent organisms. These complex processes, called physiological functions, have underlying physical and chemical bases, as well as signaling and control mechanisms that are essential to maintaining life.


Alternative definitions

From a
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
perspective, living beings are
thermodynamic systems A thermodynamic system is a body of matter and/or radiation, confined in space by walls, with defined permeabilities, which separate it from its surroundings. The surroundings may include other thermodynamic systems, or physical systems that are ...
with an organised molecular structure that can reproduce itself and evolve as survival dictates. Thermodynamically, life has been described as an open system which makes use of gradients in its surroundings to create imperfect copies of itself. Another way of putting this is to define life as "a self-sustained chemical system capable of undergoing
Darwinian evolution Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that ...
", a definition adopted by a
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
committee attempting to define life for the purposes of
exobiology Astrobiology, and the related field of exobiology, is an interdisciplinary scientific field that studies the origins, early evolution, distribution, and future of life in the universe. Astrobiology is the multidisciplinary field that investig ...
, based on a suggestion by Carl Sagan. A major strength of this definition is that it distinguishes life by the evolutionary process rather than its chemical composition. Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are
self-organizing Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order and disorder, order arises from local interactions between parts of an initially disordered system. The process can be spon ...
and autopoietic (self-producing). Variations of this definition include
Stuart Kauffman Stuart Alan Kauffman (born September 28, 1939) is an American medical doctor, theoretical biologist, and complex systems researcher who studies the origin of life on Earth. He was a professor at the University of Chicago, University of Pennsylv ...
's definition as an
autonomous agent An autonomous agent is an intelligent agent operating on a user's behalf but without any interference of that user. An intelligent agent, however appears according to an IBM white paper as: Intelligent agents are software entities that carry out ...
or a
multi-agent system A multi-agent system (MAS or "self-organized system") is a computerized system composed of multiple interacting intelligent agents.Hu, J.; Bhowmick, P.; Jang, I.; Arvin, F.; Lanzon, A.,A Decentralized Cluster Formation Containment Framework f ...
capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle. This definition is extended by the apparition of novel functions over time.


Viruses

Whether or not viruses should be considered as alive is controversial. They are most often considered as just
gene coding The coding region of a gene, also known as the coding sequence (CDS), is the portion of a gene's DNA or RNA that codes for protein. Studying the length, composition, regulation, splicing, structures, and functions of coding regions compared to n ...
replicators rather than forms of life. They have been described as "organisms at the edge of life" because they possess
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s, evolve by natural selection, 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 In biology, abiogenesis (from a- 'not' + Greek bios 'life' + genesis 'origin') or the origin of life is the natural process by which life has arisen from non-living matter, such as simple organic compounds. The prevailing scientific hypothes ...
, as it may support the hypothesis that life could have started as self-assembling organic molecules.


Biophysics

To reflect the minimum phenomena required, other biological definitions of life have been proposed, with many of these being based upon chemical systems.
Biophysicists Biophysics is an interdisciplinary science that applies approaches and methods traditionally used in physics to study biological phenomena. Biophysics covers all scales of biological organization, from molecular to organismic and populations. ...
have commented that living things function on
negative entropy In information theory and statistics, negentropy is used as a measure of distance to normality. The concept and phrase "negative entropy" was introduced by Erwin Schrödinger in his 1944 popular-science book '' What is Life?'' Later, Léon Bril ...
. In other words, living processes can be viewed as a delay of the spontaneous
diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
or
dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...
of the internal energy of biological molecules towards more potential
microstates A microstate or ministate is a sovereign state having a very small population or very small land area, usually both. However, the meanings of "state" and "very small" are not well-defined in international law.Warrington, E. (1994). "Lilliputs ...
. In more detail, according to physicists such as John Bernal,
Erwin Schrödinger Erwin Rudolf Josef Alexander Schrödinger (, ; ; 12 August 1887 – 4 January 1961), sometimes written as or , was a Nobel Prize-winning Austrian physicist with Irish citizenship who developed a number of fundamental results in quantum theo ...
,
Eugene Wigner Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his co ...
, and John Avery, life is a member of the class of phenomena that are
open Open or OPEN may refer to: Music * Open (band), Australian pop/rock band * The Open (band), English indie rock band * ''Open'' (Blues Image album), 1969 * ''Open'' (Gotthard album), 1999 * ''Open'' (Cowboy Junkies album), 2001 * ''Open'' ( ...
or continuous systems able to decrease their internal
entropy Entropy is a scientific concept, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...
at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form. The emergence and increasing popularity of biomimetics or biomimicry (the design and production of materials, structures, and systems that are modelled on biological entities and processes) will likely redefine the boundary between natural and
artificial life Artificial life (often abbreviated ALife or A-Life) is a field of study wherein researchers examine systems related to natural life, its processes, and its evolution, through the use of simulations with computer models, robotics, and biochemistry ...
.


Living systems theories

Living systems are open
self-organizing Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order and disorder, order arises from local interactions between parts of an initially disordered system. The process can be spon ...
living things that interact with their
environment Environment most often refers to: __NOTOC__ * Natural environment, all living and non-living things occurring naturally * Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
. These systems are maintained by flows of information,
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
, and matter. Budisa, Kubyshkin and Schmidt defined cellular life as an organizational unit resting on four pillars/cornerstones: (i)
energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
, (ii)
metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
, (iii)
information Information is an abstract concept that refers to that which has the power to inform. At the most fundamental level information pertains to the interpretation of that which may be sensed. Any natural process that is not completely random ...
and (iv)
form Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens. Form also refers to: *Form (document), a document (printed or electronic) with spaces in which to write or enter data ...
. This system is able to regulate and control metabolism and energy supply and contains at least one subsystem that functions as an information carrier (
genetic information A nucleic acid sequence is a succession of bases signified by a series of a set of five different letters that indicate the order of nucleotides forming alleles within a DNA (using GACT) or RNA (GACU) molecule. By convention, sequences are usu ...
). Cells as self-sustaining units are parts of different
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
s that are involved in the unidirectional and irreversible open-ended process known as
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
. Some scientists have proposed in the last few decades that a general
living systems Living systems are open self-organizing life forms that interact with their environment. These systems are maintained by flows of information, energy and matter. In the last few decades, some scientists have proposed that a general living syst ...
theory is required to explain the nature of life. Such a general theory would arise out of the
ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
and
biological sciences Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...
and attempt to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into components, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.


Gaia hypothesis

The idea that Earth is alive is found in philosophy and religion, but the first scientific discussion of it was by the Scottish scientist James Hutton. In 1785, he stated that Earth was a superorganism and that its proper study should be
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
. Hutton is considered the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century. The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock, suggests that life on Earth functions as a single organism that defines and maintains
environmental A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
conditions necessary for its survival. This hypothesis served as one of the foundations of the modern
Earth system science Earth system science (ESS) is the application of systems science to the Earth. In particular, it considers interactions and 'feedbacks', through material and energy fluxes, between the Earth's sub-systems' cycles, processes and "spheres"— atmo ...
.


Nonfractionability

Robert Rosen devoted a large part of his career, from 1958 onwards, to developing a comprehensive theory of life as a self-organizing complex system, "closed to efficient causation". He defined a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." He identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines." He summarised his views in his book ''Life Itself''. Similar ideas may be found in the book ''Living Systems'' by
James Grier Miller James Grier Miller (19167 November 2002, California) was an American biologist, a pioneer of systems science and academic administrator, who originated the modern use of the term "behavioral science", founded and directed the multi-disciplinary ...
.


Life as a property of ecosystems

A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence," and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it, life is a property of an
ecological system An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
rather than a single organism or species. He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one.
Robert Ulanowicz Robert Edward Ulanowicz ( ) is an American theoretical ecologist and philosopher of Polish descent who in his search for a ''unified theory of ecology'' has formulated a paradigm he calls ''Process Ecology''. He was born September 17, 1943 in B ...
(2009) highlights mutualism as the key to understand the systemic, order-generating behaviour of life and ecosystems.


Complex systems biology

Complex systems biology (CSB) is a field of science that studies the emergence of complexity in functional organisms from the viewpoint of dynamic systems theory. The latter is also often called systems biology and aims to understand the most fundamental aspects of life. A closely related approach to CSB and systems biology called relational biology is concerned mainly with understanding life processes in terms of the most important relations, and categories of such relations among the essential functional components of organisms; for multicellular organisms, this has been defined as "categorical biology", or a model representation of organisms as a category theory of biological relations, as well as an
algebraic topology Algebraic topology is a branch of mathematics that uses tools from abstract algebra to study topological spaces. The basic goal is to find algebraic invariants that classify topological spaces up to homeomorphism, though usually most classify ...
of the functional organisation of living organisms in terms of their dynamic, complex
networks Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...
of metabolic, genetic, and epigenetic processes and
signalling pathway In biology, cell signaling (cell signalling in British English) or cell communication is the ability of a cell to receive, process, and transmit signals with its environment and with itself. Cell signaling is a fundamental property of all cellula ...
s. Alternative but closely related approaches focus on the interdependence of constraints, where constraints can be either molecular, such as enzymes, or macroscopic, such as the geometry of a bone or of the vascular system.


Darwinian dynamic

It has also been argued that the evolution of order in living systems and certain physical systems obeys a common fundamental principle termed the Darwinian dynamic. The Darwinian dynamic was formulated by first considering how macroscopic order is generated in a simple non-biological system far from thermodynamic equilibrium, and then extending consideration to short, replicating
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
molecules. The underlying order-generating process was concluded to be basically similar for both types of systems.


Operator theory

Another systemic definition called the operator theory proposes that life is a general term for the presence of the typical closures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell and that an organism is any system with an organisation that complies with an operator type that is at least as complex as the cell. Life can also be modelled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
formed by the potential of expansion and reproduction.


History of study


Materialism

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 Empedocles (; grc-gre, Ἐμπεδοκλῆς; , 444–443 BC) was a Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city in Sicily. Empedocles' philosophy is best known for originating the cosmogonic theory of the ...
(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.
Democritus Democritus (; el, Δημόκριτος, ''Dēmókritos'', meaning "chosen of the people"; – ) was an Ancient Greek pre-Socratic philosopher from Abdera, primarily remembered today for his formulation of an atomic theory of the universe. No ...
(460 BC) thought that the essential characteristic of life is having a
soul In many religious and philosophical traditions, there is a belief that a soul is "the immaterial aspect or essence of a human being". Etymology The Modern English noun '' soul'' is derived from Old English ''sāwol, sāwel''. The earliest atte ...
(''psyche''). Like other ancient writers, he was attempting to explain what makes something a ''living'' thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing. He elaborates on fire because of the apparent connection between life and heat, and because fire moves. The
mechanistic The mechanical philosophy is a form of natural philosophy which compares the universe to a large-scale mechanism (i.e. a machine). The mechanical philosophy is associated with the scientific revolution of early modern Europe. One of the first expo ...
materialism that originated in
ancient Greece Ancient Greece ( el, Ἑλλάς, Hellás) was a northeastern Mediterranean Sea, Mediterranean civilization, existing from the Greek Dark Ages of the 12th–9th centuries BC to the end of Classical Antiquity, classical antiquity ( AD 600), th ...
was revived and revised by the French philosopher
René Descartes René Descartes ( or ; ; Latinized: Renatus Cartesius; 31 March 1596 – 11 February 1650) was a French philosopher, scientist, and mathematician, widely considered a seminal figure in the emergence of modern philosophy and science. Ma ...
(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 Julien Offray de La Mettrie (; November 23, 1709 – November 11, 1751) was a French physician and philosopher, and one of the earliest of the French materialism, French materialists of the Age of Enlightenment, Enlightenment. He is best known fo ...
(1709–1750) in his book ''L'Homme Machine''. In the 19th century, the advances in
cell theory In biology, cell theory is a scientific theory first formulated in the mid-nineteenth century, that living organisms are made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from pre ...
in biological science encouraged this view. The
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary theory of
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...
(1859) is a mechanistic explanation for the origin of species by means of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
. At the beginning of the 20th century
Stéphane Leduc Stéphane Leduc (1 November 1853 – 8 March 1939) was a French biologist who sought to contribute to understanding of the chemical and physical mechanisms of life. He was a scientist in the fledgling field of synthetic biology, particularly in re ...
(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'' was widely dismissed during his lifetime, but has incurred a resurgence of interest in the work of Russell, Barge and colleagues.


Hylomorphism

Hylomorphism is a theory first expressed by the Greek philosopher
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
(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 In many religious and philosophical traditions, there is a belief that a soul is "the immaterial aspect or essence of a human being". Etymology The Modern English noun '' soul'' is derived from Old English ''sāwol, sāwel''. The earliest atte ...
(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. 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. This account is consistent with
teleological Teleology (from and )Partridge, Eric. 1977''Origins: A Short Etymological Dictionary of Modern English'' London: Routledge, p. 4187. or finalityDubray, Charles. 2020 912Teleology" In ''The Catholic Encyclopedia'' 14. New York: Robert Appleton ...
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 The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''gigaannum'') and ev ...
of a species, which led to the natural selection of the features in question.


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. The theory of spontaneous generation was proposed by
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ph ...
, 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. Disproof of the traditional ideas of spontaneous generation is no longer controversial among biologists.


Vitalism

Vitalism is the belief that the life-principle is non-material. This originated with
Georg Ernst Stahl Georg Ernst Stahl (22 October 1659 – 24 May 1734) was a German chemist, physician and philosopher. He was a supporter of vitalism, and until the late 18th century his works on phlogiston were accepted as an explanation for chemical processes.K ...
(17th century), and remained popular until the middle of the 19th century. It appealed to philosophers such as Henri Bergson,
Friedrich Nietzsche Friedrich Wilhelm Nietzsche (; or ; 15 October 1844 – 25 August 1900) was a German philosopher, prose poet, cultural critic, philologist, and composer whose work has exerted a profound influence on contemporary philosophy. He began his ...
, and Wilhelm Dilthey, anatomists like
Xavier Bichat Marie François Xavier Bichat (; ; 14 November 1771 – 22 July 1802) was a French anatomist and pathologist, known as the father of modern histology. Although he worked without a microscope, Bichat distinguished 21 types of elementary tissues ...
, and chemists like Justus von Liebig. Vitalism included the idea that there was a fundamental difference between organic and inorganic material, and the belief that
organic material Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
can only be derived from living things. This was disproved in 1828, when Friedrich Wöhler prepared
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
from inorganic materials. This
Wöhler synthesis The Wöhler synthesis is the conversion of ammonium cyanate into urea. This chemical reaction was described in 1828 by Friedrich Wöhler. It is often cited as the starting point of modern organic chemistry. Although the Wöhler reaction concerns ...
is considered the starting point of modern
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
. It is of historical significance because for the first time an
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
was produced in
inorganic In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemist ...
reactions. During the 1850s,
Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (31 August 1821 – 8 September 1894) was a German physicist and physician who made significant contributions in several scientific fields, particularly hydrodynamic stability. The Helmholtz Associatio ...
, anticipated by
Julius Robert von Mayer Julius Robert von Mayer (25 November 1814 – 20 March 1878) was a German physician, chemist, and physicist and one of the founders of thermodynamics. He is best known for enunciating in 1841 one of the original statements of the conservation ...
, demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle. These results led to the abandonment of scientific interest in vitalistic theories, especially after Buchner's demonstration that alcoholic fermentation could occur in cell-free extracts of yeast. Nonetheless, the belief still exists in pseudoscientific theories such as
homoeopathy Homeopathy or homoeopathy is a pseudoscientific system of alternative medicine. It was conceived in 1796 by the German physician Samuel Hahnemann. Its practitioners, called homeopaths, believe that a substance that causes symptoms of a di ...
, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.


Origin

The age of Earth is about 4.54
billion years A billion years or giga-annum (109 years) is a unit of time on the petasecond scale, more precisely equal to seconds (or simply 1,000,000,000 years). It is sometimes abbreviated Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or ...
. Evidence suggests that life on Earth has existed for at least 3.5 billion years, Early edition, published online before print. with the oldest physical
traces Traces may refer to: Literature * ''Traces'' (book), a 1998 short-story collection by Stephen Baxter * ''Traces'' series, a series of novels by Malcolm Rose Music Albums * ''Traces'' (Classics IV album) or the title song (see below), 1969 * ''Tra ...
of life dating back 3.7 billion years; however, some hypotheses, such as
Late Heavy Bombardment The Late Heavy Bombardment (LHB), or lunar cataclysm, is a hypothesized event thought to have occurred approximately 4.1 to 3.8 billion years (Ga) ago, at a time corresponding to the Neohadean and Eoarchean eras on Earth. According to the hypot ...
, suggest that life on Earth may have started even earlier, as early as 4.1–4.4 billion years ago, and the chemistry leading to life may have begun shortly after the Big Bang, 13.8 billion years ago, during an epoch when the
universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...
was only 10–17 million years old. More than 99% of all species of life forms, amounting to over five billion species, that ever lived on Earth are estimated to be extinct. Although the number of Earth's catalogued species of lifeforms is between 1.2 million and 2 million, the total number of species in the planet is uncertain. Estimates range from 8 million to 100 million, with a more narrow range between 10 and 14 million, but it may be as high as 1 trillion (with only one-thousandth of one per cent of the species described) according to studies realised in May 2016. The total number of related DNA base pairs on Earth is estimated at 5.0 x 1037 and weighs 50 billion tonnes. In comparison, the total mass of the
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
has been estimated to be as much as 4 TtC (trillion tons of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
). In July 2016, scientists reported identifying a set of 355
gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s from the
Last Universal Common Ancestor The last universal common ancestor (LUCA) is the most recent population from which all organisms now living on Earth share common descent—the most recent common ancestor of all current life on Earth. This includes all cellular organisms; th ...
(LUCA) of all organisms living on Earth. All known life forms share fundamental molecular mechanisms, reflecting their common descent; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor, from simple
organic molecule In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...
s via pre-cellular life to
protocell A protocell (or protobiont) is a self-organized, endogenously ordered, spherical collection of lipids proposed as a stepping-stone toward the origin of life. A central question in evolution is how simple protocells first arose and how they could ...
s and metabolism. Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories. There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the
Miller–Urey experiment The Miller–Urey experiment (or Miller experiment) is a famous chemistry experiment that simulated the conditions thought at the time (1952) to be present in the atmosphere of the early, prebiotic Earth, in order to test the hypothesis of the ...
and the work of Sidney Fox, which show that conditions on the primitive Earth favoured chemical reactions that synthesize
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s and other organic compounds from inorganic precursors, and phospholipids spontaneously form
lipid bilayer The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
s, the basic structure of a
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
. Living organisms synthesize proteins, which are
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
s of amino acids using instructions encoded by deoxyribonucleic acid (DNA). Protein synthesis entails intermediary ribonucleic acid (RNA) polymers. One possibility for how life began is that genes originated first, followed by proteins; the alternative being that proteins came first and then genes. However, because genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently. Therefore, a possibility, first suggested by Francis Crick, is that the first life was based on
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
, which has the DNA-like properties of information storage and the
catalytic Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
properties of some proteins. This is called the
RNA world hypothesis The RNA world is a hypothetical stage in the evolutionary history of life on Earth, in which self-replicating RNA molecules proliferated before the evolution of DNA and proteins. The term also refers to the hypothesis that posits the existenc ...
, and it is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical cofactors ( ATP, Acetyl-CoA,
NADH Nicotinamide adenine dinucleotide (NAD) is a coenzyme central to metabolism. Found in all living cells, NAD is called a dinucleotide because it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an aden ...
, etc.) are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, but they were confirmed by
Thomas Cech Thomas Robert Cech (born December 8, 1947) is an American chemist who shared the 1989 Nobel Prize in Chemistry with Sidney Altman, for their discovery of the catalytic properties of RNA. Cech discovered that RNA could itself cut strands of RNA, ...
in 1986. One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor
phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...
present throughout the reaction. This study makes the RNA world hypothesis more plausible. Geological findings in 2013 showed that reactive
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
species (like phosphite) were in abundance in the ocean before 3.5 Ga, and that Schreibersite easily reacts with aqueous glycerol to generate phosphite and glycerol 3-phosphate. It is hypothesized that Schreibersite-containing meteorites from the Late Heavy Bombardment could have provided early reduced phosphorus, which could react with prebiotic organic molecules to form phosphorylated biomolecules, like
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
. In 2009, experiments demonstrated
Darwinian evolution Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that ...
of a two-component system of RNA enzymes (ribozymes) ''in vitro''. The work was performed in the laboratory of Gerald Joyce, who stated "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system." Prebiotic compounds may have originated extraterrestrially.
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
findings in 2011, based on studies with meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules) may be formed in outer space. In March 2015, NASA scientists reported that, for the first time, complex DNA and
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...
s of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
-rich chemical found in the
universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...
, may have been formed in red giants or in Cosmic dust, interstellar dust and gas clouds, according to the scientists. According to the panspermia hypothesis, microorganism, microscopic life—distributed by meteoroids, asteroids and other Small Solar System body, small Solar System bodies—may exist throughout the universe.


Environmental conditions

The diversity of life on Earth is a result of the dynamic interplay between genetic opportunity, metabolic capability, environment (biophysical), environmental challenges, and symbiosis. 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 The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...
, thereby affecting the path of evolution of subsequent life. 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.


Biosphere

The biosphere is the global sum of all ecosystems. It can also be termed as the zone of life on Earth, a closed system (apart from solar and cosmic radiation and heat from the interior of the Earth), and largely self-regulating. By the most general Geophysiology, biophysiological definition, the biosphere is the global ecological system integrating all living beings and their relationships, including their interaction with the elements of the lithosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere. Life forms live in every part of the Earth's
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
, including soil, hot springs, endolith, inside rocks at least deep underground, the deepest parts of the ocean, and at least high in the atmosphere. Under certain test conditions, life forms have been observed to thrive in the Weightlessness#Effects on non-human organisms, near-weightlessness of space and to Panspermia#Research in outer space, survive in the vacuum of outer space. Life forms appear to thrive in the Mariana Trench, the deepest spot in the Earth's oceans. Other researchers reported related studies that life forms thrive inside rocks up to below the sea floor under of ocean off the coast of the northwestern United States, as well as beneath the seabed off Japan. In August 2014, scientists confirmed the existence of life forms living below the ice of Antarctica. According to one researcher, "You can find microbes everywhere—they're extremely adaptable to conditions, and survive wherever they are." The biosphere is postulated to have evolution, evolved, beginning with a process of origin of life, biopoesis (life created naturally from non-living matter, such as simple organic compounds) or biogenesis (life created from living matter), at least some 3.5 billion years ago. The earliest evidence for life on Earth includes Biogenic substance, biogenic graphite found in 3.7 billion-year-old Metasediment, metasedimentary rocks from Western Greenland and microbial mat fossils found in 3.48 billion-year-old sandstone from Western Australia. More recently, in 2015, "remains of Biotic material, biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In 2017, putative fossilised microorganisms (or Micropaleontology#Microfossils, microfossils) were announced to have been discovered in hydrothermal vent, hydrothermal vent precipitates in the Nuvvuagittuq Greenstone Belt, 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 Origin of water on Earth#History of water on Earth, ocean formation 4.4 billion years ago, and not long after the Age of the Earth, formation of the Earth 4.54 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the
universe The universe is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. The Big Bang theory is the prevailing cosmological description of the development of the universe. ...
." In a general sense, biospheres are any closed, self-regulating systems containing ecosystems. This includes artificial biospheres such as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons.


Range of tolerance

The inert components of an ecosystem are the physical and chemical factors necessary for life—energy (sunlight or biochemistry, chemical energy), water, heat, Earth's atmosphere, atmosphere, gravitational biology, gravity, nutrients, and ultraviolet ozone layer, solar radiation protection. 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." 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.


Extremophiles

To survive, selected microorganisms can assume forms that enable them to withstand psychrophile, freezing, xerophile, complete desiccation, oligotroph, starvation, high levels of radioresistance, radiation exposure, and other physical or chemical challenges. These microorganisms may survive exposure to such conditions for weeks, months, years, or even centuries. Extremophiles are microbe, microbial life forms that thrive outside the ranges where life is commonly found. They excel at exploiting uncommon sources of energy. While all organisms are composed of nearly identical molecules, evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. Characterization of the morphology (biology), structure and metabolic diversity of microbial communities in such extreme environments is ongoing. microbe, Microbial life forms thrive even in the Mariana Trench, the deepest spot in the Earth's oceans. Microbes also thrive inside Rock (geology), rocks up to below the sea floor under of ocean. Expeditions of the International Ocean Discovery Program found Unicellular organism, unicellular life in 120 °C sediment that is 1.2 km below seafloor in the Nankai Trough subduction zone. Investigation of the tenacity and versatility of life on Earth, 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, life beyond Earth. For example, lichen could survive for a month in a Life on Earth under Martian conditions, simulated Martian environment.


Chemical elements

All life forms require certain core chemical elements needed for biochemistry, biochemical functioning. These include
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
, hydrogen, nitrogen, oxygen,
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
, and sulfur—the elemental nutrient, macronutrients for all organisms—often represented by the acronym CHNOPS. Together these make up nucleic acids, proteins and lipids, the bulk of living matter. Five of these six elements comprise the chemical components of DNA, the exception being sulfur. The latter is a component of the amino acids cysteine and methionine. The most biologically abundant of these elements is carbon, which has the desirable attribute of forming multiple, stable covalent bonds. This allows carbon-based (organic) molecules to form an immense variety of chemical arrangements. 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 Chirality (chemistry), chiralities or other chemical properties.


DNA

Deoxyribonucleic acid is a molecule that carries most of the genetics, genetic instructions used in the growth, development, functioning and reproduction of all known living organisms and many viruses. DNA and
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
are nucleic acids; alongside proteins and Polysaccharide, complex carbohydrates, they are one of the three major types of macromolecules, 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 Nucleic acid double helix, double helix. The two DNA strands are known as polynucleotides since they are composed of monomer, simpler units called nucleotides. Each nucleotide is composed of a nitrogenous base, nitrogen-containing nucleobase—either cytosine (C), guanine (G), adenine (A), or thymine (T)—as well as a monosaccharide, 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 backbone chain, sugar-phosphate backbone. According to base pairing rules (A with T, and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. The total amount of related DNA base pairs on Earth is estimated at 5.0 x 1037, and weighs 50 billion tonnes. In comparison, the total Biomass (ecology), mass of the
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also ...
has been estimated to be as much as 4 TtC (trillion tons of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...
). DNA stores biological information. The DNA backbone is resistant to cleavage, and both strands of the double-stranded structure store the same biological information. Biological information is replicated as the two strands are separated. A significant portion of DNA (more than 98% for humans) is non-coding DNA, non-coding, meaning that these sections do not serve as patterns for protein sequences. The two strands of DNA run in opposite directions to each other and are, therefore, antiparallel (biochemistry), anti-parallel. Attached to each sugar is one of four types of nucleobases (informally, ''bases''). It is the Nucleic acid sequence, sequence of these four nucleobases along the backbone that encodes biological information. Under the genetic code,
RNA Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...
strands are translated to specify the sequence of
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha a ...
s within proteins. These RNA strands are initially created using DNA strands as a template in a process called transcription (genetics), transcription. Within cells, DNA is organised into long structures called chromosomes. During
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes. Eukaryote, Eukaryotic organisms (animals, plants, Fungus, fungi, and
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts. In contrast, prokaryotes (bacteria and archaea) store their DNA only in the
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
. Within the chromosomes, chromatin proteins such as histones compact and organise DNA. These compact structures guide the interactions between DNA and other proteins, helping control which parts of the DNA are transcribed. DNA was first isolated by Friedrich Miescher in 1869. Its molecular structure was identified by James Watson and Francis Crick in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Rosalind Franklin.


Classification


Antiquity

The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384–322 BC), who classified all living organisms known at that time as either a plant or an animal, based mainly on their ability to move. He also distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of vertebrates and invertebrates respectively, and divided the blooded animals into five groups: viviparous quadrupeds (mammals), oviparous quadrupeds (reptiles and amphibians), birds, fishes and Cetacea, whales. The bloodless animals were also divided into five groups: cephalopods, crustaceans, insects (which included the spiders, scorpions, and centipedes, in addition to what we define as insects today), shelled animals (such as most molluscs and echinoderms), and "zoophytes" (animals that resemble plants). Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.


Linnaean

The exploration of the Americas revealed large numbers of new plants and animals that needed descriptions and classification. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. In the late 1740s, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Linnaeus attempted to improve the composition and reduce the length of the previously used many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and precisely defining their meaning. The Linnaean classification has eight levels: domains, kingdoms, phyla, class, order, family, genus, and species. The fungi were originally treated as plants. For a short period Linnaeus had classified them in the taxon Vermes in Animalia, but later placed them back in Plantae. Herbert Copeland, Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging their special status. The problem was eventually solved by Robert Whittaker (ecologist), Whittaker, when he gave them their own Kingdom (biology), kingdom in his five-kingdom system. Evolutionary history of life, Evolutionary history shows that the fungi are more closely related to animals than to plants. As new discoveries enabled detailed study of cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created. These new organisms were originally described separately in protozoa as animals and thallophyte, protophyta/thallophyta as plants, but were united by Ernst Haeckel, 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. However, the classification of eukaryotes, especially of protists, is still controversial. As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and
viroid Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms (flowering plants), and most cause diseases, whose respective economi ...
s. Whether these are considered alive has been a matter of debate; viruses lack characteristics of life such as cell membranes, metabolism and the ability to grow or respond to their environments. Viruses can still be classed into "species" based on their biology and genetics, but many aspects of such a classification remain controversial. In May 2016, scientists reported that 1 trillion
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
are estimated to be on Earth currently with only one-thousandth of one per cent described. The original Linnaean system has been modified over time as follows:


Cladistic

In the 1960s cladistics emerged: a system arranging taxa based on clades in an phylogenetic tree, evolutionary or phylogenetic tree.


Cells

Cells are the basic unit of structure in every living thing, and all cells arise from pre-existing cells by Cell division, division. Cell theory was formulated by Henri Dutrochet, Theodor Schwann, Rudolf Virchow and others during the early nineteenth century, and subsequently became widely accepted. The activity of an organism depends on the total activity of its cells, with Cellular respiration, energy flow occurring within and between them. Cells contain hereditary information that is carried forward as a genetics, genetic code during cell division. There are two primary types of cells. Prokaryotes lack a Cell nucleus, nucleus and other membrane-bound organelles, although they have circular DNA and ribosomes. Bacteria and Archaea are two domain (biology), domains of prokaryotes. The other primary type of cells are the eukaryotes, which have distinct nuclei bound by a nuclear membrane and membrane-bound organelles, including mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, and vacuoles. In addition, they possess organised chromosomes that store genetic material. All species of large complex organisms are eukaryotes, including animals, plants and fungi, though most species of eukaryote are
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
microorganisms. 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. The molecular mechanisms of cell biology are based on proteins. Most of these are synthesised by the ribosomes through an Enzyme catalysis, 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. In eukaryotic cells, these proteins may then be transported and processed through the Golgi apparatus in preparation for dispatch to their destination. Cells reproduce through a process of
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ar ...
in which the parent cell divides into two or more daughter cells. For prokaryotes, cell division occurs through a process of Fission (biology), 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. Multicellular organisms may have first evolved through the formation of Colony (biology), 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 Clone (cell biology), 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. In January 2016, scientists reported that, Timeline of the evolutionary history of life, about 800 million years ago, a Evolutionary developmental biology, minor genetic change in a single molecule, called GK-PID, may have allowed organisms to go from a Unicellular organism, single cell organism to Multicellular organism, one of many cells. 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.


Extraterrestrial

Though life is confirmed only on Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable. 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 Microorganism, microbial life include the subsurface of Life on Mars (planet), Mars, the upper atmosphere of Life on Venus, Venus, and subsurface oceans on some of the Natural satellite habitability, moons of the giant planets. Beyond the Solar System, the region around another main sequence, 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. 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 In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
-damaging supernova events, are predicted to have a higher probability of hosting planets with complex life. The variables of the Drake equation are used to discuss the conditions in planetary systems where civilisation is most likely to exist. Use of the equation to predict the amount of extraterrestrial life, however, is difficult; because many of the variables are unknown, the equation functions as more of a mirror to what its user already thinks. As a result, the number of civilisations in the galaxy can be estimated as low as 9.1 x 10−13, suggesting a minimum value of 1, or as high as 15.6 million (1.56 x 108); for the calculations, see Drake equation#Range of results, Drake equation. A "Confidence of Life Detection" scale (CoLD) for reporting evidence of life beyond Earth has been proposed.


Artificial

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


Death

Death is the termination of all vital functions or life processes in an organism or cell. It can occur as a result of an accident, violence, medical conditions, biological interaction, malnutrition, poisoning, senescence, or suicide. After death, the remains of an organism re-enter the biogeochemical cycle. Organisms may be necrophagy, consumed by a predator or a scavenger and leftover
organic material Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
may then be further decomposed by detritivores, organisms that recycle detritus, returning it to the environment for reuse in the food chain. One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment life ends, or when the state that follows life begins. However, determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems. Such determination, therefore, requires drawing conceptual lines between life and death. This is problematic, however, because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of afterlife or reincarnation for the
soul In many religious and philosophical traditions, there is a belief that a soul is "the immaterial aspect or essence of a human being". Etymology The Modern English noun '' soul'' is derived from Old English ''sāwol, sāwel''. The earliest atte ...
, or resurrection of the body at a later date.


Extinction

Extinction is the process by which a group of taxa or
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
dies out, reducing biodiversity. The moment of extinction is generally considered the death of the last individual of that species. Because a species' potential range (biology), 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 In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
or against superior competition. In History of the Earth, Earth's history, over 99% of all the species that have ever lived are extinct; however, mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.


Fossils

Fossils are the preserved remains or
traces Traces may refer to: Literature * ''Traces'' (book), a 1998 short-story collection by Stephen Baxter * ''Traces'' series, a series of novels by Malcolm Rose Music Albums * ''Traces'' (Classics IV album) or the title song (see below), 1969 * ''Tra ...
of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossil-containing rock (geology), rock formations and sedimentary rock, sedimentary layers (stratum, strata) is known as the ''fossil record''. A preserved specimen is called a fossil if it is older than the arbitrary date of 10,000 years ago. Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archean, Archaean Eon, up to 3.4 1000000000 (number), billion years old.


See also

*
Biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, the study of life * Astrobiology * Biosignature * History of life * Lists of organisms by population * Phylogenetics * Viable system theory * Central dogma of molecular biology * Epigenetics * Synthetic biology * Hypothetical types of biochemistry * Carbon-based life


Notes


References


Further reading

*


External links


Life
(Systema Naturae 2000)
Vitae
(BioLib)
Biota
(Taxonomicon) * species:Main Page, Wikispecies – a free directory of life
Resources for life in the Solar System and in galaxy, and the potential scope of life in the cosmological future



''Stanford Encyclopedia of Philosophy'' entry

The Kingdoms of Life
{{Authority control Life, Main topic articles