The state or property of organisms which, by their metabolic processes, use substances from their environment for the purposes of growth, the maintenance of their functional systems, the repair of their own structure, and for reproducing themselves. All life forms on Earth are based on nucleic acids, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), which carry their hereditary genetic information. Life on Earth, in the form of simple bacteria, is thought to have originated between about 3·5 and 3·8 thousand million years ago in conditions in which the primordial atmosphere contained the basic constituents of organic matter (methane, ammonia, hydrogen, and water vapour). These underwent a process of chemical evolution using energy from the Sun and electric storms, combining into more and more complex molecules until self-replicating nucleic acids developed.
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An entity with the above properties is considered to be organic life. However, not every definition of life considers all of these properties to be essential. For example, the capacity for descent with modification is often taken as the only essential property of life. Broader definitions of life may also include theoretical non-carbon-based life and other alternative biology.
Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years . Currently the entire Earth contains about 75 billion tons of biomass (life), which lives within various environments within the biosphere.
Definition by Opinion
Life itself is a set of processes that are carried out by an organism causing it to survive.
A conventional definition
Although there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
Homeostasis: Regulation of the internal environment to maintain a constant state; Organization: Being composed of one or more cells, which are the basic units of life. Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.Exceptions to the conventional definition
It is important to note that life is a definition that applies primarily at the level of species, so even though many individuals of any given species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence, sterile hybrid species such as the mule are considered life although not themselves capable of reproduction.
For similar reasons, viruses and aberrant prion proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.
Some individuals contest such definitions of life on philosophical grounds, and offer the following as examples of life: viruses which reproduce;
Still, most scientists would not call such phenomena expressive of life. Generally all six characteristics are required for a population to be considered a life form.
Descent with modification
A useful characteristic upon which to base a definition of life is that of descent with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. In all known life forms (assuming prions are not counted as such), the genetic material is primarily DNA or the related molecule, RNA.
Unlike other definitions, this definition of life includes viruses, as they are replicators with a genotype and phenotype, making them capable of natural selection and evolution.
Also difficult for this definition is organisms which cannot reproduce directly, such as worker bees—which may also continue their gene-line by helping to produce siblings, and sterilised organisms, such as spayed or neutered pets, which are no longer capable of descent.
More abstract concepts may also be considered alive by this definition, including memes and the artificial life of computer software, such as self-modifying computer viruses and programs created through genetic programming.
Other definitions
The systemic definition is that living things are self-organizing and autopoietic (self-producing).
Variations of this definition include Stuart Kauffman's definition of life as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.
Another definition is: "Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation."
Yet another definition: "Life is a self-organizing, cannibalistic system consisting of a population of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve."
Self reproduction and energy consumption is only one means for a system to promote its own continuation.
Origin of life
There is no truly "standard" model for the origin of life, but most currently accepted scientific models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:
Plausible pre-biotic conditions result in the creation of the basic small molecules of life.The possibility of extraterrestrial life
Earth is the only planet in the universe known to harbor life. The Drake equation has been used to estimate the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation. Depending on those values, the equation may either suggest that life arises frequently or infrequently.
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