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Life is the characteristic state of organisms. Properties common to terrestrial organisms (plants, animals, fungi, protists and bacteria) are that they are cellular, carbon-and-water-based with complex organization, having a metabolism, a capacity to grow, respond to stimuli, reproduce and—through natural selection—adapt.
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. This definition notably includes viruses, which do not qualify under narrower definitions as they are acellular and do not metabolise. Broader definitions of life may also include theoretical non-carbon-based life and other alternative biology.
The entire Earth contains about 75 billion tons of biomass (life), which lives within various environments within the biosphere.
Life itself is a set of processes that are carried out by an organism causing it to survive.
A conventional definition
While there is no universal agreement on the definition of life, scientists generally accept that the biological manifestation of life exhibits the following phenomena:
- Organization - Living things are composed of one or more cells, which are the basic units of life.
- Metabolism - Metabolism produces energy by converting nonliving material into cellular components (synthesis) and decomposing organic matter (catalysis). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
- Growth - Growth results from a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.
- Adaptation - Adaptation is the accommodation of a living organism to its environment. It is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.
- Response to stimuli - A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. A response is often expressed by motion: the leaves of a plant turning toward the sun or an animal chasing its prey.
- Reproduction - The division of one cell to form two new cells is reproduction. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
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. It is also worth noting that non-reproducing individuals may still help the spread of their genes through such mechanisms as kin selection.
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; flames which "grow"; certain computer software programs which are programmed to mutate and evolve; future software programs which may evince (even high-order) behavior; machines which can move; and some forms of proto-life consisting of metabolizing cells without the ability to 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. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. 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. The definition may also include other replicating elements, including plasmids, which are otherwise considered part of a larger organism.
Taken to the extreme, a characteristic conveyed by a single gene of an organism may be considered to have its own life, as it has descent with modification,—an idea explored by Richard Dawkins in The Selfish Gene. The usefulness of this definition becomes less clear when the link between genotype and phenotype is more abstract, such as for individual base pairs or satellite DNA.
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.
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." This definition does not include flames, but does include worker ants, viruses and mules. Without 'most of', it does not include viruses.
Self reproduction and energy consumption is only one means for a system to promote its own continuation. This explains why bees can be alive and yet commit suicide in defending their hive. In this case the whole colony works as such a living system.
Origin of life
- Main article: 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. This was demonstrated in the Miller-Urey experiment.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
- Procedures for producing random RNA molecules can produce ribozymes, which are able to produce more of themselves under very specific conditions.
There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.
The possibility of extraterrestrial life
- Main articles: Extraterrestrial life, Astrobiology
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.
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from 
- Lynn Margulis and Dorion Sagan - What is Life? (1995). Simon & Schuster. ISBN 0-684-81087-5
- Erwin Schrodinger - What is Life? (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8
- "In the Beginning..." (The Economist)
- Today life and stress
- "The Adjacent Possible: A Talk with Stuart Kauffman"
- Stanford Encyclopedia of Philosophy entry
- The Biologist: Biology
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