This article is confined to the primary meanings in the life sciences; links to articles on life in other senses are included in the section Related articles below.
Life has a number of senses with a biological meaning —
It would be relatively straightforward to offer a practical set of guidelines if one's only concern was life on Earth as we know it (see biosphere), but as soon as one considers questions about life's originss on Earth, or the possibility of extraterrestrial life, or the concept of artificial life, it becomes clear that the question is fundamentally difficult and comparable in many respects to the problem of defining intelligence.
In biology, an entity has traditionally been considered to be alive if it exhibits all the following phenomena at least once during its existence:
Defining the concept of life
The remainder of this section focuses on the last sense — how can one tell when an entity is a living thing? A conventional definition
These criteria are not without their uses, but their disparate nature makes them unsatisfactory from a number of perspectives; in fact, it is not difficult to find counterexamples and examples that require further elaboration. For example, according to the above definition, one could say:
Biologists who are content to focus on terrestrial organisms often note some additional signs of a "living organism", including these:
All life on Earth is based on the chemistry of carbon compounds. Some assert that this must be the case for all possible forms of life throughout the universe; others describe this position as 'carbon chauvinism'.
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. Another exception might be the software code of certain forms of viruses and programs created through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.
Note that many individual organisms are incapable of reproduction and yet are still generally considered to be "alive"; see mules and ants for examples. However, these exceptions can be accounted for by applying the definition of life on the level of entire species or of individual genes. (For example, see kin selection for information about one way by which non-reproducing individuals can still enhance the spread of their genes and the survival of their species.)
Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and proto-life, consisting of metabolizing cells without reproduction apparatus, can have existed. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered alive.
Main article: Origin of life
There is no truly "standard" model of the origin of life, however most currently accepted models build in one way or another upon the following discoveries, which are listed in a rough order of postulated emergence:
As of 2003, Earth is the only planet in the universe known by humans to support life. The question of whether life exists elsewhere in the universe remains an open question, although the probability that Earth is the only location in the universe, or even the galaxy, that harbors life, is extremely low. There have been a number of false alarms of life elsewhere in the universe, but none of these apparent discoveries have so far survived scientific scrutiny.
Currently, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars. There also may be simple life forms on Jupiter's moons.
Other definitions
Other definitions include:
Descent with modification: a "useful" characteristic
Exceptions to the common definition
Origin of life
There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many of the possibilities have tended to fall into either "genes-first" or "metabolism-first", a recent trend is the emergence of hybrid models that combine aspects of both.The possibility of extraterrestrial life
Related articles
References