Life

Alternate meanings: Conway's Game of Life, Hasbro's Game of Life, personal life, Life magazine, Life imprisonment

Life is a multi-faceted concept with no simple definition, in part because the word is often used in an intentionally open-ended way, as exemplified by phrases such as "eternal life", "artificial life", and "extraterrestrial life". In fact, WordNet identifies fourteen different senses of the word "life".

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.

Attempts to define the concept of life

Life has a number of senses with a biological meaning —

"life" may refer to the ongoing process of which living things are a part;
"life" may refer to the period between birth and death of an organism;
"life" may refer to the state of something that has been born and has yet to die, i.e., that which makes a living thing alive.

The remainder of this section focuses on the last sense — how can one tell when an entity is a living thing?

It would be relatively straightforward to offer a practical set of guidelines if ones only concern was life on Earth as we know it (see biosphere), but as soon as one considers questions about life's origins 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.

A conventional definition

In biology, an entity has traditionally been considered to be alive if it exhibits all the following phenomena at least once during its existence:

Growth
Metabolism, consuming, transforming and storing energy/mass; growing by absorbing and reorganizing mass; excreting waste
Motion, either moving itself, or having internal motion
Reproduction, the ability to create roughly exact copies of itself
Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions.

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:

fire is alive. (This could be remedied by adding the requirement of locality, where there is an obvious feature that delineates the spatial extension of the living being, such as a cell membrane.)
a male mule is not alive — mules are sterile, and cannot reproduce.
viruses are not alive — they do not grow.

Biologists who are content to focus on terrestrial organisms often note some additional signs of a "living organism", including these:

Living organisms contain molecular components such as: carbohydrates, lipids, nucleic acids, and proteins.
Living organisms require both energy and matter, in order to continue living.
Living organisms are composed of, at least, one cell.
Living organisms maintain homeostasis.
Species, of living organisms, will evolve.

Other definitions

Other definitions include:

Lynn Margulis's definition of life as an autopoietic (self-producing), water based, lipid-protein bound, carbon metabolic, nucleic acid replicated, protein readout system
"a system of inferior negative feedbacks subordinated to a superior positive feedback" (J. theor Biol. 2001)
"functional organization for sustaining self and kind, involving active use of energy and information replication (respectively)" (Human Knowledge: Foundations and Limits, which classifies about twenty-five categories of replicating or self-sustaining phenomena)
Tom Kinch's definition of life as a highly organized auto-cannibalizing system naturally emerging from conditions common on planetary bodies, and consisting of a population of replicators capable of mutation, around each set of which a homeostatic metabolizing organism, which actively helps reproduce and/or protect the replicator(s), has evolved
Stuart Kauffman's definition of life as an autonomous agent or autonomous agents capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle

Descent with modification: a "useful" characteristic

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 it, but that also have the possibility of random variations. This characteristic alone is sufficient to allow evolution, assuming the variations in the offspring allow for differential survivability. The study of this form of heritability is called genetics, and in all known life forms, with the exception of prions, the genetic material is primarily DNA, or the related molecule, RNA. Another exception might be the software code of certain forms of virii and programs created through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.

Exceptions to the common definiton

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.)

Virii 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 six characteristics are required for a population to be considered alive.

The possibility of extraterrestrial life

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 facts

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'.

The most successful animal of the earth, in terms of biomass, is the Antarctic krill, Euphausia superba, with a biomass of probably over 500 million tonnes, roughly twice the total biomass of humans.

Lifespan is the length of life in each species. Death is the termination of life in a living system, or in part thereof. Some people think that life was created by God or gods.

Reference

Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [[1]]

External link

"The Adjacent Possible: A Talk with Stuart Kauffman"