Life

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

Life is a primarily biological concept that has no simple definition. Something is usually defined to be alive if it matches the following conditions, at least once during its existence:

• Growth
• Metabolism, the uptake of food, conversion of food into energy, and disposal of waste products
• Motion, either moving itself, or having internal motion
• Reproduction, the ability to create more-or-less exact copies of itself
• Stimulus response, the ability to measure properties of its surrounding environment, and act on certain conditions.

Controversially, according to this definition,

• fire is alive
• mules are not (cannot reproduce)
• viruses are not (cannot grow)

Lynn Margulis defined life as an autopoietic (self-directed), water based, lipid-protein bound, carbon metabolic, nucleic acid replicated, protein readout system. Other definitions are

• "a system of inferior negative feedbacks subordinated to a superior positive feedback" (J. theor Biol. 2001 (http://www.mol.uj.edu.pl/~benio/cyber_def_life.pdf));
• "functional organization for sustaining self and kind, involving active use of energy and information replication (respectively)" (Human Knowledge: Foundations and Limits, which classifies (http://humanknowledge.net/Thoughts#life) about 25 categories of replicating or self-sustaining phenomena).

Perhaps a more 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 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 one way that non-reproducing individuals can still enhance the spread of their genes and the survival of their species).

Currently (2003), the 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. 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 gotten to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars.

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'.

Most successful animal of the earth in terms of biomass: Antarctic krill, Euphausia superba, with a biomass probably over 500 million tonnes.

Lifespan is the length of life in each species.

Death is the termination of life in a living system, or in part thereof.

Life insurances, including pensions and life annuities, provide payments depending on life or death of a particular person. Accordingly, documents that may be required for payment are:

• a life certificate stating that a person was alive at the date of issue;
• a death certificate stating that a person died on a particular date.

See also: Meaning of life, Vitalism, Materialism, Artificial life, Value of life, Afterlife