Life

Life is a primarily biological concept that has no simple definition.

Attempts to define the concept of life

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 and produce a mule)
   * viruses are not (cannot grow)

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 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).

As of 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.
There may be simple life forms on Jupiter's moons. 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 probably over 500 million tonnes.

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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.

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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.

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