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Life has no simple definition. Apart from countless religious definitions and explanations, 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

As all other known definitions of life, this is inadequate. According to this definition,

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 computer virus and programs created through genetic programming, but whether computer programs can be alive even by this definition is still a matter of some contention.

Currently (2001) all known forms of life are found on the planet Earth. The question of life 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.

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'.