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| *] are not | *] are not | ||
| ] has provided what may be the most satisfying definition of life. She defined life as an autopoietic (self-directed), ] based, ]-] bound, ] metabolic, ] replicated, protein readout ]. | ] has provided what may be the most satisfying definition of life. She defined life as an autopoietic (self-directed), ] based, ]-] bound, ] metabolic, ] replicated, protein readout ]. Other definitions are | ||
| * "a system of inferior negative feedbacks subordinated to a superior positive ]" (); | |||
| * "functional organization for sustaining ] and ], involving active use of ] and ] ] (respectively)" (], which 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 ], assuming the variations in the offspring allow for differential survivability. The study of this form of heritability is called ], and in all known life forms with the exception of ]s the genetic material is primarily ] or the related molecule ]. Another exception might be the ] code of certain forms of ] and programs created through ], but whether ] programs can be alive even by this definition is still a matter of some contention. | 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 ], assuming the variations in the offspring allow for differential survivability. The study of this form of heritability is called ], and in all known life forms with the exception of ]s the genetic material is primarily ] or the related molecule ]. Another exception might be the ] code of certain forms of ] and programs created through ], but whether ] programs can be alive even by this definition is still a matter of some contention. | ||
Revision as of 21:37, 22 January 2003
- Alternate meanings: Conway's Game of Life, Hasbro's Game of Life, personal life, Life magazine
Life is a biological concept that 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 are all other known definitions of life, this one is inadequate. According to this definition,
Lynn Margulis has provided what may be the most satisfying definition of life. She 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);
- "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).
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'.
See also: Meaning of life, Vitalism, Materialism, Artificial life