Life

:For other uses, see Life and Living

Defining the concept of life

How can one tell when an entity is a lifeform? It would be relatively straightforward to offer a practical set of guidelines if one's only concern were 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. Also, loosely speaking, some theories are grounded in the basic assumption that "ideas have a life of their own".

Related Topics:
Earth - Biosphere - Origin - Artificial life - Intelligence - "ideas have a life of their own"

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A conventional definition

In biology, a lifeform has traditionally been considered to be a member of a population whose members can exhibit all the following phenomena at least once during their existence:

Related Topics:
Biology - Existence

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• 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 entities that are similar to, yet separate from, itself
• Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions.

Exceptions to the conventional definition

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:

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• Mules and people who are infertile cannot reproduce and thus would not qualify as lifeforms. Also worker bees and other organisms living in colonies would not qualify; only the queen and the drones (or the whole colony) can be considered 'alive'.
• Fire and stars could be considered lifeforms.
• A virus does not grow and cannot reproduce outside of a host cell and thus would not qualify as a lifeform.

Many individual organisms are incapable of reproduction and yet are still considered to be lifeforms; see mules and ants for examples. This is because the term "lifeform" applies 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.) It is important to keep in mind the difference between a "lifeform" and "a being that is alive." One example of sterility does not render the rest of the species a non-lifeform, any more than one dead animal renders the rest of the species dead.

Related Topics:
Mule - Ant - Species - Gene - Kin selection - Sterility

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Note also that the two cases of fire and stars fitting the definition of life can be simply remedied by defining metabolism in a more biochemically exact way. Fundamentals of Biochemistry by Donald Voet and Judith Voet (ISBN 0471586501) defines metabolism as follows: "Metabolism is the overall process through which living systems acquire and utilize the free energy they need to carry out their various functions. They do so by coupling the exergonic reactions of nutrient oxidation to the endergonic processes required to maintain the living state, such as the performance of mechanical work, the active transport of molecules against concentration gradients, and the biosynthesis of complex molecules." This definition, in use by most biochemists, makes it clear that fire is not alive, because fire releases all the oxidative energy of its fuel as heat.

Related Topics:
Fundamentals of Biochemistry - Donald Voet - Judith Voet

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This could also be remedied by adding the requirement of locality, where there is an obvious structure that delineates the spatial extension of the living being, such as a cell membrane.

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A conceptual problem with saying that fire is life is that it collapses the distinction between "growth" and "reproduction." It is possible to think of a spreading flame as either growing or reproducing, but what would it mean to say that the same act is both growth and reproduction?

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Viruses 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 five characteristics are required for a population to be considered a lifeform.

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Other definitions

Biologists who are content to focus on terrestrial organisms often note some additional signs of life, including these:

Related Topics:
Biologist - Organism

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• 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 for some period of time.
• Species of living organisms will evolve.

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'.

Related Topics:
Chemistry - Carbon - Carbon chauvinism

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The systemic definition is that living things are self-organizing and autopoietic (self-producing). These objects are not to be confused with dissipative structures (e.g. fire). Variations of this definition include:

Related Topics:
Systemic - Autopoietic - Dissipative structures

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• Francisco Varela and Humberto Maturana's definition of life (also widely used by Lynn Margulis) 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)
• 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 a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle
• Robert Pirsig's definition of life, found in his book ', as that which maximizes its range of possible futures, in other words, that which makes decisions that result in the most future choices, or that which strives to keep its options open.
• A system converting entropy to negentropy, using flow of energy.

Related Topics:
Decision tree - Decision making - Deterministic system

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Other definitions:

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• That which seeks to continue its own existence (attributed to Clifford A. Schaffer).

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 its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. In all known life forms (assuming prions are not counted as such), 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.

Related Topics:
Descent - Chance - Evolution - Genetics - Prion - DNA - RNA - Software - Viruses - Genetic programming - Computer

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