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For other uses, see Life and Living

Life is a multi-faceted concept. Life may refer to an ongoing process of which living things are a part, the period between the conception (or a point at which the entity can be considered to be an individualized being) and death of an organism, the condition of an entity that has been born (or reached the point in its existence at which it can be established to be alive) and has yet to die, and that which makes a living thing alive.


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".

A conventional definition

Unsolved problems in biology: How did life start? Is life a cosmic phenomenon? Are the conditions necessary for the origin of life narrow or broad? How did life originate and diversify in hundreds of millions of years? Why, after rapid diversification, do microorganisms remain unchanged for millions of years? Why have so many biological systems developed sexual reproduction? How do organisms recognize like species? How are the sizes of cells, organs, and bodies controlled?

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:

  1. Growth, full development, maturity
  2. Metabolism, consuming, transforming and storing energy/mass; growing by absorbing and reorganizing mass; excreting waste
  3. Motion, either moving itself, or having internal motion
  4. Reproduction, the ability to create entities that are similar to, yet separate from, itself
  5. Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions. This property is also called homeostasis.

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:

  • 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.

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.

(Note: Actually, the definition does not help much at all. For it is circular. What we are looking for, after all, is a definition of "living entity." We agreed that part of the definition is "capable of metabolism." We then tried to define "metabolism" in order to get clear on which entities are capable of it and which not. But the definition of "metabolism" just offered is in terms of living systems. And those are exactly what we are trying to define!)

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.

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?

Viruses reproduce, flames grow, some software programs mutate and evolve, future software programs will probably evince (even high-order) behavior, machines move, and some form of proto-life consisting of metabolizing cells without the ability to reproduce presumably existed. Still, some would not call these entities alive. Generally, all five characteristics are required for a population to be considered a lifeform.

Other definitions

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

  1. Living organisms contain molecular components such as: carbohydrates, lipids, nucleic acids, and proteins.
  2. Living organisms require both energy and matter in order to continue living.
  3. Living organisms are composed of at least one cell.
  4. Living organisms maintain homeostasis for some period of time.
  5. 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'.

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:

Other definitions:

  • 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.

Origin of life

Main article: Origin of life

There is no truly "standard" model of the origin of life, but most currently accepted models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:

  1. Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Urey-Miller experiment.
  2. Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
  3. Procedures for producing random RNA molecules can produce ribozymes, which are able to produce more of themselves under very specific conditions.

There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.

The possibility of extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

As of 2005, 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 open, but analyses such as the Drake equation have been used to estimate the probability of such life existing. There have been a number of claims of the discovery of life elsewhere in the universe, but none of these have yet survived scientific scrutiny.

Today, the closest that scientists have come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars (via the ALH84001 meteorite). Searches for extraterrestrial life are currently focusing on planets and moons believed to possess liquid water, at present or in the past. Recent evidence from the NASA rovers Spirit and Opportunity supports the theory that Mars once had surface water. See Life on Mars for further discussion.

Jupiter's moons are also considered good candidates for extraterrestrial life, especially Europa, which seems to possess oceans of liquid water.

Other highly speculative and somewhat doubtful places for present or past life include the atmosphere of Venus, Titan cyrovolcanoes, or even Enceladus.

See also


  • Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [1]

External links

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