- This article concerns the primary meanings of "life" in biology. For other uses, see Life (disambiguation) and Living (disambiguation).
Life is a multi-faceted concept. Life may refer to the ongoing process of which living things are a part; the period between the birth (or a point at which the entity can be considered to be living) 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:
- 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 seperate 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:
- Mules and people who are impotent cannot reproduce and thus would not qualify as lifeforms.
- Fire and Stars could be considered lifeforms.
- Viruses do not grow and cannot reproduce outside of a host cell and thus would not qualify as lifeforms. (Although, this is not a very solid counterexample as viruses are no more than a nucleic acid.)
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.
This could also be remedied by adding the requirement of locality, where there is an obvious feature that delineates the spatial extension of the living being, such as a cell membrane (although this would then discount fungi, and grasses from being life forms).
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? (Although it is colloquial to speak of flames "getting bigger," there are no discernable "generations" of flames of which to speak.)
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.
Other definitions
Biologists who are content to focus on terrestrial organisms often note some additional signs of life, including these:
- 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'.
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:
- 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 Lila: An Inquiry into Morals, 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.
Other definitions:
- That which seeks to continue its own existence (attributed to Clifford A. Schaffer).
Descent with modification: a "useful" characteristic
Life - A sexually transmitted disease: side effects and symptoms vary but common features include damage to the environment, high financial cost and the production of excrement; usually incurably terminal - only 3 reported cases of recovery on record so far.
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, however most currently accepted models build in one way or another upon the following discoveries, which are listed in a rough order of postulated emergence:
- Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Urey-Miller experiment.
- Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
- 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 of the possibilities have tended to fall into either "genes-first" or "metabolism-first"; a recent trend is the emergence of hybrid models that combine aspects of both.
The possibility of extraterrestrial life
Main articles: Extraterrestrial life, Xenobiology
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.
See also
References
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [1]
External link
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