Life: Encyclopedia - Life

Life

Life is a multi-faceted concept that may refer to

• the ongoing process of which living things are a part
• the period between the conception (the point at which the entity can be considered to be an individualized being) and death of an organism
• the condition of an entity between conception and death
• that which makes a living thing alive.

Life - Defining the concept of life

Life - A conventional definition

In biology, a life form 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 of individuals to create entities that are similar to, but separate from, themselves
5. Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions. This property is also called homeostasis.
6. Cells, a basic unit of reproduction

Life - 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:

• (most) 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'.
• 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 extinct.

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 six characteristics are required for a population to be considered a lifeform.

Life - 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:

• 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).
• A self-replicating system that evolves through mutation.

Life - 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.

Life - Origin of life

Main article: Origin of life

There is no truly "standard" model of the origin of life, but most currently accepted scientific 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.

Life - The possibility of extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

At this time, 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 cryovolcanoes, or even Enceladus.

See also

• Animal
• Artificial life
• Bacteria
• Biology
• Death
• Fungi
• Biological kingdom
• Biological life cycle
• Monera
• Odic force
• Plant
• Prehistoric life
• Protista

Adapted from the Wikipedia article "Life", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki