So ... What Exactly is this 'Life' thing?

E. Moritz notes - started on Life Web Project 2/17/2004 2200 CST

We'll get to it ... in the meanwhile  - you might be interested in the following ...

1.Erwin Schrodinger, one of the originators of Quantum Mechanics, wrote What is Life? This book inspired Francis Crick of Crick and Watson to research DNA ... you might know the rest of the story ... We'll amplify on this as time proceeds.  (A retrospective on his work and its significance, What is Life? The Next Fifty Years : Speculations on the Future of Biology, is now in print)

2. This could well be a rather inexhaustible topic (see for example this extensive bibliography) ...

3. One of our main interests is in the special status accorded to the term 'life'by the Framer's of the Declaration of Independence.

4. Our culture (Western - Republic, Democratic) values life.  There are cultures around that do not value life to the extent we do.  This situation is one that puts our Nation at peril.

5. Life is finite and is precious beyond words ... once gone, its gone (for the individual).

6. There are those actively working on Life Extension.  There are a whole host of moral, ethical, philosphical, and economic issues associated with Life Extension.  These need to be discussed.

n. Wikipedia, the free encyclopedia, has [at 03:21, 16 Feb 2004] the following discussion:

Alternative meanings: Conway's Game of Life, Hasbro's Game of Life, Life magazine
Life is a multi-faceted concept with no simple definition, in part because the word is often used in an intentionally open-ended way, as exemplified by phrases such as "eternal life", "artificial life", and "extraterrestrial life". In fact, WordNet identifies fourteen different senses of the word "life" and the Longman Web Dictionary enumerates thirty-five.

Defining the concept of life
Life has a number of senses with a biological meaning

"life" may refer to the ongoing process of which living things are a part;
"life" may refer to the period between birth and death of an organism;
"life" may refer to the state of something that has been born and has yet to die, i.e., that which makes a living thing alive.
The remainder of this section focuses on the last sense how can one tell when an entity is a living thing?

It would be relatively straightforward to offer a practical set of guidelines if one's only concern was 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.

A conventional definition
In biology, an entity has traditionally been considered to be alive if it exhibits all the following phenomena at least once during its existence:

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 roughly exact copies of itself
Response to stimuli - the ability to measure properties of its surrounding environment, and act upon certain conditions.
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:

fire is alive. (This could 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.)
male mules are not alive as they are sterile and cannot reproduce.
viruses are not alive as they do not grow.
Biologists who are content to focus on terrestrial organisms often note some additional signs of a "living organism", 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.
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'.

Other definitions include:
Lynn Margulis's definition of life 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)
"functional organization for sustaining self and kind, involving active use of energy and information replication (respectively)" (Human Knowledge: Foundations and Limits, which classifies about twenty-five categories of replicating or self-sustaining phenomena)
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 autonomous agents capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle

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.

Exceptions to the common definition
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 information about one way by which non-reproducing individuals can still enhance the spread of their genes and the survival of their species.)

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 alive.

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
As of 2003, 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, although the probability that Earth is the only location in the universe, or even the galaxy, that harbors life, is extremely low. 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 come to finding extraterrestrial life is fossil evidence of possible bacterial life on Mars. There also may be simple life forms on Jupiter's moons.


The noun "life" has the following 14 senses in WordNet.

1. life -- (a characteristic state or mode of living; "social life"; "city life"; "real life")
2. life -- (the course of existence of an individual; the actions and events that occur in living; "he hoped for a new life in Australia"; "he wanted to live his own life without interference from others")
3. life, living -- (the experience of living; the course of human events and activities; "he could no longer cope with the complexities of life")
4. animation, life, living, aliveness -- (the condition of living or the state of being alive; "while there's life there's hope"; "life depends on many chemical and physical processes")
5. life, lifetime, lifespan -- (the period during which something is functional (as between birth and death); "the battery had a short life"; "he lived a long and happy life")
6. life -- (the period between birth and the present time; "I have known him all his life")
7. liveliness, life, spirit, sprightliness -- (animation and energy in action or expression; "it was a heavy play and the actors tried in vain to give life to it")
8. biography, life, life story, life history -- (an account of the series of events making up a person's life)
9. life -- (the period from the present until death; "he appointed himself emperor for life")
10. life -- (a living person; "his heroism saved a life")
11. life -- (living things collectively; "the oceans are teeming with life")
12. life -- (a motive for living; "pottery was his life")
13. life -- (the organic phenomenon that distinguishes living organisms from nonliving ones; "there is no life on the moon")
14. life sentence, life -- (a prison term lasting as long as the prisoner lives; "he got life for killing the guard")

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