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Life
*** Shopping-Tip: Life
:''For other uses, see
Life (disambiguation) and
Living (disambiguation) Living''
{{wiktionarypar2|life|living}}
'''Life''' is a multi-faceted concept that may refer to the ongoing process of which living things are a part or the period between
fertilisation or
mitosis and
death.
Defining the concept of life
How can one tell when an entity is alive? 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
Origin of life 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 (trait) intelligence.
A conventional definition
Although there is no universal agreement on the definition of life, the generally accepted biological manifestations are that life exhibits the following phenomena:
#'''
Organism Organization''' - Living things are comprised of one or more
cell (biology) cells, which are the basic units of life.
#'''
Metabolism''' - Metabolism produces
energy by converting nonliving material into cellular components (
synthesis) and decomposing organic matter (
catalysis). Living things require energy to maintain internal organization (
homeostasis) and to produce the other phenomena associated with life.
#'''
Growth''' - Growth results from a higher rate of synthesis than catalysis. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
#'''
Adaptation''' - Adaptation is the accommodation of a living organism to its environment. It is fundamental to the process of
evolution and is determined by the individual's
heredity.
#'''Response to stimuli''' - A response can take many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals. Plants also respond to stimuli, but usually in ways very different from animals. A response is often expressed by motion: the leaves of a plant turning toward the sun or an animal chasing its prey.
#'''
Reproduction''' - The division of one cell to form two new cells is reproduction. Usually the term is applied to the production of a new individual (either
asexual reproduction asexually, from a single parent organism, or
sexual reproduction sexually, from two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
Common features of life on Earth
The conventional definition of life defines the common features that life is expected to exhibit wherever it may be encountered in the universe. On Earth, all life exhibits at least one additional common feature, namely that life on this planet 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, a position referred to as ''
carbon chauvinism''.
Exceptions to the conventional definition
It is important to note that life is a definition that applies at the level of species, so even though many individuals of any given species do not reproduce, possibly because they belong to specialised sterile castes (such as
ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence,
sterile hybrid species such as the
mule are considered life although not themselves capable of reproduction. It is also worth noting that non-reproducing individuals may still help the spread of their genes through such mechanisms as
kin selection.
For similar reasons,
viruses and aberrant
prion proteins are often considered
replicators rather than forms of life: they cannot reproduce without very specialised substrates such as host cells or proteins, respectively. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.
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 life form.
Other definitions
The
Systems Theory systemic definition is that living things are self-organizing and
autopoiesis autopoietic (self-producing).
These objects are not to be confused with
Dissipative system dissipative structures (e.g. fire).
Variations of this definition include
Stuart Kauffman's definition of life as an
agent autonomous agent or a
multi-agent system 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
common descent 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.
This argument would, however, include replicating elements which have been observed to evolve, but are not generally accepted as being alive. Examples include
virus viruses,
plasmid plasmids,
transposon transposons,
satellite DNA, and even
meme memes. Another exception might be the
software code of certain forms of
computer virus computer viruses and programs created through
genetic programming. 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 scientific models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:
#Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the
Miller experiment 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 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
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
bacterium bacterial life on
Mars (planet) 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
MER-A Spirit and
MER-B Opportunity supports the theory that Mars once had surface water. See ''
Life on Mars'' for further discussion.
Jupiter (planet) Jupiter's moons are also considered good candidates for extraterrestrial life, especially
Europa (moon) 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 (moon) Titan cryovolcanoes, or even
Enceladus (moon) Enceladus.
See also
*
Artificial life
*
Extraterrestrial life
*
Kingdom (biology) Biological kingdom
*
Origin of life
*
Mimivirus is the largest known virus.
References
*Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [http://www.edge.org/3rd_culture/kauffman03/kauffman_index.html]
*
Lynn Margulis and
Doris Sagan - ''What is Life?'' (1995). Simon & Schuster. ISBN 0-684-81087-5
*
Erwin Schrodinger - ''What is Life?'' (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8
External links
{{wikiquote}}
-
"In the Beginning..." (''The Economist'')
-
"The Adjacent Possible: A Talk with Stuart Kauffman"
-
Stanford Encyclopedia of Philosophy entry
-
The Collaborative International Dictionary of English: Life
-
The Deep Hot Biosphere Theory (Thomas Gold)
-
What is Life ? The Process from Birth to Death
-
Is Unraveling of the Mystery of Life and Death Possible Through Science?
Category:Life
Category:Biology
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ja:生命
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see
Life (magazine)
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{| cellspacing="0" style="width: 238px; background: ;"
| style="width: 45px; height: 45px; background: blue; text-align: center; font-size: {{{5|{{{id-s|14}}}}}}pt; color: {{{id-fc|black}}};" | '''Life'''
| style="font-size: {{{info-s|8}}}pt; padding: 4pt; line-height: 1.25em; color: {{{info-fc|black}}};" | This user believes that given the existence of
humanity, and the size of the universe, there must be
Exobiology life on other worlds
|}
*** Shopping-Tip: Life
