Text 96, 173 rader
Skriven 2004-09-18 16:52:00 av Peter F (1:278/230)
Ärende: Re: what is life
========================
"TomHendricks474" <tomhendricks474@cs.com> wrote in message
news:chnea6$30ff$1@darwin.ediacara.org...
> << We should rethink about the definition of life: which property is
> > more fundamental? I prefer self-replication or reproduction.
>
> I agree with that way of looking at it. >>
>
>
> I do not. The debate over replication or
> metabolism, nucleic acids or proteins -
> has gone on for decades and has led nowhere.
Found this relevant article at
http://www.space.com/scienceastronomy/astrobio_life_030415.html
Interview with Carol Cleland
Q: What is your opinion of attempts to define "life?"
I argue that it is a mistake to try to define "life." Such efforts reflect
fundamental misunderstandings about the nature and power of definitions.
Definitions tell us about the meanings of words in our language, as opposed to
telling us about the nature of the world. In the case of life, scientists are
interested in the nature of life; they are not interested in what the word
"life" happens to mean in our language. What we really need to focus on is
coming up with an adequately general theory of living systems, as opposed to a
definition of "life."
But in order to formulate a general theory of living systems, one needs more
than a single example of life. As revealed by its remarkable biochemical and
microbiological similarities, life on Earth has a common origin. Despite its
amazing morphological diversity, terrestrial life represents only a single
case. The key to formulating a general theory of living systems is to explore
alternative possibilities for life. I am interested in formulating a strategy
for searching for extraterrestrial life that allows one to push the boundaries
of our Earth-centric concepts of life.
Q: In the category of what is "alive," would you exclude what you call the
"borderline" cases - viruses, self-replicating proteins, or even
non-traditional objects that have some information content, reproduce, consume,
and die (like computer programs, forest fires, etc.)?
This is a complex question. Language is vague, and all terms face borderline
cases. Is an unmarried twelve-year-old boy a "bachelor?" How about an eighteen
year old? How many hairs does it take to turn a "bald" man into a man who is
"not bald?" 20 or 100 or 1,000 hairs?
The fact that there are border line cases -- that we can't come up with a
precise cut-off -- doesn't mean there isn't a difference between a bachelor and
a married man, or a bald man and a man who is not bald. These difficulties
don't represent profound difficulties; they merely represent the fact that
language has a certain degree of flexibility. So I don't think that entities
like viruses provide very interesting challenges to definitions of "life."
On the other hand, I don't think that defining "life" is a very useful activity
for scientists to pursue since it is not going to tell us what we really want
to know, which is "what is life." A scientific theory of life (which is not the
same as a definition of life) would be able to answer these questions in a
satisfying way.
As an analogy, the medieval alchemists classified many different kinds of
substances as water, including nitric acid (which was called "aqua fortis").
They did this because nitric acid exhibited many of the sensible properties of
water, and perhaps most importantly, it was a good solvent. It wasn't until the
advent of molecular theory that scientists could understand why nitric acid,
which has many of the properties of water, is nonetheless not water. Molecular
theory clearly and convincingly explains why this is the case: water is H2O -
two hydrogen atoms and one oxygen atom. Nitric acid has a different molecular
composition.
A good theory of life would do the same for the cases that you mention, such as
computer programs. Merely defining "life" in such a way that it incorporates
one's favorite non-traditional "living" entity does not at all advance this
project.
Q: What is your favored theory for how life could have arisen on Earth -clay
crystals, RNA world, membranes, or some other option?
It seems to me that all theories of the origin of life face two major hurdles.
The biggest one is explaining the origin of the complex cooperative schema
worked out between proteins and nucleic acids -- the controlled production of
self-replicating catalytic systems of biomolecules. All of the popular accounts
of the origin of life strike me as side stepping this issue. Instead, they
focus on the other hurdle: producing amino acids and nucleotides, and getting
them to polymerize into proteins and nucleic acids (typically, RNA). But it
seems to me that none of them have provided us with a very satisfying story
about how this happened.
All the scenarios that have been proposed for producing RNA under plausible
natural conditions lack experimental demonstration, and this includes the RNA
world, clay crystals, and vesicle accounts. No one has been able to synthesize
RNA without the help of protein catalysts or nucleic acid templates, and on top
of this problem, there is the fragility of the RNA molecule to contend with.
But I still think that the more serious problem is the next stage of the
process, the coordinating of proteins and RNA through a genetic code into a
self-replicating catalytic system of molecules. The probability of this
happening by chance (given a random mixture of proteins and RNA) seems
astronomically low. Yet most researchers seem to assume that if they can make
sense of the independent production of proteins and RNA under natural
primordial conditions, the coordination will somehow take care of itself.
I suppose that if I had to pick a favorite theory, it would be Freeman Dyson's
double origin theory, which postulates an initial protein world that eventually
produced an RNA world as a by-product of an increasingly sophisticated
metabolism. The RNA world, which starts out as an obligatory parasite of the
protein world, eventually produces the cooperative schema, and hence life as we
know it today. I like the fact that this account attempts to deal with the
origin of the cooperative schema.
Q: Do you think there could have been multiple origins of life, or that life
could have come to Earth from somewhere else?
Life arising more than once from nonliving materials could occur elsewhere than
Earth, but it could also have occurred on Earth. It is possible that
extraterrestrial life exists and that all life nonetheless has a common
ancestor. Scientists now believe that microbes can survive interplanetary
journeys ensconced in meteors produced by asteroid impacts on planetary bodies
containing life. In other words, we could all be the descendants of Martians --
or Martians, if they happen to exist, could share a common ancestor with us! In
short, the mere discovery of extraterrestrial life doesn't guarantee that life
had more than one origin.
Q: As one of the great mysteries and challenges in science, do you think we can
determine the origin of life through experimentation?
I hope so! But until we have an adequate theory of life to drive the
formulation of the right experiments, it will be difficult to tell. I suppose
it is always possible that life is not a natural category, and thus no
universal theory of life can be formulated. But I doubt it.
It is also possible that life on Earth is the product of a very complex
historical process that involves too many contingencies to be readily
accessible to definitive experimental investigations. An adequately general
theory of life would make this clear, however. Besides, historical research is
quite capable of obtaining empirical evidence that can resolve historical
questions of this sort-evidence that is just as convincing as that provided by
classical experimental research! So even if we can't produce life in the lab
from nonliving materials, it doesn't follow that we will never know how life
originated on Earth.
What's Next
As an example of how one's definition of life can directly shape exploratory
science, the European Space Agency will launch a Mars mission in early summer
2003. Current plans are for its lander, Beagle 2, to perform biological
experiments designed to search directly for evidence of life on Mars. The
scientific payload highlights the common features thought to classify what
might previously have indicated life growing there. For instance, Beagle 2 will
look for the presence of water, the existence of carbonate minerals, the
occurrence of organic residues, and any isotopic fractionation between organic
and inorganic phases. Each of these will provide clues when matched against the
prevailing environmental conditions like martian temperature, pressure, wind
speed, UV flux, oxidation potential, and dust environment.
For life to flourish, the prerequisites are nearly as lengthy as the definition
itself.
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