"TomHendricks474"  wrote in message
news:chnea6$30ff$1{at}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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