The precambrian is divinded into various periods among them Archean and
Algonkian. But not being a precambrian expert I prefer the more general term.
>
> I certainly agree that "if EVERY life form had a unique genetic code,
>one would be hard pressed to hold to a theory of common descent," though if
the
>number of radically different genetic codes were much less than the number of
>life forms, one might still plausibly believe in evolution within each type of
>genetic code.
Agreed this would be possible.
>
> Incidentally, is there any discussion or consensus on the following
>related questions?
>
> Can the fact that all self-reproducing life (e.g., excluding viruses)
>on earth are based on DNA (if this is indeed true) give us any information on
>how probable it would be for life to develop on an earth-like planet in the
>universe? I have in mind the following crude argument: Surely life could
have
>been based on something other than DNA, which presumably is an historical
>accident for life on earth. But then if the probability of life were high
on a
>per planet basis, and if DNA-type life is not the only kind of life for which
>it is high, one might think that several forms of life would have developed on
>earth. So does the existence of only DNA-life on earth suggest that the
>probability of life on a per-planet basis might be low compared to unity?
I don't know the answer to your question. I do remember from years ago
someone speculating on a life system which is moved down one row on the
periodic table. Instead of oxygen, sulfer and instead of carbon, silicon.
But I think that this was eventually ruled out. I prefer the belief that
God programmed the processes necessary for life to arise into the fabric of
the universe. since the change in the genome is driven by the 2nd law in
spite of what antievolutionists say, I wonder what else we are missing.
> I should say that I see no firm lower bound on the probability of life
>per planet, since our cosmological theories of the universe (particularly
>theories of inflation) suggest that the universe may be extremely much larger
>than what we can see of it (which encompassses about 10^22 stars), and perhaps
>infinitely large, so even if life occurs only extremely rarely within it, the
>selection principle that we are alive would put us on one of the very rare
>planets in which life occurs. (Does anyone know of any convincing lower
>bound?)
>
zero. :-)
> One problem with this crude argument is that the probability of non-DNA
>life on earth is presumably not independent of the existence of DNA life on
>earth, and this in particular is what I wanted to ask about. In other words,
>the DNA life on earth presumably makes the earth either more or less
hospitable
>to non-DNA life. Is is known which it is, and roughly how strong this
>dependence might be? For example, it might be imagined that DNA life so
>dominates the resources on earth that non-DNA life would have a hard time
>surviving with this competition. Or maybe the DNA life, if somewhat dominant,
>would eat up all the non-DNA life. On the other hand, one might imagine that
>DNA life has made the earth more hospitable for other types of life as well,
>rather as one kind of DNA life on earth (e.g., oxygen-producing plants) may
>make the earth more hospitable for another kind of DNA life (e.g.,
>oxygen-breathing animals). Which do biologists think it is?
>
fascinating question! Its beyond me though.
> My questions are indirectly related to a lecture and paper by my
>gravitation-physics colleague Brandon Carter, "The Anthropic Principle and Its
>Implications," Phil Trans. R. Soc. London A 310, 347 (1983), which argued,
>among other things, that the fact that we are near the middle of the solar
>lifetime suggests that the number n of independent highly improbable steps of
>evolution to life of our intelligence on earth is of order 1, since if it were
>0, we should have appeared earlier, but if it were large, we should be in
>roughly the last 1/n fraction of the solar lifetime. If one applies this
>argument just to life, it would seem to imply that its probability of arising
>on the earth was not small, since it appeared relatively quickly after the
>earth cooled enough to become hospitable.
>
> But this last argument assumes that our appearing now is rather
>independent of the timing of the appearance of the first life on earth. If
the
>first life had to appear before the earth cooled down too much, or before
>something else settled down to a state that has not changed too much over
>billions of years (i.e., over a time comparable to the total solar lifetime,
>estimated to be of order ten billion years), then one could not deduce that
>that appearance was probable for the earth (i.e., without putting in the
>selection effect that we are here now). In other words, we might be much more
>probable now if the first life had developed quickly than if it had taken much
>longer, though I am certainly not sure about this possibility. (What do
people
>think?) Therefore, I wondered about the fact (if a fact) of no non-DNA life
>and whether it might suggest anything about the probability of life developing
>on a random earth-like planet.
>
> I have also been curious as to what would happen IF the earth presently
>had two biospheres, one of DNA life, and one of non-DNA life, both of roughly
>equal complexity and not geographically separated. (Never mind the possible
>improbability of getting to this situation.) What fraction of the eating
might
>be across the DNA barrier? In other words, what fraction of non-DNA life
might
>DNA life eat, in comparison to the total that DNA life eats, assuming that
each
>creature has equal access to eating both DNA and non-DNA life?
If it were RNA life then they both might eat each other and one would win
out in an unstable battle.
>
> What might the fraction be if the non-DNA life were just like the DNA
>life, except for having all its chiral molecules have the opposite handedness?
>Having heard that humans can taste but not digest sugar of the opposite
>handedness (which would apparently thus be the ideal diet sweetener, if it
>weren't so expensive), I have wondered whether we could even survive if all we
>had to eat were DNA-like life of the opposite handedness. And if we needed to
>eat at least some true DNA life to survive, could we get any nutritional
>benefit at all from eating the DNA-like life of the opposite handedness?
>(I.e., would it in any sense be better to eat the DNA-like life than to go
>hungry?)
Usually the thought is that one molecular handedness would win out early in
the chemical evolution of life. And we must remember that left handed amino
acid molecules have now been found in meteors giving a decided handedness to
the biological molecules which might have started life. see (~I.
Peterson, "Left-handed Excess in Meteorite Molecules", Science
News, Feb 22, 1997, p. 118; John R. Cronin and Sandra Pizzarello,
"Enantiomeric Excesses in Meteoritic Amino Acids," Science
275(February 14, 1997):951-955;Christopher F. Chyba, "A left-handed Solar
System? " Nature 389, 234-235 (1997); and M. H. Engel and S. A. Macko,
"Isotopic evidence for Extraterrestrial non-racemic Amino Acids int he
Murchison Meteorite," Nature 389:265-268.
This discovery rules out the argument against evolution given by Bradley and
Thaxton p. 189 of Moreland's Creation Hypothesis. In fact, one could see
this coming from previous papers prior to Bradley and Thaxton's publising
their argument. Consistent reports of left-handed amino acids have been
appearing in the literature since the early 80's.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm