Thanks for the clarification in your posting to asa@calvin.edu of Mon,
17 Nov 1997 21:41:07 -0600. I hadn't realize what you meant by "earliest
precambrian." (I must admit that I can't remember all the names for the
geological epochs. I thought "precambrian" was the period just before the
cambrian and that there were many periods containing life before that, but
presumably "precambrian" means all the periods before the cambrian.)
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.
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 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?)
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?
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?
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?)
Thanks to anyone for any information or suggested reading you can give
me on this specific issue or the more general one of how probable on a
per-planet basis is the development of life of various degrees of
sophistication or intelligence.
Don Page