On Sat, 11 Sep 1999 firstname.lastname@example.org wrote:
Given that we get a stable crust ~4 billion years ago and life seems
to appear at 3.86 billion years ago, that says that (a) it either
> But actually assembling some kind of self-replicating
> system or cycle is still not well understood.
Given that we get a stable crust ~4 billion years ago and life seems to appear at 3.86 billion years ago, that says that (a) it eithercame from space (panspermia); or (b) it is easy.
> There is still a very hard step here, going from the building blocks
> to self-replication.
If it came from space, then it is (a) ubiquitous; or (b) we are a very special case.
> Until we have come up with some convincing way to bridge this gap
> I don't think we can say that scientific evidence suggests that
> life is likely to be common.
What we really need to know is the relative sizes of the life "phase space" and the non-life "phase space". It would appear, given the millions of compounds that can be made out of C/H/N/O (some of the most aboundant elements), that the phase space for fundamental building blocks (amino acids, RNA, etc.) is large. Since scientists have come up with scenarios for other information carriers (I think for example peptide nucleic acids), we may be based on a "difficult" to evolve information carrier, but that doesn't imply that other life forms must be.
You may want to look up: "The orgin of life--a review of facts and speculations", by L. E. Orgel, Trends Biochem Sci. Dec 1998 23(12):491-5.