As was explored last year, this "best evidence" is not based on
rocks, as we do not have them from this "window" of earth history.
It is based on the adoption of a certain theory of planetary
formation, by analogy with the Jovian planets, and by various other
inferences based on observed isotopic data. (I've just reread Brian
Harper's post to you on 23 November which reviews some of these
points.)
> As for the implied need for "unlimited time" I would remind everyone that
> 800 million years covers virtually the entirety of metazoan evolution, from
> the simplest hypothetical organisms through the Cambrian explosion on up to
> modern times. That is a heck of alot of evolution for so short a time.
And if you consider that most of it was stasis, this is an even more
remarkable situation!
> Considering the ease with which biomolecules, including catalytic proteins
> and RNA, plus vesicles with lipid bilayers, can form, I don't consider 500
> million years to be any serious limitation to abiogenesis.
"Ease" is not the word I would use. They can form under certain
specific conditions, and they are also very quickly destroyed.
However, the main point I would wish to make is that the challenge
for abiogenisis advocates is to form a replicating bio-structure.
Without replication, there is nothing for selection forces to act on
and whatever structure is formed is quickly degraded. This is my
understanding of why abiogenesis needs time: the hypothesis is that
the more time there is, the greater the probability of arriving at a
replicating structure.
This is also my answer to Pim's comment:
"Then again the question really is, is such "unlimited time" really
required ? The remaining time is still quite long."
One last point: Kevin says that 500 million years is not a serious
limitation. But have you got 500 million years? For most of this
time, the Great Bombardment of the Earth is thought to have occurred
- and few would wish to invoke abiogenesis in that environment. (No
warm little ponds during this time!). (Maybe this is the
breakthrough abiogenesis is looking for: introduce extreme energy
sources from bolide impacts and up pops a self-replicating cell!
Perhaps Hoyle and Wickramasinghe were way out in their 'tornado
forming a plane out of scrap' story!)
Best regards,
David J. Tyler.