🚀 go-pugleaf

nickh@CS.CMU.EDU (Nick Haines) tried to write:
> [... stuff, including reference, about how life probably emerged here ...]
> My guess for P(life evolving | suitable planet) is 1e-12.

One way to estimate P(life | suitable planet), while adjusting for the
weak local anthropic principal ("how strange that we should be on
a planet which has life---NOT!" :-)) is to look at how long it took life to
evolve, and compare that to the length of the 'window of opportunity'
in whch life like ours could have started--not so early that the
Earth got resterilized by continued planetery bombardment and not
so late that there is no time to develop Usenet before the Sun
becomes inhospitable.

It turns out that life occurred just about as soon as the Earth was
tolerable, like within 100 million years (this number is approximate, I
don't know offhand the actual number. Nobody does.)  (This leads to
speculation that life may have developed one or more times earlier as
well, but was wiped out each time when the next planetesimal was
accreted.)  This is very near the start of the window, which is roughly
5 billion years long as a rough number.

This is compatible with a P(life | suitable planet) ~ 1.  If P(life |
suitable planet) were small, then on those planets which did, against
the odds, develop life, there would be no preference for the beginning
of the window rather than the end, and much preference for the big, wide
middle.

However, until about 600 Million years ago, life didn't seem to do
much evolving, (from a fossil-viewing, anthropocentric, politically
incorrect dead-white-European-male viewpoint :-)).  If the Cambrian
explosion (see this Month's Scientific American for a report on this)
had been delayed by a factor of three (which is nothing in terms of
astrophysics and the statistics of small numbers) brains might
not have evolved before old Mr. Sun decided to stretch out a bit
and eat the inner planets, a few billion years from now.  (Sorry
about the tenses in the last sentence.  I don't know how to
conjugate English verbs in the combined contrafactual subjunctive
past-factual future tense, if that is indeed the tense I should have used.)

This suggests that P(brains | life) is not close to 1, and so
life does not imply that brains (or even creatures more complex than
a jellyfish) will eventually arise.

However, P(brains)  can be pretty low without making SETI hopeless.

-- 
		David Palmer		 palmer@alumni.caltech.edu