On Tue, 21 Mar 2000, Billy Brown wrote:
> <snip> ...
> you have to have simple organisms that don't have such adaptations.
Agreed. The simplest organisms, living off either solar energy or
energy produced by deep sea vents, got established *very* early,
within a few hundred million years after the asteroid/cometary
bombardment declined to non-sterilizing levels.
> You need an environment
> in which life can arise, solve the basic survival problems like energy
> production and reproduction, and reach a level of complexity that allows
> complex adaptations to evolve. That implies that a world that has a stable
> period followed by wild climate swings may support interesting life, but one
> that has always had extreme shifts may not evolve life in the first place.
Perhaps. But the evidence suggests we have suggests that primitive
organisms (bacteria and archea) that are able to survive over a much
wider range of environments frozen to ~100C (at high pressures) than
can complex animals at this time. So the adaptations to these
environments could occur in simple organisms and then be incorporated
into more complex organisms. On our planet the vectors seems to have
taken two directions. One, that evolves greater complexity and a second
that seems to have evolved adaptation to environmental extremes. Now,
it may be that the extreme environment vector for life was a direct
result of extinction events (such as Snowball Earth). There was
only a limited "complexification" vector for a long time, perhaps
because it kept getting truncated. Only after Snowball Earth have
things been stable enough for complexification to generate an
explosion of species. However on planets that always have extreme
environmental swings, the only vector that can develop is one
that includes first extreme tolerance followed by complexification.
It may take longer since you probably have some energy and reproduction
costs associated with the environmental adaptations. Its difficult
to say however, whether the penalty for this is only millions or many
billions of years.
>
> Which leads me to what I found interesting about this whole theory. IMO it
> should be rather unusual (to put it mildly) for a planet to undergo the
> kinds of shifts the authors are talking about without completely wiping out
> all life.
I disagree. For bacteria and yeast, they freeze quite well. So you
simply can't remain frozen *too* long or have massive radiation exposures
while frozen. Archea on the other and can seek out the environmental
extremes and live off unconventional energy sources, e.g. deep sea vents.
Only if you vaporize the surface of the planet down to the bottom of
the oceans and heat the first several km of rock (where bacteria may
be living in buried organic material) do you wipe out all life. So the
questions with all extinction events, are, "How severe is it?", and
"How much does it roll back the development of complexity?".
> You need a combination of surface gravity, water abundance, solar
> energy levels and geography that falls within very narrow limits to get the
> kind of behavior they describe. Disturb any one parameter by very much and
> you either can't get large climate shifts, can't recover from one, or don't
> have any enclaves where complex life can survive from one warm era to the
> next.
Aha, but they (in Rare Earth) are careful to say that they are considering
complex life primarily *as we know it*. For example, I question their
assumption that you need oxygen. Yes, all complex life on earth requires
oxygen. However microbiology shows what you really need is free energy
sources. As I've mentioned in other discussions, I could see adapting
humans to be solar powered. We might have a slower life pace, but we
would still be intelligent. Whether complexity has to wait for the
accumulation of oxygen is unclear. The vector for the development of
intelligent plants could have gone much further much sooner. In our
case, the Snowball events certainly would have reset evolutionary
progress back to a low level.
> Which, of course, would have interesting implications for the expected
> abundance of sentient life in the universe.
Agreed.
Robert
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