Robert J. Bradbury wrote:
> 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.
I can see I wasn't being clear. What I meant by "complex" was simply
organisms sufficiently advanced to have features like DNA, ribosomes,
mitochondria, cell walls, etc. Primitive life that hasn't solved these
basic problems of biochemistry yet is in a poor position to evolve defenses
against serious environmental hazards. Of course, as you suggested, these
featues seem to evolve in only a few hundred million years, so the window of
vulnerability isn't all that big in terms of geological time.
However, there are a lot of types of worlds that can't meet this initial
condition. If there is never a period in a planet's history where the
climate is hospitable to primitive, unspecialized life, then you're stuck
with a lifeless rock. For familiar forms of life that means you have to
have stable bodies of liquid water, which is a pretty stiff limitation as
planetary environments go. There may be other biochemistries which can
operate under different conditions, but they seem to be few and far between
(or at least, we haven't thought of very many possibilities, and most of the
planets we can see appear lifeless).
Thus, a lot of the ideas about extreme-environment life found in science
fiction seem problematic to me, because life could not have gotten started
in the first place.
> 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.
Interesting observation. I think you may be on to something here. OTOH, it
is pretty much the opposite of what the "Rare Earth" theory hypothesizes -
that you don't get complex life at all without repeated extinctions.
Ah, if only we could run experiments...
> > 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.
Well, I'm not talking about the typical naive scenario of "everything
freezes for a thousand years, and when it eventually thaws there is nothing
left alive".
What I'm thinking is that most planets would not produce repeated
extinction/repopulation cycles. Either there are no big really big climate
changes in the first place, or the first one that happens will push the
planet into a new equilibrium state in which liquid water can't exist.
Either way, you don't get the hypothesized evolutionary kick-start.
For example, what happens if you just move Earth out from the sun a bit?
There is a band at the outer edge of the liquid water zone where you need
both greenhouse gases and a reasonable albedo to keep the temperature high
enough. If a world in that band has a big freeze it can't ever recover -
the CO2 alone can't raise the temperature enough to start a warm spell. You
may or may not actually render life extinct, but you will never get a
repopulation cycle. Therefore, this theory would predict that multicellular
life would not arise on such a world before its sun reached the end of its
life cycle.
> > 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.
>
> 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, but that doesn't change the important point. We could be talking about
critters made of liquid helium living on Pluto, and the same principle would
apply. Repeated environmental perturbations of this size are a very
unlikely event, regardless of what the baseline environment is.
Billy Brown
bbrown@transcient.com
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