From: Robert J. Bradbury (bradbury@aeiveos.com)
Date: Sat Mar 18 2000 - 13:36:09 MST
For those of you who didn't catch it, the January 2K issue of Scientific
American had an interesting article on "Snowball Earth":
http://www.scientificamerican.com/2000/0100issue/0100hoffman.html
The snowball earth events where the CO2 recycler got jammed by the
continents clustering around the equator (maybe), causing the earth to go
through freezings (-50C) followed by heat waves (+50C) lasting tens of
millions of years. During the ice periods all of the continents and oceans
would have been buried in several km of ice. There is no evaporation
from the oceans covered in ice and even if they weren't, the cold would
produce low evaporation rates. Volcanoes spewing out CO2, which accumulates
since it isn't washed out by the rain), eventually get the greenhouse running
again, but it way overshoots a comfortable temperature. So you get continual
rain and massive erosion as the glaciers melt dumping tons of nutrients
from the land into the seas. Until the continents move away from
the equator (where high mountain glaciers are reflecting back valuable
solar energy), the cycle repeats.
Now, speculative mode...
Standard measures of DNA mutation rates as well as geological
evidence, put the development of Eukaryotes ~2 billion years ago --
about 1400 million years before the higher life forms gat started
(~600 Myr ago). They speculate the stress of repeated Snowball events
created many isolated pockets of deep sea vent organisms. When conditions
got mild enough they could get access to sunlight & nutrients,
this would allow evolution of new life forms before they got
knocked off by the next freezout. The mixing of genes from
widely separated pockets (as they spread out repopulating
the unfrozen oceans) would potentially allow for rapid evolution
of new life forms.
Plus ca change, plus c'est le difference.
Since, there was almost 3 times the amount from the start of eukaryotes
to the Neoproterozoic glaciation events as from those events till now.
That raises the possibility, that complex life could have arisen 2-3 times
and have been completely erased by freeze/glacier/thaw/rain/flood cycles.
This kind of activity is going to do a pretty good job of removing any large
scale fossils we might look for.
Though there isn't much geological evidence for this, but that may be
because there isn't much crust around that is 0.7-2 billion years old
or because geologists don't expect to find complex life in those eras
and they haven't gone looking for it. So there may be a discoveries
waiting to happen for the clever person that figures out where to look.
Alternatively, and this is perhaps more interesting, if stresses like
these are what it takes to get complex life, then the fire & ice
planets that have highly eliptical orbits, might evolve complex, and
even intelligent life, much quicker than did the Earth. That would
imply that the "Rare Earth" arguments are pretty irrelevant. (But
since the authors aren't biologists they may not discuss it [I've
only gotten through the first couple of chapters so far].)
I would think the strategies of hibernation, low-temperature
antifreeze proteins, etc. combined with DNA preservation strategies
found in Deinococcus radiodurans would allow the evolution of species
that can tolerate the highly eliptical orbits. So complex life
might be quite abundant. Those that do get a leg up in such
environments are going to be hyper-evolving species compared
with the rather pokey rates found on Earth.
So we have, yet another area not much thought has been devoted
to. In addition to my favorite, "What are the speed limits on
self-replication?", we now have to add "What are the speed limits
on evolution?".
Robert
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