I'd like to raise the precision of this discussion a bit, focusing on what
seems to me the most likely candidate for the Great Filter in the past. In
the latest version of your paper, you mention that of the itemized steps,
the one that took the longest span of time (about two billion years, if
prokaryotes originated on Earth) was the jump from simple to complex
single-cell life. Am I safe in assuming that this means the same thing as
the jump from prokaryotic to eukaryotic life?
Given the Mars life evidence, which seems to be purely prokaryotic, we
could even conjecture that the universe is strewn with prokaryotic life,
and that the stuff has been scattering around by panspermia since shortly
after the Big Bang cooled.
Then the only prima facie evidence we seem to have indicates that the Great
Filter is probably the jump from prokaryotic to eukaryotic life. The only
evolutionary model of this jump that I know of is Lynn Margulis's
endosymbiotic theory.
For a non-authoritative background discussion, you might look at
http://dekalb.dc.peachnet.edu/~pgore/students/w96/joshbond/symb.htm
noting that Kwang Jeon's results were obtained with amoebas, organisms
which had *already* learned the trick of endosymbiosis. Spontaneous origin
of endosymbiosis among *prokaryotes* seems unlikely to me for the following
hunch-ridden reasons.
Consider that all prokaryotic life is going to have some sort of immune
system or digestive system (these specialized terms, devised for
multicellular eukaryotes, would both metaphorically apply to the single and
much simpler moneran system I'm talking about here). Consider that ordinary
selection pressure would push for the ruthless effectiveness of that
system. Consider that the ability to utilize "internal symbiotes" such as
mitochondria and the like could develop only in the event of what amounts
to a drastic *failure* of that system. It seems to me that by the very
nature of "immune systems", especially in very simple organisms such as
prokaryotes, the vast majority of such failures are going to be lethal
mutations.
The jump from prokaryotic to eukaryotic life, then, would be a mutation or
a sequence of mutations that resulted in a nonlethal total failure of the
prokaryotic immune system. This notion seems sufficiently paradoxical to me
that I'm content, for the time being, to identify the jump from prokaryote
to eukaryote as the Great Filter.
But then, perhaps I'm just being a dynamic optimist. ;) Counterexamples
to my notion might include deliberate suppression of the human immune
system to enable organ transplants. But this, of course, is a consequence
of memetic evolution. Within the pure context of genetic evolution, I
suspect that nonlethal total failures of the "immune system" among
organisms as simple as prokaryotes would be extremely rare.
The other possible counterargument, as Robin points out, is that the
mutation took only two billion years (on Earth) to happen. But if the
prokaryotic panspermia idea is right, and there are prokaryotes as thick as
mud throughout the universe, then we could say that the Great Filter
mutation took more like ten to fifteen billion years to take place, and
just happened to take place on Earth. From this, it would seem that
universally-panspermic prokaryotes (and the Mars life evidence) would
strengthen my argument identifying the Great Filter with the evolution of
eukaryotic endosymbiosis.
If there are any expert students of protists and monerans out there who are
really up on their phylogenetic studies, please correct any errors I've
made.
Eric Watt Forste <arkuat@pobox.com> http://www.c2.org/~arkuat/