Re: Question: Population Bottleneck and "out of Africa"

On Oct 2, 12:08 pm, nos...@xxxxxxxxxx (Paul Ciszek) wrote:
The way the "out of Africa" theory has been presented to me, the flow
of genes is supposed to have always been one-way: Homo Erectus leaves
Africa and learns to survive in a variety of different places, but we're
descended from any of *those* Homo Erecti. Later, archaic Homo Sapiens
migrates out of Africa, but we're not descended from any of *those*
archaic Homo Sapiens, etc. This never seemed right to me; if we were
capable of travelling all over the world before we were even human, then
why aren't we descended from some of those travelling pre-humans? If
anything, natural selection should *favor* geographic mobility. Then
I read about the Toba catastrophy hypothesis in Wikipedia.

Quoting from the wiki entry: "genetic evidence suggests that all humans
alive today, despite their apparent variety, are descended from a very
small population, perhaps between 1,000 and 10,000 breeding pairs."
Let's leave aside for now the discussion of whether this is what
actually happened, and speculate that it did. If the number was toward
the low end, I assume that these 1,000 or so breeding pairs had to be
close enough to each other geographically that their families could meet
up in time to save each other from the effects of inbreeding. Now, my
question: Would a bottleneck event like that erase all genetic evidence
of previous geographical distribution, and produce the "Out of Africa"
effect? Of course all modern humans are descended from people who
migrated out of Africa 70,000 years ago--that's all that was left! If
cosmopolitan, globe-trotting pre-humans had, in previous millenia,
occasionaly returned to woo the naive, provincial gals back home, would
there be any evidence for or against this in the post-bottleneck genome?

Of course, this hypothesis could be supported or debunked by *other*
sorts of evidence, like distribution of tool innovations, but that is
more a topic for sci.anthro.paleo.

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We can only go with the data. If populations outside of Africa
contributed to the gene pool they didn't contribute very much.

Most of the genetic variation in the human population is still in
Africa, but even that shows that there was a bottleneck where the
effective population size of humanity was pretty small. I've seen
estimates of on the order of 1,000. Several sub populations
apparently made it out of Africa after this bottle neck and show the
expected reduction in genetic variation of taking a part of a larger
population and moving it somewhere else. The populations outside of
Africa do not show detectable mixing from populations of Homo that may
have been in the areas that they were moving into. They mainly show a
part of the variation that exists in Africa.

If there was extensive mixing we would have to explain why the
populations outside of Africa do not show evidence of it. We would
expect the previous populations of Homo to have genetic variation
different from that left in Africa after the bottleneck there because
these populations branched off before the bottleneck. If you look at
humans we have about 1/5 the genetic variation as other species like
chimps or deer. This is the evidence for a bottleneck. Humans lost
quite a lot of their genetic diversity. Mixing with other populations
outside of Africa should have increased the diversity of those
populations to include a lot of genetic diversity not found in Africa
instead of decreasing it to a subset of what was left in Africa.

Ron Okimoto


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