Gene flow out of Africa is very restricted. The Sahara to the North kept most Africans on the mid to southern part of the continent. Then there's the Sinai, which is a very narrow land bridge linking Africa with the greater land mass of Eurasia.
One thing many people seem to be missing out on in terms of migration is the small numbers involved. Up until just a few thousand years ago, the total Human population numbered only a few tens of millions globally. Most of these people stayed in place with only some small groups actually moving outward. In the early days literally centuries or longer would pass before there was any contact between them and other groups. It is this geographic isolation and small gene pools which led to regional physical differences in various populations. In order for evolution to work its magic those two conditions must be met. Variations can not develop and become established in a population if the flow of genes is as fluid as you suggest.
Then there's the whole notion of blood types and Rh factors keeping successful intermixing to low levels. That probably goes along way in explaining the extremely high mortality rates for both mother and child in the past. It's a problem that was of major concern up until very recently with the advent of modern medicine.
Space/Science » in reply to whirlpools versus trees
You're missing the bottleneck
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