well, as I said, pa__ul, I don't think the 6000-year-old theory is widely accepted these days. Yes, I know creationists deny evolution; but I don't think they're particularly widespread, except perhaps in some parts of the southern USA and Africa, and not influential. (They may run for the US presidency; I don't believe they'll ever win it.)
Looking into it a bit more (and may God forgive me for linking to the Mail), it seems there may also have been later migrations out of Africa, which might have refreshed the gene pool
http://www.dailymail....bounding-rapidly.html
but even that story suggests that at one stage humanity outside Africa was reduced to only 1200 people - again, quite a small number.
Then again, apparently 1000 years ago there was just one breeding pair of cheetahs in the world, but they're still hanging on. I think the suggestion is that a very small gene pool doesn't necessarily make you all mentally deficient from inbreeding, but it does make you susceptible to mass infection: one illness could wipe out all cheetahs.