"Forty generations ago, almost everyone living today had ancestors in Europe, Asia, and Africa, and many present-day Asians, Europeans, and Africans had ancestors in the Americas because of the continual exchange of mates across the Bering Strait...Say you go back 120 generations, to about the year 1000 B.C...your ancestors then included
everyone in the world...And if you compared a list of your ancestors with a list of anyone else's ancestors, the names on the two lists would be identical...
It means that you and I are descended from all of the Africans, Australians, Native Americans, and Europeans who were alive three millenniums ago and still have descendants living today."
- Steve Olson in
Slate (with internal links to
Nature, explanations of the mathematics behind it, etc).
I've got more to say on this, but this is enough blowing your minds for right now.