[tlg]

"You are descended from almost every single human being alive 1,000 years ago."

If a generation is about 30 years then you will have close to 33 ancestors lines in your family tree over those 1000 years. That makes 2^33 ancestors or 10 billion, that's impressive (well there is probably a large overlap)

[lutusp]

Yes, but remember the "bottleneck" from 70,000 years ago, when the human race was nearly wiped out. At that time, because of a nuclear winter brought on by a volcanic supereruption, there may have been fewer than 10,000 humans in the entire world.

http://en.wikipedia.org/wiki/Toba_catastrophe_theory

So, based on that, we're descended from fewer people in the past than most people realize. Another way to say it is that most of our ancestors were cousins.

[demallien]

That overlap is more than large, it's actually the most important factor in the calculation. You only need to go back about 6 generations (200 years), and there is practically no interchange between geographically separated groups, which means that I probably don't have any genes from some random asian living 1000 years ago (although we both almost certainly share genes that we inherited from some far more ancient common ancestor).