David Reich

But the ecology with deserts and so on makes certain areas permeable or impermeable.

And so in some periods of time, the Near East gets reclaimed by Eurasia somehow ecologically.

And in other periods of time, it gets reclaimed by Africa.

So it's kind of a place of movement of flora and fauna out and in again and again and again and again.

So I think the simplest model—I'm not an expert on this—but the simplest model would be one in which an extension of the modern human substructure leading to us, the ones that some of those lineages coalesced to form people living today, the great majority of the ancestors.

gets into the Near East several hundred thousand years ago, and then mixes there with the ancestors of what we have now sequenced as Neanderthals, and the skeletons that we have now are Neanderthals, and that that gene flow event occurs there, and it's modern humans from Africa or the part of the African population that extends into the Near East pushing into Neanderthals at that time.

We have evidence of modern human incursions since that time into Neanderthal parts of Western Eurasia, also in intermediate periods from the skeletal record, and maybe even claims recently in the DNA data.

But certainly the genetic data attests to a very strong event a few hundred thousand years ago.

I don't know.

So one of the things that is really interesting, we just published a couple of years ago a paper on relatively recent hunter-gatherer populations from mostly eastern and central Africa.

And this included individuals going back up to about 15,000 years ago, which is the oldest DNA from sub-Saharan Africa, which is not very old at some level in order to be able to discern these deep populations.

population exchanges that really we would like to know in order to understand human evolution, which really we would like to be able to probe 2 million years ago.

We can't.

But with 15,000 year old individuals, what you see is many, many groups at many, many places, all with very reduced diversity.

So in other words, they look like they're living in tiny populations of hundreds of people and not exchanging DNA with each other very often at all over time.

And this is again and again, we see this again and again.

And so if you take such a population, put it into a model and say, it's this small, what will happen over time?

It will lose its diversity over time and it will become very non-diverse.

So over time, Africa will have very little diversity.

But of course, Africa today has great human diversity in it.