David Reich

In people who all have the capacity to do these things,

So the bottleneck occurred, the bottleneck, by bottleneck we mean founder event, a relatively small number of people giving rise to a large number of descendants today.

It occurred well before the mixture with Neanderthals, which is probably somewhere like 50,000 years ago, plus or minus 5,000 years or something like this.

So we don't know where it occurred.

Maybe it occurred somewhere in Arabia.

Maybe it occurred somewhere in the Nile Valley.

Maybe it occurred somewhere else.

But maybe thousands or even tens of thousands of years before the encounter with Neanderthal that pushed in some Neanderthal DNA into modern humans.

So one way to see this is, in fact, this was not an unusual thing, that this was not an unusual thing to have a group with low diversity.

In fact, the great majority of African groups would have had very low diversity, and it's just the one that started expanding into Eurasia also had low diversity, but it was so successful it didn't mix with very many other groups and recharge its diversity by remixing with other groups.

So, and maybe it also expanded inside of Africa.

So there's lots of reasons to think that the expansion of the early modern human group outside of Africa would have been accompanied by a within Africa expansion of the same group and that it would not have been unidirectional.

So one way to look at the expansion of modern humans into different parts of Eurasia where we have data is almost as a kind of sort of forest fire of some kind where it throws sparks into different parts of Eurasia.

Right.

And it interacts with the local people.

So, for example, if you look at the first modern humans of African and Near Eastern origin who get to, for example, Europe, where we have the best data, we have a number in Western Siberia where we have the best data so far.

We have a number of these very early ones from about 45,000 to 40,000 years ago, which are called initial Upper Paleolithic humans.

And a good fraction of them have had Neanderthal ancestors in their last two to four to eight generations.

So that's a kind of crazy result.

So we have only a couple of dozen or a dozen or so of these very early humans and a very large fraction of them recently mixed with Neanderthals in their ancestry.