David Reich

When we get lucky in ancient DNA and sample them, they're quite isolated from each other, and they have reduced diversity in the last tens of generations.

The great majority of them go extinct.

The great majority of them are wiped out by encounters with natural disasters or other groups of humans or other animals.

And so what you have is a vast experiment with an archipelago of these groups.

And

what might be happening is that you just have a process of accumulation of cultural knowledge and loss of cultural knowledge.

But since there's many of these experiments going on, maybe something takes off somewhere and maybe that's what happens 50 to 100,000 years ago.

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.