David Reich

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.

And so what seems to be happening is that the whole continent of Sub-Saharan Africa, and probably Eurasia at this time, is full of hundreds, thousands, tens of thousands of little groups that are communicating hardly at all with each other, are in very small sizes, are losing diversity.

And when we sample them, this is a group that leaves hardly any descendants at all, maybe none, amongst modern people.

And what's actually happening is occasionally these groups merge together and recharge their diversity.

So the diversity is maintained in the ensemble of rarely mixing groups.

And you can't really appreciate the diversity by studying any one group, but rather you actually have to think about the whole ensemble of hundreds or thousands of tens of thousands of them as preserving the diversity.

So there's some question about the migration rate amongst these groups, which are archipelago of little groups losing diversity, going extinct at some level.

But together, there is enough recontact to recharge the diversity and then create the incredibly diverse populations you see today, for example, in Southern Africa or Western Africa or Central Africa.

So that's super interesting question.

And I think there's a lot of insight and ideas about this topic.

And I think it's an area to which genetics right now has contributed almost nothing.

So I think, you know, in the book that I have, this book that I wrote

who we are and how we got here, ancient DNA and the new science of the human past.

And it's a bit of a misleading title or a kind of bait and switch title.