David Reich

That's two populations 70,000 years separated.

There's no biological incompatibilities between West Africans and Europeans.

There's no natural selection against biological incompatibilities.

So we know when Neanderthals and modern humans met and mixed,

There were biological incompatibilities.

That was at 700,000 years ago.

And so as populations become more apart, there begins to be biological incompatibilities rapidly developing, probably as the square of the distance separation, because you need pairs of interacting genes, and therefore it's the square of the separation.

So here, it would have been maybe only 400,000 years separated between this lineage and this lineage.

Interesting.

Here, it's like 1.2 million years.

It's a lot.

So these are at the edge of not being able to produce children.

So this is quite different humans.

These are actually three times closer than these.

And like, if you look at mixtures of humans today, there are mixtures in Southern Africa today are people who are half this distance.

If you look at Khoisan and Bantu people mixing in Southern Africa, like the Xhosa, which is the population of, for example, Nelson Mandela, this is groups that are separated by almost 200,000 years, which is half of this.

Totally compatible.

And so...

Like what you're seeing is this is a group that's actually completely permeable genetically or nearly completely permeable.

This one almost certainly has substantial biological incompatibilities because 300,000 years later, two or 300,000 years later, we see the interbreeding between Neanderthals and modern humans or between Denisovans and modern humans, there's clear evidence of incompatibility at that point.