David Reich

I don't know.

I think it may be a numerical issue.

If you look at the part of the world where we have the best data in the Holocene, the last 10,000 years, there are places of long-term survival of hunter-gatherers for a few thousand more years than elsewhere.

In the Netherlands, for example, hunter-gatherers survive for several thousand more years than in the surrounding areas, probably because they're exploiting the wetlands, but they're gone soon enough once something happens.

Mammoths go extinct mostly.

14,000 years ago, but they survive on Wrangel Island north of Siberia until 4,000 years ago.

At some point, each of these places is encountered by the spread of modern humans at high densities.

The other thing is it's not even clear to me what expansion means.

If you want to make a strong argument, you might argue that non-Africans today are Neanderthals who just have waves and waves of modern humans from Africa mixing with them.

Who are the ancestors?

So that might sound like a silly kind of philosophical statement, but like genealogically, I don't know if this happened before or after my books, you probably don't know about this, but there was a super interesting series of papers that came out which made it clear, many things became clear, but one of them was that actually the proportion of non-Africans ancestors who are Neanderthals is not 2%, which is the proportion of their DNA in our genomes today if you're a non-African person.

It's more like 10 or 20% of your ancestors are Neanderthals.

And what actually happened was when Neanderthals and modern humans met and mixed, the Neanderthal DNA was not as biologically fit.

And the reason was that Neanderthals had lived in small populations for about half a million years since separating from modern humans, which had lived in larger populations.

and had accumulated a large number, thousands of slightly bad mutations, such that in the mixed populations, there was selection to remove the Neanderthal ancestry.

And that would have happened very, very rapidly after the mixture process.

And there's now overwhelming evidence that that must have happened.

And so if you actually look at the ancestors, if you count of your ancestors, if you're of non-African descent,

how many of them were Neanderthals, say, 70,000 years ago, it's not gonna be 2%, it's gonna be 10 or 20%, which is a lot.

And maybe the right way to think about this is that you have a population in the Near East, for example, that is just encountering waves and waves of modern humans mixing.