David Reich

I was part of a group of scientists who had established that non-Africans were a simple subset of African variation and that there was no evidence at all of Neanderthal interbreeding into the ancestors of modern humans or other archaic interbreeding.

different analyses that I and very much more other people had done made it look like non-African variation was just a subset, a small sample of that in Africa, and that could fully explain the data.

And so that when I was involved in analyzing the Neanderthal DNA sequences,

What happened was I found this very strong evidence of Neanderthals being more closely related to non-Africans than to Africans, and it was very surprising, and I thought it must be a mistake.

I was quite incredulous.

I thought it was unlikely to be true because other evidence that had

that had been found before seemed to point in the other direction.

And so I spent several years trying to make these results go away, as did my colleagues, and we just couldn't make the results go away.

They just kept getting stronger.

And this experience working on natural selection was the same.

So what we had felt here was that what we were convinced of was that natural selection had been pretty quiescent in our species over the last several hundred thousand years.

Therefore, if we look at patterns of variation in non-African people today, or in any people today, we should see not a lot of selection going on.

And indeed, the first ancient DNA studies, beginning in 2015 with this paper that we were involved in with Ian Matheson and colleagues,

Indeed, these papers seem to show relatively small numbers of genetic positions associated with natural selection.

So in 2015, we analyzed data from about 200 Europeans and Middle Easterners to try to understand frequency changes over time.

And we compared those ancient people who were the sources of modern Europeans to people in Europe today.

And we looked at frequency differences that were too extreme to be due to chance.

And we were very excited to find 12 positions that we were convinced were highly different in frequency between Europeans today and what we would expect based on the history that

that we and others had identified was the history relating modern to ancient Europeans.

And so some of these were known and some of these were not known, and this was very exciting.