David Reich: uncertainty

Transcript context

…One thing I didn't realize until I read your book is how small the population that expanded out into Eurasia was, and how small even generally the human population was 50,000 to 100,000 years ago. I remember one of the papers you cited said that there might have been a population bottleneck around this time period. People talk about the Toba eruption. I don't know if that's the cause, but there's many potential causes. Anyways, I remember from somewhere that the ancestors of everybody in Eurasia was initially like 1000 to 10,000 people. How small was the human population that was the seed of this modern period? By bottleneck we mean founder event, a relatively small number of people giving rise to a large number of descendants today. The bottleneck occurred well before the mixture with Neanderthals, which is probably somewhere like 50,000 years ago, plus or minus 5000 years or something. 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 it occurred maybe thousands, or even tens of thousands, of years before the encounter with Neanderthals that pushed some Neanderthal DNA into modern humans. One way to see this is that in fact, this was not an unusual thing. This was not unusual to have a group with low diversity. The great majority of African groups would have had very low diversity. 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. Maybe it also expanded inside of Africa. There are 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. 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. It throws sparks into different parts of Eurasia and interacts with the local people. Look at the first modern humans of African and Near Eastern origin who get to Europe, where we have the best data. We have a number in western Siberia, where we have the best data so far, of these very early ones from about 45,000 to 40,000 years ago which are called Initial Upper Paleolithic. A good fraction of them had Neanderthal ancestors in their last 2-8 generations. That's a kind of crazy result. We have only a couple of dozen or so of these very early humans. A very large fraction of them recently mixed with Neanderthals in their ancestry. So a model that might explain the data is that you have sparks coming out of a kind of forest fire of humans expanding in the Middle East or Near East. They come in and they start going to places like western Siberia or parts of South Asia or parts of Europe. They mix with the Neanderthals. They produce these mixed populations, like these initial Upper Paleolithic groups we sample in the record, and they all go extinct including the modern human ones. There's just extinction after extinction of the Neanderthal groups, the Denisovan groups, and the modern human groups. But the last one standing is one of the modern human groups. That's what we happen to see, the interbreeding event that we see. The great majority of the ancestors of modern humans, for example in Eurasia, are not from the initial Upper Paleolithic ones. roups. That's what we happen to see, the interbreeding event that we see. The great majority of the ancestors of modern humans, for example in Eurasia, are not from the initial Upper Paleolithic ones. They’re from a later wave from the core in the Near East after 39,000 years ago, that repeoples a place that's been affected by these sparks coming out of the same region. Those groups too disappear.…

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