David Reich: 70,000 years ago, there are half a dozen different human species all around the world that are pretty different. And then this group that's descended from the people in subsaharan Africa just explodes all across the world. So, something seems like it changed. What do you reckon it was? I'm very sympathetic to the idea that it's hardly genetic.

David Reich: So I think that this is cultural Innovation. It's very natural to think that this is cultural Innovation. The whole continent of subsaharan 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. In every group of human beings of hundreds of people, which is the size of a band or sometimes a thousand of people, they accumulate shared cultural knowledge, shared knowledge to knowledge about tools, knowledge about Life Strategies. And they build up a shared knowledge more and more and more and more. But if you have a limited size group of people that's not interacting with other sufficiently other large group of people, either occasionally this group has an information loss, you know, there's a natural disaster, key elders in the group die and knowledge gets lost and there's not a critical mass of shared knowledge. But once it goes above some kind of critical mass, the group can get larger. The the the amount of shared knowledge becomes greater and then you have a runaway process where uh where where an increasing body of shared knowledge of how to make particular tools, how to innovate, uh patterns of innovation, and so on, language, conceptual ideas, uh run a mock.

David Reich: So what you actually have in the world is 50,000 years ago is tens or hundreds or thousands or tens of thousands of different human groups, each of them possessing local knowledge, rarely exchanging with each other. When we get lucky in ancient DNA and sample them, they're quite isolated from each other and they have reduced diversity. In the last tens of generations, the great majority of them go extinct. The great majority of them are wiped out by encounters with natural disasters, or other groups of humans, or other animals. And so what you have is a vast experiment with an archipelago of these groups. And what might be happening is that you just have an a process of accumulation of cultural knowledge and loss of cultural knowledge. But since there's many of these experiments going on, maybe something takes off somewhere. And maybe that's what happened 50 to 100 thousand years ago. And people who all have the capacity to do these things. So I'm attracted to that idea.

David Reich: Without being an expert on this topic, I I understand if like there new cultural Technologies developed by this near East early group tribe that like then they expand out through Eurasia and I get like that might enable them to be so um dominant. What I don't understand is how is it that none of the other ones survived? Like not even one Tribe of denisovans, uh or like one group of Neanderthals, one group of Hobbits like there's no, there was no niche in which they could just like fend off everywhere. Uh This One Tribe of humans, um, One Tribe of African humans just dominated.

David Reich: I don't know. I mean, I think it may be a numerical issue. I mean, if you look at the part of the world where we have the best data in the holis, 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 gathers 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 happ, at some point, each of these places is encountered by the spread of modern humans, high densities.

David Reich: The other thing is, it's not even clear to me what expansion means. So, you know, if you want to make a strong arguments, you might argue that non-Africans today are Neanderthals who just have waves and waves of modern humans from Africa mixing with them. Like who's who are the ancestors? So that might sound like a silly kind of philosophical statement.

David Reich: But like genealogically, I don't know if this happened before, after my books, you 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 um in our in our genomes today. If you're a non-African person, it's more like 10 or 20% of your ancestors were Neanderthals.

David Reich: And what actually happened was with when Neanderthals and modern humans met and mixed, the Neanderthal DNA be 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.

David Reich: 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 going to be 2%. It's going to be 10 or 20%. Um, 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. There's so many of them that over time, it stays Neanderthal, stays local, but it just becomes over time more and more modern human and eventually gets taken over from the inside by modern human ancestry.

David Reich: It's not even obvious that non-Africans today are modern are are modern are modern humans. They're just maybe they're Neanderthals who became modernized by waves and waves of admixture.