David Reich: evaluation

Transcript context

…That's really interesting. Going back to archaic humans, we talked a lot about Neanderthals but obviously there were two different species of Denisovans. I don't know if species is the right word, but there were two different kinds of Denisovans and also the hobbits in Asia. I don't know if there are more, but we're talking about half a dozen different distinct groups and only one survives. I understand if new cultural technologies are developed by this Near East early tribe, then they expand out through Eurasia. I get that might enable them to be so dominant. What I don't understand is how none of the other ones survived, not even one tribe of Denisovans or Neanderthals or hobbits. There was no niche in which they could just fend off. Everywhere this one tribe of African humans just dominated. How did none of them survive? I don't know. 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 4000 years ago. At some point, each of these places is encountered by the spread of modern humans at high densities. The other thing is that 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? That might sound like a silly kind of philosophical statement, but genealogically… I don't know if this happened before or after my book. You probably don't know about this. There was a super interesting series of papers. They made many things clear but one of them was that actually the proportion of non-Africans ancestors who are Neanderthals is not 2%. That’s the proportion of their DNA in our genomes today if you're a non-African person. It's more like 10-20% of your ancestors are Neanderthals. What actually happened was that when Neanderthals and modern humans met and mixed, the Neanderthal DNA was not as biologically fit. The reason was that Neanderthals had lived in small populations for about half a million years since separating from modern humans—who had lived in larger populations—and had accumulated a large number, thousands of slightly bad mutations. In the mixed populations, there was selection to remove the Neanderthal ancestry. That would have happened very, very rapidly after the mixture process. There's now overwhelming evidence that that must have happened. If you actually count 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 10-20%, which is a lot. 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. It stays local. But it just becomes, over time, more and more modern human. Eventually it gets taken over from the inside by modern human ancestry. This is what happens to northern European hunter-gatherers. They become farmer over time, but they are intact on the male line. Culturally they stay on the male line intact. I'm not trying to be politically correct, I'm just saying that you can actually have scenarios where this happens, for example in elephants. he male line. Culturally they stay on the male line intact. I'm not trying to be politically correct, I'm just saying that you can actually have scenarios where this happens, for example in elephants. If you look at forest elephants, which are the smaller of the two species of elephants in Africa, they're very matrilocal. They have these female lines that are very intact over a long period of time. If you look at the savanna elephants, which are the bigger elephants in eastern and southern Africa, they have savanna elephant DNA overall. But their mitochondrial sequences are forest elephant, which are the smaller West African elephants. The interpretation of this is that you just have waves and waves of dominant male bulls from the savanna coming into populations and eventually just replacing all of the genome in waves and waves of an intact forest population. So all that's left is the mitochondrial sequence, which is passed in the maternal line. It’s not even obvious that non-Africans today are modern humans. Maybe they're Neanderthals who became modernized by waves and waves of admixture.…

Stored transcript either side of the excerpt. The highlighted words are the published quote; the surrounding text is unedited source, never generated.