David Reich — How one small tribe conquered the world 70,000 years ago

**Dwarkesh Patel** (0:00)
Today, I have the pleasure of speaking with David Reich, who is a geneticist of ancient DNA at Harvard. And David's work and his lab's work and his field's work has transformed, like really transformed our understanding of human history and human evolution. I mean, it's really fascinating stuff from many perspectives. In its own light, it's very interesting. From the perspective of AI, which I plan on asking you about, it's interesting to understand human evolution and what that implies about what the future of AI might look like. Anyways, I'll stop doing the introduction. David, we were just chatting before we started recording about what new information you've been studying since the book came out about archaic humans and the relationship between modern humans and Neanderthals. Can you explain again what you're studying these days?

**David Reich** (0:49)
Well, I think what's very interesting is that what we have data from now are modern humans, the sequences of people living today. We also have data from Neanderthals who are archaic humans who lived in Western Eurasia for the last couple of hundred thousand years. We have now sequences from many Neanderthals. We also have DNA from Denisovans. Denisovans are archaic humans who were discovered from the DNA, from a finger bone that was found in a cave in Siberia, not anticipated to be a new group of humans, but were sequenced. We have DNA from these different sources plus bits of DNA from these sources mixed into modern populations. Based on this, in the last 10 years or 14 years, we collectively have been piecing together an understanding of how modern humans are related to our closest relatives who are now no longer with us in unmixed form, the Neanderthals, Denisovans, and maybe others who are no longer not yet sampled.
The model that we have is really a model based on accretion. We start with the modern humans and then we add the Neanderthals once we obtain that sequence and we add the Denisovans. Then the model doesn't quite fit and we add other mixtler events to make the model fit. At this point, there's a number of these mixtler events that seem increasingly implausible. They feel to me a little bit like, I don't know if you know the history of models of how the Earth and the Sun relate to each other in ancient Greek times, but there's these epicycles that were attached by the Greek Hellenistic astronomer, Ptolemy, to make it still possible to describe the movements of the planets and the stars, given that a model where the Sun revolved around the Earth. We've added all of these epicycles to make things fit. One wonders whether there's some pretty fundamental differences that might explain the patterns that are observed. Just to give you an example of this, that standard model is basically this, that modern humans separated from a group that is ancestral to Denisovans and Neanderthals, these two groups for which we have sequences, somewhere between maybe 500,000 to 750,000 years ago. That's what the genetic papers beginning in about 2012 and 2014 said, and that's still used as the explanation for the vast majority of the genealogies, the DNA lineages connecting them. So, maybe except for 5% of the DNA, that's what we think is going on. Modern humans are one group, and then there's a sister of modern humans, the Denisovans and Neanderthal group, and they separated 500,000 to 750,000 years ago. But what's become very, very clear in a really important series of papers since that time is that, in fact, there are exceptions to this. And one exception to this is the mitochondrial sequence, what you get from your mother and she gets from her mother and so on, going back in time. And there, the shared ancestor between Neanderthals and modern humans is only maybe 300,000 or 400,000 years ago, which is after the split that's very well estimated from the whole genome. And what we've also learned is that's also true for the Y chromosome, so that's inherited from your father and his father and so on. And that's true, it too is only maybe 300,000 or 400,000 years separated between Neanderthals and modern humans. And like the mitochondrial DNA, the Denisvins are much more distant, maybe 800,000 years, 700,000 years, a million years. So the story told by these two parts of the genome is one that's really, really different from the rest of the genome and incompatible with the main story, too recent sharing. And we know in these papers that maybe a few percent, 5%, 3%, 8% of the DNA of Neanderthals comes from a gene flow event, a migration event into the ancestors of Neanderthals from the modern human lineage a few hundred thousand years ago.