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

1. 0:00 – 21:13

   Archaic and modern humans gene flow

   1. DRDavid Reich0:00

      ... there's just extinction after extinction after extinction of the Neanderthal groups, of the Denisovan groups, and of the modern human groups. But the last one standing is one of the modern human groups. It's not even obvious that non-Africans today are modern humans. Maybe they're Neanderthals who became modernized by waves and wave of admixture. Farmers who were just on the verge of encountering people from the steppe, a huge fraction of them have Black Death. It's killing, like, a scarily large fraction of the population. A lot of people I know dropped off the paper. They just didn't want to be associated with it because it was so weird and they just thought it might be wrong. But it's stood up as far as I can tell.

   2. DPDwarkesh Patel0:33

      70,000 years ago, there are half a dozen different human species and then this group, initially, like, 1,000 to 10,000 people, explodes all across the world.

   3. DRDavid Reich0:42

      I think there's been an assumption where Africa has been at the center of everything. Models that are considered to be standard dogma are now low probability.

   4. DPDwarkesh Patel0:50

      Today, I have the pleasure of speaking with David Reich, who is a geneticist at, of har- ancient DNA at Harvard. And, um, 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. Uh, I mean, and it's really fascinating stuff from many perspectives. Uh, 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. Uh, can you, can you explain again what the, uh, what, what you're studying these days?

   5. DRDavid Reich1:39

      Well, I think what's very interesting is that what we have data from now are modern humans, the sequences of people living today, and we also have data from Neanderthals, who are archaic humans that lived in Western Eurasia for the last couple of 100,000 years, and we have now sequences from many Neanderthals. And we also have DNA from Denisovans. Denisovans are archaic humans who were discovered from the DNA, from a, a, a, a finger bone that was found in a cave in Siberia and not anticipated to be a new group of humans, but were sequenced. So we have DNA from these different sources, plus bits of DNA from these sources mixed into modern populations. And based on this, in the last 10 years or 14 years, we collectively have been piecing together a tr- an understanding of how, uh, 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, uh, not yet sampled.

   6. DPDwarkesh Patel2:35

      Mm.

   7. DRDavid Reich2:36

      And, uh, the model that we have is really a model based on accretion. So we start with the modern humans and then we add the Neanderthals once we obtain that sequence and we add the Denisovans. And then the model doesn't quite fit and we add, uh, other mixture events to make the model fit. And at this point, there's a number of these mixture events that seem increasingly implausible. They feel to me a little bit like... I don't know if you know the history of, uh, 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, uh, Greek Hellenistic astronomer Ptolemy to make it still possible for, to describe the movements of the planets and the stars given that, a model where the sun revolved around the Earth. And we've added all of these epicycles to make things fit, and one wonders whether there's some pretty fundamental differences that might explain the patterns that are observed. So just to give you an example of this, that standard model is basically this, that modern humans separated from the, a group that is ancestral to Denisovans and Neanderthals, these two groups for which we have sequences somewhere between maybe 500 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 Denisova Neanderthal group, and they separated 500 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 3 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 true, it too is only maybe 3 or 400,000 years separate tr- separated between Neanderthals and modern humans. And like the, the, the, uh, mitochondrial DNA, the Denisovans 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-

   8. DPDwarkesh Patel5:25

      Mm.

   9. DRDavid Reich5:25

      ... into the ancestors of Neanderthals from the modern human lineage a few hundred thousand years ago. And it's tempting to think that both the Neanderthal mitochondrial DNA and Y chromosome come from that event, but the probability of that happening by chance is only 5% squared, which is a very, very small number. And people have evoked epicycles, for example, natural selection for the mitochondrial DNA coming from modern humans or natural selection coming from the Y chromosome coming from modern humans somehow being more advantageous and pushed up in frequency. But that would have to really happen on both these parts of the genome to produce this pattern.

   10. DPDwarkesh Patel6:00

       Yeah.

   11. DRDavid Reich6:00

       And it just seems surprising. So the, what's been put together is a complicated model and epicycles, ideas like natural selection, to kind of make it work. It's not impossible, it may be the case, but one ima- one, one wonders whether profoundly different models might actually explain the data, and so that's something that we and others have been thinking about.... can there be other models? An example of another model that might be able to explain the data that we've been playing with is one where there's much more DNA in Neanderthals from modern humans than the 3% or 5% that's been estimated.

   12. DPDwarkesh Patel6:34

       Mm.

   13. DRDavid Reich6:34

       And we can get such models to fit, but here it's 30% or 50% or 70%. So in that view, Neanderthals and Denisovans are not sisters. In fact, modern humans and Neanderthals are just as qualified to be sisters as Neanderthals and Denisovans. And in that case, maybe it's not clear what's modern and what's archaic. Are modern humans archaic? Are modern humans-

   14. DPDwarkesh Patel6:57

       (laughs)

   15. DRDavid Reich6:57

       ... modern? Are Neanderthals archaic? Neanderthals are modern. What's also become clear in the last few years in a separate thread of research, not based on ancient DNA but based on using more and more powerful and sophisticated ways of pattern finding and modern data is that modern humans are also highly substructured. We can see that even without having ancient DNA yet.

   16. DPDwarkesh Patel7:16

       Hm.

   17. DRDavid Reich7:16

       Of course, once one has ancient DNA, it's so much clearer. But it's very clear that you can't explain, for example, modern African DNA without invoking very extreme substructure as deep as the mixtures that produced Neanderthal contributed in a mix between Neanderthals and modern humans. And so that mixture, which of those groups were archaic? Which of them were modern? Were they both archaic? Was one of them modern? Was one of those more closely related to Neanderthals, uh, and the possibly higher proportion of ancestry? It's not obviously wrong that the model is very, very different from the standard one that we currently have.

   18. DPDwarkesh Patel7:49

       Interesting. So, uh, I mean, uh, from your book, I remember that there are lineages of human, modern humans that are over 200,000 years separated from other groups like the San hunter-gatherers from, um, everybody in Eurasia today, or at l- everybody descended from Eurasia. Um, so the, um, then you're saying that 100,000 years before that is when we have a sister lineage with Neanderthals. Uh, a- actually, I'm not sure what, what the, the new findings, we're finding about how closely related Neanderthals are to us and how much, um, uh, mitochondrial and Y-chromosome DNA they share. What, what model do you think is the most plausible to explain why there's, uh, so much shared ancestry?

   19. DRDavid Reich8:30

       I'm very agnostic. I really, really don't know.

   20. DPDwarkesh Patel8:33

       But the ones you... uh, sorry, the ones you... the models you were just talking about, it seem- sounded like you thought they were low probability. Is there one you think is, like, more-

   21. DRDavid Reich8:39

       I think the models that are considered to be standard dog- dogma are now low, low probability.

   22. DPDwarkesh Patel8:43

       Yeah.

   23. DRDavid Reich8:44

       So there's a standard dogma that's, uh, that's developed over an accretion of papers where the data, the, the history gets patched. So someone sequences a genome, someone performs an analysis, someone proves something that wasn't known before, and so they, we claim a mixture event, we didn't know about it before, an event that we didn't know before, and that gets patched onto the current model-

   24. DPDwarkesh Patel9:05

       Mm-hmm.

   25. DRDavid Reich9:05

       ... which is now a series of patches.

   26. DPDwarkesh Patel9:06

       Interesting.

   27. DRDavid Reich9:07

       And nobody has really rethought the whole thing very hard. And the whole thing is not obviously very, very different. So you can actually reassemble the whole model in a new way without doing it from the ground up, the, the... or from the simple model up but in fact thinking about it again and seeing if it can be all related in new ways. And in fact, it might be actually quite different in the way that I just described.

   28. DPDwarkesh Patel9:31

       Where did the gene flow f- between, uh, the most recent gene flow between Neanderthals and humans? I guess not the most recent, because the most recent was 60,000 or whatever years ago, but, like, the one you're referring to here, where did, where physically did that happen?

   29. DRDavid Reich9:42

       Uh, even that's not clear, but probably such a thing would have occurred somewhere, uh, in, uh, the Near East or in Western Eurasia somehow. And it's not even clear where the modern human lineage at that time was residing. So probably the modern human lineage was, uh, l- leading to the great majority of the ancestors of people today-

   30. DPDwarkesh Patel10:04

       Yeah.
2. 21:13 – 40:49

   How early modern humans dominated the world

   1. DRDavid Reich21:13

      idea.

   2. DPDwarkesh Patel21:13

      That, that's fascinating. So, um, uh, people, quote unquote... By, by the way, it's so interesting that, like, it's, it's, it's hard to think of the correct terminology of, like, what do... when we say people, which kind of people are we talking about? But anyways, so, um, uh, the, uh, the ancestors of modern humans are, uh, uh, at least in a position to have gene flow with other archaic humans in the Near East, but they... at least it doesn't seem like they expanded out hundreds of thousands of years ago. And if you're right that they had the brain size... or at least, you know, like, a lot of the brain size had already been accumulated before this wo- with Neanderthals, then they should have been pretty smart hundreds of thousands of years ago, but they're not expanding out. And then something happens 60,000 years ago, and then, um, they, they... like, this group that's descended from the people in sub-Saharan Africa just explodes all across the world. So something seems like it changed. W- what do you reckon it was?

   3. DRDavid Reich22:08

      So this is outside my area of expertise. I'm being very much like a scientist right here. But, uh, I'm very sympathetic to the idea that it's hardly genetic.

   4. DPDwarkesh Patel22:16

      Mm-hmm.

   5. DRDavid Reich22:16

      So, I think that this is cultural innovation. It's very-

   6. DPDwarkesh Patel22:19

      Interesting.

   7. DRDavid Reich22:20

      ... natural to think that this is cultural innovation. And humans sometimes develop a new, uh, technique of storing information, sharing information, and so on. For example, writing, which allows you to record collective knowledge in a library, or computational knowledge, or large storage devices and so on and so forth. Language, uh, conceptual language, uh, which allow you to create a cultural body of knowledge.

   8. DPDwarkesh Patel22:48

      Yeah. You know, you talk in the book about how the FOXP2 gene, which modulates, uh, language ability in a- not only in humans but other animals. It... obviously all hu- living humans have it, um, and so it's at least 200,000 years old when the human lineage starts to split off. So, everybody has language, uh, so it can't have been l- like, wh- what do we think it was that-

   9. DRDavid Reich23:08

      Well, I don't know what we... what the language was.

   10. DPDwarkesh Patel23:10

       Mm-hmm.

   11. DRDavid Reich23:10

       I mean, it's almost certainly the case that Neanderthals were using lang- uh, sounds and, and, uh, communicating in ways that are probably pretty complicated, complex, and amount to some kind of language. Uh, but some people think that language in its modern form is not that old and might coincide with the, uh, later Stone Age, Upper Paleolithic revolution 50 to 100,000 years ago-

   12. DPDwarkesh Patel23:31

       Mm-hmm.

   13. DRDavid Reich23:32

       ... and might be specific to our lineage, and that there might be a qualitative shift in the type of language that's being used.

   14. DPDwarkesh Patel23:37

       Interesting.

   15. DRDavid Reich23:37

       There's been one incredibly interesting and weird, uh, line of genetic evidence that was so weird that a lot of people I know dropped off the paper. They just didn't want to be associated with it-

   16. DPDwarkesh Patel23:49

       Oh.

   17. DRDavid Reich23:49

       ... because it was so weird, and they just thought it might be wrong. But it's, it's stood up as far as I can tell. It's just so weird. So this is one of the things that surprises that genetics keeps delivering. I think that that's probably gonna come across in my conversa- in this conversation, which is, I am pretty humbled by the type of data that I'm involved in collecting. It's very surprising, this type of data. Again and again, it's not what we expect. And so, it just makes me think that things are going to be surprising the next time we look at something that's really not looked at before.

   18. DPDwarkesh Patel24:23

       Mm-hmm.

   19. DRDavid Reich24:23

       So the line of evidence I'm talking about is one based on epigenetic modification of genomes. So, just to explain what that means, uh, the genome is not just a sequence of letters, DNA letters, adenines, thymines, guanines, and cytosine, A, C, T, G. It also is decorated in anybody's cells by, uh, modifications that tell the genes when to be on and off in what conditions. So an example of such a modification is methylation, uh, in cytosine guanine pairs. So, this turns down a gene and makes it not functional in certain tissues. And this methylation is bestowed by cellular environments and differs in different cells and also in different species to identify which genes are more active or more passive, and it's not directly encoded by the A, C, T, Gs locally. It's encoded by something else and sometimes even passed on by your parent directly. So it's really very interesting. So this can be read off ancient genomes.... the methylation pattern survives in Denisovan and Neanderthal genomes, and we can actually learn which genes were turned down and turned up.

   20. DPDwarkesh Patel25:30

       Mm.

   21. DRDavid Reich25:30

       So work by David Gokhman and Liron Carmel and colleagues ident, created these maps of where in the Neanderthal genome, where in the Denisovan genome, and where in modern human genomes genes are turned on and off. And there's a lot of technical complexity to this problem, but they identified differentially methylated regions, several thousand parts of the, uh, sections of the genome that were consistently and very differently turned down or turned up in Neanderthals and modern humans. And when they looked at the set of differentially methylated region, roughly a thousand of them, that were systematically different on the modern human lineage and asked what characterized them, was there particular biological activities that was very unusual on the modern human-specific lineage, there was a huge statistical signal that was very, very surprising and very, very unexpected. And it was the vocal tract. So it was-

   22. DPDwarkesh Patel26:23

       Mm-hmm.

   23. DRDavid Reich26:24

       ... the laryngeal and pharyngeal tract. And because you can actually learn from little kids with congenital malformations when you knock out a gene, when a gene gets knocked out by a inborn error of, uh, genetic transformation, of genetic, uh, inheritance, you can... kids will have, for example, a, a sha, a face that looks different or a vocal tract that looks different and so on. You know what the effect of-

   24. DPDwarkesh Patel26:47

       Yeah.

   25. DRDavid Reich26:48

       ... knocking out these genes is. We can actually imply directionality to how the modern human-specific changes are. And the directionality is to change the shape of the vocal tract, which is soft tissue not preserved in the skeletal record, to be like the way ours is distinctive from chimpanzees.

   26. DPDwarkesh Patel27:05

       Mm.

   27. DRDavid Reich27:05

       So in the shape that we know is very helpful for the articulation of the range of sounds we use that chimpanzees-

   28. DPDwarkesh Patel27:12

       Yeah.

   29. DRDavid Reich27:12

       ... don't have in their laryngeal and pharyngeal tract. So even though we don't have surviving hard tissue like skeletons from this part of the body, we now have this methylation signature which suggests that these changes have occurred specifically on our lineage and are absent in both the Neanderthal and Denisovan lineages. So if you think this change in the vocal tract is important in language, which is, seems possibly reasonable, then maybe that's telling you that there's very important changes that have la, happened in the last half million or few hundred thousand years, specifically on our lineage that were absent in Neanderthals and Denisovans.

   30. DPDwarkesh Patel27:47

       But it, but what's, what's even, uh, what's significant is that the, um, that... so there have, humans have, to the extent that humans have had it for hundreds of thousands of years, it's not clear then why humans weren't able to expand out of Africa and-
3. 40:49 – 50:52

   How the bubonic plague rewrote history

   1. DPDwarkesh Patel40:49

      Yeah, okay. So le- let, let's jump forward to then, since you mentioned this, the, um, the way in which after the, uh, after agriculture was developed in the Middle East, eh, I don't know, 10,000, 12,000 years ago, and then after that, y- uh, the, um, the, the way the Native Americans, uh, the, the, the population of Native Americans declined was because of disease. And one of the hypotheses that you talk about in the book is potentially this happened with respect to people in Europe from, uh, th- uh, the Yamnaya with literally the, the, the bacteria that causes the bubonic plague, uh, Yersinia pestis. The question I'm trying to ask is the, going back a bit, so the, um, uh, James Scott, who I, uh, I think just died a couple weeks ago, in his book Against The Grain, the whole book is like, you know, agriculture sucked but we were forced to adopt it because it allowed some humans to organize nation-states. So they were very abusive, but did allow them to get the barbarians and co-opt them because they needed the labor to, uh, do this monotonous activity. And one of the things he talks about is b- well, um, one thing I didn't realize until I read that book is just how new, uh, all the diseases or most of the diseases that afflict humans today are. Eh, everything from cholera to typhus to tuberculosis. I mean, the, the, if you just go down the list. And because of agriculture, because of, um, domestication of animals, and because of the density that created. And so the, uh, the theory he says, uh, talks about in the book is that potentially the reason that hunter-gatherers, uh, the quote unquote barbarians couldn't fight back against these early nation-states was because they were getting killed off by the diseases. Um, and I don't know how much evidence there is for this. Basically I, uh, the qu- the question I'm trying to ask is the way in which Europeans encountered Native Americans in the New World, did that just happen again and again throughout history? Basically the way if you go back to Eur- Europe, uh, 9,000 years ago or 5,000 years ago, is that just what human history has been like? That wasn't a one-off event.

   2. DRDavid Reich42:50

      There's a, there's a, there's an amazing book by Kyle Harper called, I think it's called The Fate of Rome, and-

   3. DPDwarkesh Patel42:55

      Yeah.

   4. DRDavid Reich42:55

      ... it's an argument about the history as a, he's a historian, a Roman historian. And it's a history of, uh, three major plagues in the Roman period, a couple of whi- two of which are really not even very well known-

   5. DPDwarkesh Patel43:07

      Yeah.

   6. DRDavid Reich43:07

      ... and argues that, that the decline of the Roman Empire is due to just weakening of the, as the result of plagues and other climatic, biological, climatological worsening events. And there is a lot of reason to think that some of these events have been recurrent throughout history, and that it's not just a difference between farmers and hunter-gatherers, uh, but actually a lot of different types of interactions that are occurring. So the example that you mentioned is something that's been a big shock from the ancient DNA revolution.

   7. DPDwarkesh Patel43:40

      Mm-hmm.

   8. DRDavid Reich43:40

      So, uh, this is now maybe eight years, nine years old. So when the first large number of DNA sequences from people who lived five, and six, and 4,000 years ago from the, in the steppe north of the Black and Caspian Seas and in Europe were being published about in 2015, this group in Denmark...... led by Eske Willerslev and Kristian Kristiansson and colleagues. Sequence- looked at their DNA and they (laughs) discovered in their sequence, from the 100 or so humans they sequenced, that there was also pathogen DNA. And in five to 10% of the random people they sequenced, around four or five thousand years ago, there was Yersinia pestis, which is the agent of the Black Death. But actually without the pa, uh, without the plasmid that, uh, contributes to bubonic plague that's required for flea-rat transmission. So it must've been, for example, pneumonic plague with the aerosolized transmission or something. But five to 10% of, uh, random deaths means that actually the percent of people who were dying was- must've been even higher-

   9. DPDwarkesh Patel44:38

      Mm.

   10. DRDavid Reich44:38

       ... because they weren't detecting everything that was there. So a study by another group, uh, Johannes Krause and colleagues, of people in plague pits in London from the 1300s epidemic, found that they only... When you apply this method to people we know died of Black Death, you only find a quarter of the people, so the rate was even higher. And if people are bacteremic when they die, when they, if they have bacteria in their teeth, they probably or almost certainly died of that-

   11. DPDwarkesh Patel45:02

       Yeah.

   12. DRDavid Reich45:03

       ... agent. So a paper just came out a few weeks ago, um, in Scandinavia, uh, sh- uh, looking at these tombs from about 5,000 years ago of farmers who were just on the verge of count- encountering people from the steppe, and a huge fraction of them have Black Death when they die. They're buried-

   13. DPDwarkesh Patel45:19

       Wow.

   14. DRDavid Reich45:19

       ... in tombs of normal. Even higher than five or 10%. So this whole pedigree with many, many generations, so it's not all at the same time, just like the parents' generation, generations, uh, a very large fraction, like well more than 10%, have Black Death. And d- you know, have, have, have Yersinia infection. So it looks like this particular agent has been killing people for 5,000 years, 4,000 or 5,000 years, in Western Eurasia, and in fact is killing like a scarily large fraction of the population. Like as a quantitative person, which I am, reading this literature, I think people are embarrassed by the implication. The implication is that a third, a quarter, half of deaths (laughs) in this entire period are from this, this... And the people are just, it's so unbelievable, so ridiculous that such a high proportion of people over such a long period of people- time are dying from this one agent that people don't even say it. They just publish one paper after the other-

   15. DPDwarkesh Patel46:15

       (laughs)

   16. DRDavid Reich46:15

       ... publishing more sequences. And they just don't think about the implications of such a high rate of death. And yet, it's really hard to imagine that people have r- the bacteria in their blood and they're not dying of these things. It's not, doesn't seem that people are ignoring the hu- people are selectively picking tombs. These are tombs that are buried properly. They're not grave pits. So the implication seems to be this one agent that we happen to be able to detect is killing a very large fraction of people in Western Eurasia over this period. So what's the implication of that? One thing is that maybe it seems to b- be coming from steppe rodents probably, and so maybe the people on the steppe are somewhat more... I mean, (laughs) they're still dying of it, but they're somewhat more protected of it, then it spreads into farming Europe maybe 5,000 years ago, which is when we start to see it. And maybe this results in disorganization of the population given such high rate of death, and maybe it creates a type of situation that the Europeans encountered when they got to the Americas where societies were disrupted. So in, you know, in the last few years we had COVID-19 that killed .5% of the world population or something like that, and it was so disruptive. So if this thing is killing a third (laughs) of people or half-

   17. DPDwarkesh Patel47:23

       Yeah.

   18. DRDavid Reich47:23

       ... of people, you know, randomly, randomly killing people with cultural knowledge, randomly-

   19. DPDwarkesh Patel47:31

       Yeah.

   20. DRDavid Reich47:32

       ... ripping into structures like in, you know, I don't know, was it Montezuma died or one of the, or one of his parents, you know, resulting in civil wars in the Inca when the Europeans-

   21. DPDwarkesh Patel47:41

       Yeah.

   22. DRDavid Reich47:41

       ... encountered them, just disrupting the cultures that were there. Maybe this would've created a situation where there was disruption in the old ways of life, and maybe combined with other things or even just by itself could have created an opportunity for people to move in from elsewhere even though they were not as densely spread. Because the big observation we haven't talked about, and it's something that we as an ancient DNA community have been looking into again and again now f- and keep making progress on, is that about 5,000 to 4,500 years ago in Europe there's a radical transformation in the ancestry of Europeans. An example of this is what happens in Britain. So about 4,500 years ago the farmers who are there, they arrived there 6,000 years ago, they build Stonehenge 40, uh, the last big stones of Stonehenge go up 4,500 years ago, and then within a hundred years ag- uh, year, years, 90% of them are gone.

   23. DPDwarkesh Patel48:31

       Wow.

   24. DRDavid Reich48:31

       And they're replaced by migrants from the continent bearing pr- majority ancestry from the steppe north of the Black and Caspian Seas. This is one place where we know what happened very well, but we see it all over Europe. We see it in Spain, we see it in Portugal, we see it in the Netherlands, we see it in Germany, we see it in Czechia, we see it in Italy, we see it in Switzerland, we see it everywhere. This wave of people from the East arrives and it displaces these successful, impressive, densely packed farmers, uh, with new people who have this ancestry from the East who are not as focused on farming, although some of them are, as the people who came before.

   25. DPDwarkesh Patel49:07

       Th- this is so crazy. So, um, j- just for the audience, the, the, if you're keeping tally, this one, uh, one bacteria, Yersinia pestis, is responsible... I mean, we learned in grade school that it's responsible for killing a third of Europeans, um, more recently causing the Black Death, right? And then there's even theories that this was, uh, this helped with the Industrial Revolution because it drove wages up in Britain and because of higher wages they had to make machines and da, da, da. The, the, Robert Allen, the economist, has a theory about this. So potentially caused the Industrial Revolution. That one's more 10 to-

   26. DRDavid Reich49:38

       Also causes inflation.

   27. DPDwarkesh Patel49:39

       Yeah.

   28. DRDavid Reich49:39

       So it ends, it ends... I mean, in, in the, the medieval one crea- creates a lot of inflation, and the serfs, as I understand it, were sort of on fixed wages and so they had to be paid more. It basically inflated out their sort of seigneurial responsibilities.

   29. DPDwarkesh Patel49:53

       Yeah. So th- th- that's one of my theories. Uh, the other is, um, during the Bronze Age it allows the steppe people basically to replace the existing hunter-gatherer farmer population in Europe. Like, literally all of Europe is, uh, allows a population from the eastern steppes to, uh-... like, replace the existing people who built the Stonehenge with, uh, w- doing other things in Europe. And the Kyle Harper's book talks about this, where the Plague of Justinian, I think the final one that killed off the empire, was also Yersinia pestis.

   30. DRDavid Reich50:23

       Definitely.
4. 50:52 – 1:00:18

   Was agriculture terrible for humans?

   1. DRDavid Reich50:52

      definitely not Yersinia.

   2. DPDwarkesh Patel50:53

      So, I mean, tha- that- that's crazy that not only disease but this one in particular has had this big a role in human history. I'm curious if you can talk to, um... There's anthropologists and h- historians who have different theories about what the early history of humanity looked like. Basically, like, what kind of gods did they worship? How big were the communities? What, um, um... Th- um, and this informs their political philosophy today. James Scott obviously being the main example here, right? Uh, can you... Eh, does genetics shed any light on whether, for example, the... In fact agriculture was terrible for humans, and the first nation-states were abusive and so forth? Or is this stuff that is not available through ancient DNA?

   3. DRDavid Reich51:34

      We have indirect information about some of these things. So one thing that you might hope to learn about is whether our genomes reacted to the innovation of agriculture in a disrupted way. So you might think that, uh, our genomes would've been in some kind of steady state, sort of natural selection had adapted us to the previous environments we were in. And you might expect that in reaction to a change so economically, dietarily, cognitively transformative as agriculture, the genome might shift in terms of how it adapts. And so you might actually see that in terms of adaptation on the genome. You might expect to see a quickening of natural selection or a change.

   4. DPDwarkesh Patel52:18

      Mm.

   5. DRDavid Reich52:18

      I don't think we know the answer yet to whether that's occurred, although there are beginning to be hints.

   6. DPDwarkesh Patel52:22

      Mm.

   7. DRDavid Reich52:22

      And we could learn that from the DNA data.

   8. DPDwarkesh Patel52:24

      Mm. Hint, hint, hints in which direction?

   9. DRDavid Reich52:26

      So one question is... So there's an increasing view amongst geneticists and that natural selection is a process where there's relatively little directional selection to-

   10. DPDwarkesh Patel52:41

       Mm.

   11. DRDavid Reich52:41

       ... adapt to new environments. One piece of evidence connected to this is the finding that there's very few genetic changes that are 100% different in frequency between, say, Europeans and East Asians, or West Africans and Europeans, or West Africans and East Asians. If there had been genetic variants that had had modest selective advantages, 1%, half a percent, 2%, that's actually a lot, but over year by year that had arisen, and then that's... In a few hundred generations, they would have risen from very rare to very common-

   12. DPDwarkesh Patel53:11

       Yeah.

   13. DRDavid Reich53:11

       ... and in fact gone to 100%. There's thousands of generations separating Europeans and East Asians and West Africans and Europeans and so on. So if that was a common process in evolution, we would expect many genetic changes to be 100% different in frequency between Europeans and East Asians or West Africans and Europeans. We see almost none. So what that suggests, at some level, is that there's not strong adaptation over the last 50,000 years. Because if there was, we would have seen genetic variants driving to 100% frequency difference across different groups around the world be- which have hardly been connected with each other genetically over that timeframe, frame that we're talking about. We don't see those variants, so maybe selection hasn't been important. But maybe over a shorter period of time, selection has quickened and variants have started rising in frequency in the last maybe few hundred generations or something like that. And we might be able to appreciate that. So maybe we could see whether there's been a quickening of natural selection over that time period. There's a question about, um... I think the view amongst common trait geneticists is that we've been at a kind of steady state, where almost where the natural selection that does occur is just they're pushing down slightly bad variants, not-

   14. DPDwarkesh Patel54:21

       Mm.

   15. DRDavid Reich54:21

       ... not adapting to new situation. We're at a kind of stable point. So it's not clear how that works, because over a scale of two million years, we're clearly genetically quite different from our ancestors.

   16. DPDwarkesh Patel54:32

       Yeah.

   17. DRDavid Reich54:32

       Our brains are bigger, we do some things differently, our proportions are different. And yet over the last 200,000 years, we are not profoundly different. There's not genetic changes that differ dramatically across populations. So there's, like, a kind of disconnect. It's tempting to think sele- evolution has stopped-

   18. DPDwarkesh Patel54:50

       Mm-hmm.

   19. DRDavid Reich54:50

       ... from one perspective, because there's so little fixed differences. But on the other hand, somehow there are diff- somehow it looks... If you look in the last 10,000 years in West Eurasian DNA, which we're doing now, it looks like a lot of change is happening. So it's a very confusing situation. It feels like we don't really understand what's going on, but there's a lot to learn.

   20. DPDwarkesh Patel55:12

       Do you have a sense of w- what... Um, 'cause obviously 10,000 years, we're talking about the beginning of agriculture.

   21. DRDavid Reich55:16

       Yeah.

   22. DPDwarkesh Patel55:16

       Do you have a sense of what, what those changes might look like? Or is it too early to tell?

   23. DRDavid Reich55:20

       So we... I mean, we're, we're working right now on a study which is documenting changes over the last 10,000 years in Europe and Western Eurasia, uh, based on tracing changes in about 8,500 high-quality d-

   24. DPDwarkesh Patel55:34

       Yeah.

   25. DRDavid Reich55:34

       ... DNA sequences from people from this period that has been, have been collectively accumulated by us and others. So we've been working very hard at this, led by Ali Akbari in my group. And we think we have many, many hundreds of places where the, th- there's been very strong change in frequency over time-

   26. DPDwarkesh Patel55:53

       Mm.

   27. DRDavid Reich55:54

       ... where we're confident of. And we think there are many thousands that we can see traces of. That, that is there's... The whole genome is seething with these changes.

   28. DPDwarkesh Patel56:02

       Mm-hmm.

   29. DRDavid Reich56:03

       In this time of per- in this per- period.

   30. DPDwarkesh Patel56:04

       And can you give us a sneak peek on... Do we know what phenotype any particular ones correspond to?
5. 1:00:18 – 1:16:29

   Yamnaya expansion and how populations collide

   1. DPDwarkesh Patel1:00:18

      So one thing I'm very curious about is whether we have any sense of what it looked like when different populations came into contact with each other. Because in many of these cases you're talking about 90, 95% of the population being replaced, to the extent that sometimes you refer to them as ghost populations because only in the aftermath with this modern genetic technology can we even tell that there was some other population here. Uh, we can see the trace of that. And, um, you know, for example, if like the Yamnaya, like when they're coming into Europe, I know there's obviously many different cases and many different cases look different in terms of how violent it was or what the clashes looked like, but the- the- the fact that, for example, the Yamnaya, if you focus on that one- that one example, um, m- uh, replaces, uh, like becomes a dominant group in so many different parts of Europe. It's not like Genghis Khan where it's like one empire and there's the great khan who's like the- everybody's, um, uh, ev- uh, e- everybody's pledging fealty to. So it's th- they're not organized in that way, but like, they're still organized enough that they can go from, uh, go from place to place and, "We are the Yamnaya and we're taking over." Like what- what did that concretely look like?

   2. DRDavid Reich1:01:30

      Yeah. So that's super interesting. And I'm gonna back up a little bit because i- in my book I have a section where I describe when we had these findings for the first time and the conversations we had with archeologists on- about these findings. So ancient DNA has been very disruptive to conventional understanding of the past. And what happened when we had these findings of massive disruption of the local population in Germany, uh, about 45 to 4700 years ago based on arrival of people from the steppes north of the Black and Caspian Sea, was some of our archeologist co-authors really just were very distressed by the implication.

   3. DPDwarkesh Patel1:02:10

      Mm-hmm.

   4. DRDavid Reich1:02:10

      Because after the Second World War, there had been a reaction where people said this initial idea that people had based on archeology where in the beginning of the 20th century, uh...... when people would see new types of pots in a certain layer of the excavation, they would argue that this is the arrival of a new people, uh, coming through invasion or through movement into a region-

   5. DPDwarkesh Patel1:02:33

      Yeah.

   6. DRDavid Reich1:02:34

      ... and its very disruptive event, the arrival of the Corded Ware complex-

   7. DPDwarkesh Patel1:02:37

      Yeah.

   8. DRDavid Reich1:02:37

      ... or the arrival of the Bell Beaker complex, or something like this. This is a very disruptive event mediated by invasion or, or, or so on. And the re- and that was used by, for example, the Nazis to argue that these were spreads of Arians moving across the landscape and being very disruptive and violent, for example. And the reaction after the Second World War was to say, "We don't know this." And in fact when you see the arrival of new types of, uh, material culture, pots for example, or tools, or ways of organizing life, might- you might be seeing as more the spread of culture. You might s- be seeing, for example, something like people copying use of a cell phone or something like this, which can be used by people of very different backgrounds and it's just... Or a new religion spreading, and it's not actually movement of people. And in fact, how could there be a big movement of people? You're looking at densely settled Europe with well-developed agriculture. How could it be that new people coming in from outside will unseat these people, disrupt these people, especially after a period of stasis, after a period of, uh, well, especially once... Sorry. E- especially once, when you have farmers who are densely settled and how could these be pastoralists coming from-

   9. DPDwarkesh Patel1:03:46

      Yeah.

   10. DRDavid Reich1:03:46

       ... somewhere else? They're not as dense on the ground. In India today, uh, we, uh, the British were sort of in control, the Mughals were in control for hundreds of years, but made hardly any demographic impact. How could people from outside with less density make much of a dem- demographic impact? But then you look at the genetic data and there's a 50%, 70%, 90% population disruption. You take the DNA from people after these events and almost all their ancestors are from far Eastern Europe, right across most of Europe. And so the DNA proved that that idea was wrong. It was very disruptive. So the question that you had is, what does it look like on the ground? And so the DNA results was extremely disruptive to people in archeology who had made these arguments that change wasn't possible in this very... that w- large scale migration, large scale disruption probably didn't occur in the past, and so it was a real challenge, it was a real challenge to our understanding of h- pre-history. It was sort of a case example, a prime example that's been important for me in showing that we really don't know what the past was like until we actually look at it and have hard data telling us what it's like. Our guesses, our models, our, our... including many of mine, are likely to be wrong because we can see that because when we have hard data, w- we're surprised. I'm sorry for that long preamble.

   11. DPDwarkesh Patel1:05:01

       (laughs)

   12. DRDavid Reich1:05:01

       So when, when... What's happened in the last few years is there's been something of a reconciliation after the book. Uh, archeology is trying to reconcile itself with the DNA data and it's arguing about the subtlety of these interaction events. So people talk about what's happened in Britain, for example. Well, maybe the arrival of the Beaker phenomenon, which happens about 4,500 years ago, maybe it's not an invasion. Maybe it's a kind of peaceful event. Maybe the previous people, uh, the reason we're seeing such a disruption is the previous people, we know they cremated their dead and the Beaker people buried their dead, so it looks like a much more abrupt change than it did. Maybe what ha- happens in Iberia when there's a 40% arrival of these foreigners from the East and 60% local people, but the Y chromosomes are completely replaced, so the local men don't sort of contribute their DNA to local later populations. It looks to you- it looks somehow like that must be extremely disruptive to the local male population, but people are saying, "Well, maybe this is female mate choice. Maybe this is a somehow kind of not what you think it is. Maybe it's not what happened 4,000 late- years later amongst the descendant of the Iberians in the Americas, where today in Columbia 95% of the Y chromosomes are European, 95% of the mitochondrial DNAs are Native American."

   13. DPDwarkesh Patel1:06:17

       Yeah.

   14. DRDavid Reich1:06:17

       We know what happened there. It wasn't friendly, it wasn't peaceful-

   15. DPDwarkesh Patel1:06:20

       Yeah.

   16. DRDavid Reich1:06:20

       ... it wasn't nice. But maybe what happened in Iberia 4,000 years ago amongst these ancestors of people was much more peaceful, was much more calm. If you look in detail in Iberia, what you see is the period of this change is actually over 500 years. But if you look at a microscale, now that we have better data, it's immediate each place. So in Southern Spain-

   17. DPDwarkesh Patel1:06:40

       Mm-hmm.

   18. DRDavid Reich1:06:40

       ... it's very fast, and then in Central Spain it's a little later, but very fast. And so actually, there's these rapid changes occurring in one place than the other. People thought, in Britain, maybe this was actually a slow process, but we now have data not yet published from the Netherlands, which is clearly the same population of Beaker people that's spreading in Britain, and there it's very disruptive and you actually have the whole series of people before and after. You see the earlier Corded Ware people are local, e- which is actually very unusual for Corded Ware. They're actually local people adopting the religion of the Corded Ware, but mostly local ancestry. And then the Beaker arrival is incredible disruption. There's almost no continuity, uh, very little continuity.

   19. DPDwarkesh Patel1:07:17

       Yeah.

   20. DRDavid Reich1:07:17

       So probably what's happening with the Beaker individuals is one way or the other you have some kind of people who expand demographically and displace people somehow, rapidly displace people over a period of, well, less than a century.

   21. DPDwarkesh Patel1:07:34

       And do we know whether they were organized... 'Cause more modern versions of this, um, when Cortes goes over to the New World he's like serving fealty to the emperor of Spain and so forth, or like, I don't know, the, the Mongols and Genghis Khan or something. In this case, I assume there wasn't enough hierarchical organization that something like that was available, but there was enough organized... May- I don't know if organized is the right word, but there was enough sort of like persistent, um, um, uh, persistent invasion that like, "We're gonna keep going f- town to town, settlement to settlement until we've reached the ends of Europe." And so was it just like the Yamnaya were just lots of different independent groups that were doing this at the same time? Or was it like... H- h- how organized was this, basically is what I'm try- what I, what I mean to ask?

   22. DRDavid Reich1:08:18

       So we don't know-

   23. DPDwarkesh Patel1:08:19

       Yeah.

   24. DRDavid Reich1:08:19

       ... and I think there's debates even about that. I think one example I've heard archeologists I work with think about is, um-... is the Comanche- Yeah. ... in the US Southwest where, you know, it's another horse-based, uh, expanding- Yep. ... group and they expand super dramatically, you know, in parallel to the Spanish expansion and alongside the US expansion before encountering the US, uh, sort of militarized United States- Yeah. ... at some point. And, you know, it's local, there's local bands of people expanding, they go on campaigns, they expand to certain areas. It's, it's the, the Beaker people and the Corded Ware people, they're contemporary to ancient Sumer and to a lot of the, uh, uh, you know, Egyptians that we actually have written history from, it's not so ancient. They s- they weren't writing, but they, they were contemporaries of these people not so much far to their south. Right. So we really don't know what was going on, but if you were part of a community where there is a culture where, say, the males, as we think from reconstructions from Indo-European myth, which is probably the class of cultural shared knowledge these people were operating from because we think these people were the spreaders of Indo-European languages in this part of the world. If you think about this as a world where at a certain age, men- males would band together and go on raiding parties and so on, and that would b- then maybe settle down later in life, you can imagine a process where, built into the culture, you have a process of expansion, exploitation. Mm. One thing that's really interesting that has actually emerged in the last years and was not really sort of strong at the time that, uh, I wrote my book was, uh, an understanding of the relationship between the Yamnaya and groups like the Corded Ware and- Mm. ... the Beakers. So the Yamnaya are these groups that, uh, thrived between about 5,300 and 4,600 years ago in the steppes north of the Black and Caspian Seas. They're probably the first people to domesticate the horse, or that's arguable, and, uh, they used the horse and the cart, which was newly invented, and the wheel to exploit the open steppe lands and, and, and be able to economically expand much more rapidly. They're the first, world's first extreme mobile pastoralists. But they can't get further than the steppe, so they expand into Europe, they expand into the little island of the steppe that's in the Great Hungarian Plain in the Carpathian Basin, and they stop. They can't expand their way of life to, um, to the forested parts of Europe, which is most of Europe. And somehow the ancestry of the Yamnaya gets absorbed by the Corded Ware group. Mm. And then later the Beaker group, and that takes it further through Europe. But the Corded Ware group is quite different from the Yamnaya culturally, and in fact, a lot of archeologists think that they're so different they can't be the same. They sh- have some shared features, but the Corded Ware have many different traditions. One possibility is that the Yamnaya expand and they encountered early Corded Ware, the Corded Ware learn some of the adaptations of the Yamnaya, and then they actually take Yamnaya women, absorb them into Corded Ware, mostly male communities, and create a new community and that group expands. Mm. So one of the mysteries of the Yamnaya expansion was everybody had this cognitive bias to think this is very male driven, people have these Indo-European notions- Mm. ... of sort of male-centered mythologies and so on. Yeah. So this must be an extremely male-centered migration, a mer- very male-centered migration. You look at the genetic data and you look at the Y chromosomes which track male migration and the mitochondrial sequences which are more sensitive to female migration, and it looks like the steppe expansion from the east to west is very both sexes. Mm. Both males and females expand. And people have found this confusing and there's been a lot of inc- incredulity about this. People expect to see that it's an even movement of males and females, but it's quite clear that the bias is not so strong. And we think the most likely explanation for what's happening now is that it actually is a male-biased process, but it's one that's interrupted. So the Yamnaya expansion is very male biased. Yeah. It expands to the edge of the range, they encounter the Corded Ware, complex people- Yeah. ... and then what happens is the Corded Ware complex people interact with the, the Yamnaya people, and in fact, the Yamnaya people actually are, lose out in that interaction. Ah. And in fact, the Corded Ware males absorb and take Yamnaya females and they actually also take farmer females- Mm. ... 'cause you actually see these sites in early, early Corded Ware sites in Czechia where both things are happening. Females from farmers and females from Yamnaya are being absorbed into the Corded Ware community, and then they expand further. So what you actually have is a two-step process where you have a male Yamnaya expansion and then that ancestry from the steppe is con- carried further through females being absorbed- Yeah. ... from, into the Corded Ware, and then another male-driven expansion under the Corded Ware, and so on. And that brings both female and male Yamnaya lineages west, but not always with the Yamnaya ancestry being associated with the kind of intuition that you would think it's domination. The same ex- sort of parallel thing in another part of the world is what you see in, uh, remote Oceania, in the Southwest Pacific. Mm. So if you look at Vanuatu, uh, which is the islands, uh, some of the first islands that people got about 3,000 years ago in the Southwest Pacific, so m- moving to this other part of the world. If you look at New Guinea and Australia, people are there almost a little bit after 50,000 years ago, people are in the Solomon Islands and the Bismarck Archipelago to the east of New Guinea maybe 35 to 40,000 years ago, and they stop. And the Pacific has all these fertile, uh, places that are good places for people to live, it's completely empty of people until 3,000 years ago. Suddenly these people from Taiwan go through the Philippines, they skirt the edge of New Guinea and the Bismarck Archipelago, and they get to Vanuatu and Fiji and Tonga and New Caledonia and Samoa about 3,000 years ago, super rapidly, in the guise of something called the Lapita cultural complex. And if you look at the DNA of the people from this, they're almost entirely East Asian in ancestry, they look like early Taiwanese people. Uh, and today, people in Vanuatu and Fiji and Tonga and New Caledonia...... have only 10% of this DNA.

   25. DPDwarkesh Patel1:14:38

       Mm.

   26. DRDavid Reich1:14:38

       So something else happened afterward. The first people are almost entirely East Asian, via Taiwan and the Philippines, and then you look at later DNA from the same part, and 2,500 years ago, 500 years after the initial arrival, there's mass movement in a male-driven way from New Guinea, in the Bismarck Archipelago, into Vanuatu of Papuans. People, uh, uh, with overwhelmingly Papuan ancestry from New Guinea coming into Vanuatu, and that's the origin of the ancestry that's overwhelmingly there in Vanuatu and New Caledonia today. So there's a two-step process, the initial step, which is East Asian ancestry and these people-

   27. DPDwarkesh Patel1:15:14

       Yeah.

   28. DRDavid Reich1:15:14

       ... who invented outrigger canoe k- technology and long-distance sailing. And then the technology becomes adopted by Papuans, who are using this culture for the next few hundred years. We can see them trading back and forth between the Bismarck Archipelago and Vanuatu. And by the end, there's all... this culture is carried out by Papuan ancestry, and males from this group then spread into Caledonia and take local females. But the ancestry is flipped from the way that people have this cognitive bias-

   29. DPDwarkesh Patel1:15:42

       Yeah.

   30. DRDavid Reich1:15:42

       ... that it should be. So people think, "Oh, it should be the East Asian males-
6. 1:16:29 – 1:32:22

   “Lost civilizations” and our Neanderthal ancestry

   1. DRDavid Reich1:16:29

   2. DPDwarkesh Patel1:16:29

      Mm. Th- that's really interesting. I, I guess, m- uh, speaking of this, uh, w- we're going to a totally different era, but, um, uh, something I'm curious about. So going back to our cave humans, and w- we talked a lot about Neanderthals, but obviously there were two different species of Denisovans. Or I don't know if species is the right word, but two different kinds of Denisovans, and also I think the Hobbits in Asia, right? And then, I don't know if there's more, but, like, we're talking about half a dozen different, like, distinct groups, and only one survives. Do we... Um, I, I understand w- if like a s- new cultural technology is developed by this Near East gr- early gr- tribe that, like, then they expand out through Eurasia. And I get, like, th- 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 and one group of Hobbits? Like, there's no... there was no niche in which they could just, like, fend off. You... Everywhere, uh, this one tribe of humans, um, one tribe of African humans just dominated.

   3. DRDavid Reich1:17:37

      Yeah. (laughs)

   4. DPDwarkesh Patel1:17:38

      I don't know if that asks the question and, uh-

   5. DRDavid Reich1:17:39

      Yeah.

   6. DPDwarkesh Patel1:17:39

      But, yeah, th- yeah, how did none of them survive?

   7. DRDavid Reich1:17:41

      Yeah. I, I don't know. I mean, I think it may be a numerical issue. I mean, if you look at the pl- 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 survived (laughs) for several thousand more years than in the surrounding areas, probably because they're exploiting the wetlands. But they're gone soon enough.

   8. DPDwarkesh Patel1:18:04

      Yeah.

   9. DRDavid Reich1:18:05

      Once something happens. Mammoths go extinct mostly 14,000 years ago, but they survive on Wrangel Island north of Siberia-

Episode duration: 1:57:03