David Reich - How One Small Tribe Conquered the World 70,000 Years Ago

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 very 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?

Well- came out about archaic humans and the relationship between modern humans and Neanderthals. Can you explain again what you're studying these days? 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 who lived in Western Eurasia for the last couple of hundred thousand 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 finger bone that was found in a cave in Siberia, 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 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.
And 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 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 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. 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, the 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 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 Denisva-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 three 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 three or 400,000 years separated between Neanderthals and modern humans. And like the 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 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. And it just seems surprising.
So 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 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.

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 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 in modern data, is that modern humans are also highly substructured.

We can see that even without having ancient DNA yet.

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 contributed and mixed 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 and the possibly higher proportion of ancestry.
It's not obviously wrong that the model's very, very different from the standard one that we currently have. Interesting.
So, I mean, from your book, I remember that there are lineages of modern humans that are over 200,000 years separated from other groups, like the Sun hunter-gatherers from everybody in Eurasia today, or everybody descended from Eurasia. So then you're saying that 100,000 years before that is when we have a sister lineage with Neanderthals.
Actually, I'm not sure what the new findings we're finding about how closely related Neanderthals are to us and how much mitochondrial and Y chromosome DNA they share. What model do you think is the most plausible to explain why there's so much shared ancestry? I'm very agnostic.
I really, really don't know. But the models you were just talking about, it sounded like you thought they were low probability.
Is there one you think is more... I think the models that are considered to be standard dogma are now low probability.
So there's a standard dogma that's developed over an accretion of papers where 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, which is now a series of patches.
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 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.
Where did the gene flow between, 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 physically did that happen? Even that's not clear, but probably such a thing would have occurred somewhere in 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 leading to the great majority of the ancestors of people today was in sub-Saharan Africa for the last 500,000 years, at least, and maybe might be much more. Certainly our main lineage was in Africa, probably 3 million, 5 million, 7 million years ago.
But in a period between about 2 million to 500,000 years ago, I think it's not at all clear where the main ancestors leading to modern humans were. There were humans throughout many parts of Eurasia and throughout many parts of Africa with a parallel increase in brain size and not obviously closer ancestrality to modern humans in one place than in the other.
It's not clear where the main lineages were. Maybe they were in both places and mixed to form the lineages that gave rise to people today.
So I think there's been an assumption where Africa has been at the center of everything for many, many millions of years. And certainly it's been absolutely central at many periods in human history.
But in this key period when modern humans develop from Homo habilis and Homo erectus all the way to Homo heidelbergensis and the shared ancestor of Neanderthals, modern humans and Denisovans, that time period, which is when a lot of important change happens, it's not clear, as I understand it, based on the archaeology and

also certainly based on the genetics where that occurred. So can I just say for the audience, what is so interesting here that we, I don't know, we're humans.
And like, you would think one of the things history would have figured out is how did humans come to be, right? Like, that's probably one of the biggest questions you could imagine asking of history, of archaeology, of anthropology, of genetics.

And the fact that, I don't know, this is the thing, at least a conventional model is the thing you're taught like third grade. This is one of the first things you're taught about the world, right? And the fact that many parts of it could be wrong or we're learning in greater detail what those parts look like at the very least.
And we're doing that right now because of new technology that's being used by labs like yours. I think that's really wild.
And I'm just thinking like the audience might not be aware of how much of a change this is in our understanding of the human past. And I just sort of really want to emphasize that.
So if the gene flow event you're talking about a few hundred thousand years ago happened between quote unquote modern humans and Neanderthals happened outside of Africa. Then did that lineage go back to Africa and then come back out again? How do we think about it? Well, the simplest version of this is that the main lineage leading to modern humans is in Africa at this point.
And Africa, as I understand it from talking with the archaeologists and the climatologists, is that Africa and the Near East are continuous ecological spaces at certain periods of time.

And so there's no difference between what's now the Near East and Africa.

The fauna and the flora are pumped from Africa into the Near East or pumped from the Near East into Africa.

And so the African range goes into that region.

And so it's a place of overlap between Eurasian fauna and flora and African flora and fauna. And so that's a very natural place for interactions to occur, especially in periods of climate change.
Animals, for example, from one region get pumped into the Near East, and then in another period of climate change, they get pumped into Eurasia or the rivers. Because there's a land bridge during different climactic events? I think there's always a land bridge.
But the ecology with deserts and so on makes certain areas permeable or impermeable. And so in some periods of time, the Near East gets reclaimed by Eurasia somehow ecologically.
And in other periods of time, it gets reclaimed by Africa. So it's kind of a place of movement of flora and fauna out and in again and again and again and again.
So I think the simplest model, I'm not an expert on this, but the simplest model would be one in which an extension of the modern human substructure leading to us, the ones that some of those lineages coalesce to form people living today, the great majority of the ancestors, gets into the Near East several hundred thousand years ago, and then mixes there with the ancestors of what we have now sequenced as Neanderthals, and the skeletons that we have now are Neanderthals, and that that gene flow event occurs there, and it's modern humans from Africa or the part of the African population that extends into the Near East pushing into Neanderthals at that time. We have evidence of modern human incursions since that time into Neanderthal parts of Western Eurasia, also in intermediate periods from the skeletal record, and maybe even claims recently in the DNA data.
But certainly the genetic data attests to a very strong event a few hundred thousand years ago. So how many humans are around at this time? Because to the extent that all modern humans are descendants of this group, there's like, there's, were there just, like how many different groups of humans are there such that, by groups I don't mean genetically distinct necessarily, but just like separate locations or so forth, such that there was enough gene flow between all of them that there's a shared common descent? I don't know.
So like one of the things that is really interesting, we just published a couple of years ago a paper on relatively recent hunter-gatherer populations from mostly Eastern and Central Africa. And this included individuals going back up to about 15,000 years ago, which is the oldest from sub-Saharan Africa which is not very old and at some level in order to be able to discern these deep population exchanges that really we would like to know in order to understand human evolution which really we would like to be able to probe two million years ago we can't but with 15,000 year you see is many, many groups at many, many places, all with very reduced diversity.
So in other words, they look like they're living in tiny populations of hundreds of people and not exchanging DNA with each other very often at all over time. And this is again and again, we see this again and again.
And so if you take such a population, put it into a model and say it's this small, what will happen over time?

It will lose its diversity over time.

It will become very non-diverse.

So over time, Africa will have very little diversity. But of course, Africa today has great human diversity in it.
And so what seems to be happening is that the whole continent of sub-Saharan 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, are in very small sizes, are losing diversity. And when we sample them, this is a group that leaves hardly any descendants at all, maybe none amongst modern people.
And what's actually happening is occasionally these groups merge together and recharge their diversity. So the diversity is maintained in the ensemble of rarely mixing groups.
And you can't really appreciate the diversity by studying any one group, but rather you actually have to think about the whole ensemble of hundreds or thousands of tens of thousands of them as preserving the diversity. So there's some question about the migration rate amongst these groups, which are archipelago of little groups losing diversity, going extinct at some level.
But together, there is enough recontact to recharge the diversity and create the incredibly diverse populations you see today, for example, in Southern Africa or Western Africa or Central Africa. I want to go back to what you were saying, that for hundreds of thousands of years, not just with modern humans, quote unquote, but with even the so-called archaic humans with Neanderthals and other species, that there's been selective pressure for larger brains.
And this is, despite the fact that they're in different parts of the world, if I'm right. So like if you're in Eurasia or if you're in Sub-Saharan Africa, either way, somehow these like,

finally we've got to a state where there's,

the niche we're in rewards marginal increases in intelligence

and is willing to bear the cost of that

and keep chugging on that variable.

Do we know why that was the case?

What was happening in the world

or what was happening with maybe primate brains such that the selective pressure was turning towards greater intelligence? So that's a super interesting question. And I think there's a lot of insight and ideas about this topic.
And I think it's an area to which genetics right now has contributed almost nothing. so I think you know in the book that book that I have this book that I wrote, Who We Are and How We Got Here, Ancient DNA and the New Science of the Human Past.
And it's a bit of a misleading title or a kind of bait and switch title. And the way in which it's a bait and switch title is you might read it thinking you're going to learn something about how we became whatever we think is distinctive about us relative to other animals.
And so I try very early in the book to say that, unfortunately, with the genetic data available up to this point, we don't really have very meaningful insights about what makes us distinct, how we became to be distinct from other animals. But what I'm going to tell you about is how we came to be how we are are from another perspective, through mixture and migration.
So it's very surprising how we came to be, how we are through migrations and mixtures. A lot of people used to think that we were not mixed, but in fact it's been mixture again and again in the past in many populations we didn't anticipate.
But with regard to your question, which is how it is that humans evolved into a distinctive niche, which includes having a strong reliance on a large brain, putting a large amount of metabolic energy into the brain, brain relative to body size much bigger than it is in the past. I have two things that are striking to me about that.
One of them is that I think genomics actually has promised to learn about those things,

and I think we are potentially on the verge of learning a lot about those things. I just think we don't have important new qualitative insights about that topic right now.
The other is that the large brain was already in place prior to the separation of Neanderthals and modern humans, and maybe Denisovans as well. So already the common ancestors of Neanderthals and modern humans probably had a brain as large as ours.
It's not obvious that there's parallel evolution in multiple parts of the world. It may be that it's a sufficiently interconnected group that it's not a parallel evolution event, but a single process.
So many questions there. One is, when you say that there's a single interconnected population, are you referring to basically all of not only Eurasia, but also Africa? Possibly.
So basically the whole world, even hundreds of thousands of years ago, can be thought of as having gene flow and being one global population? I think that's almost certainly true. We don't yet know the frequency of exchange between Africa and Eurasia, but this is 2 million years.
It's a lot of time. Paul Salopek is like walking around the planet in like seven years or something like this.
Like people move incredibly quickly and Africa and Eurasia are not really separated by barriers that mean anything very important to a species like ours over periods of even dozens or hundreds or thousands of years. So the idea that being in Eurasia or Africa is such a profound barrier that you would not expect people to move from one region to the other in periods of tens of thousands of years or hundreds of thousands of years, that's a strange idea.
That's fascinating. So people, quote unquote, by the way, it's so interesting that it's hard to think of the correct terminology of when we say people, which kind of people are we talking about? But anyways, so the ancestors of modern humans are 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 with neanderthals then they should have been pretty smart hundreds of thousands of years ago but they're not expanding out.
And then something happened 60,000 years ago. And then this group that's descended from the people in sub-Saharan Africa just explodes all across the world.
So something seems like it changed. What do you reckon it was? So this is outside my area of expertise.
I'm being very much like a scientist right here, but I'm very sympathetic to the idea that it's hardly genetic. So I think that this is cultural innovation.
It's very natural to think that this is cultural innovation.

And humans sometimes develop a new 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, conceptual language, which allow you to create a cultural body of knowledge.
You know, you talk in the book about how the FOXP2 gene, which modulates language ability, not only in humans, but other animals, obviously all living humans have it. And so it's at least 200,000 years old when the human lineage starts to split off.
So everybody has language. So it can't even, like, what do we think it was? Well, I don't know what the language was.
I mean, it's almost certainly the case that Neanderthals were using sounds and communicating in ways that are probably pretty complicated, complex, and amount to some kind of language. But some people think that language in its modern form is not that old and might coincide with the later Stone Age, upper Paleolithic Revolution 50 to 100,000 years ago, and might be specific to our lineage.
And that there might be a qualitative shift in the type of language that's being used. There's been one incredibly interesting and weird 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 because it was so weird, and they just thought it might be wrong. But it's stood up, as far as I can tell, as just so weird.
So this is one of the things that surprises that genetics keeps delivering. I think that that's probably going to come across 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. so the line of evidence i'm talking about is one based on epigenetic modification of genomes so just to explain what that means the genome is not just a sequence of letters d DNA letters, adenines, thymines, guanines, and cytosine, ACTG.
It also is decorated in anybody's cells by modifications that tell the genes when to be on and off in what condition. So an example of such a modification is methylation 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 ACTGs 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.
So work by David Gokman and Liron Carmel and colleagues 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 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 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 an inborn error of genetic transformation, of genetic inheritance, kids will have, for example, a face that looks different or a vocal tract that looks different and so on. You know what the effect of 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.
So in the shape that we know is very helpful for the articulation of the range of sounds we use that chimpanzees 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 seems possibly reasonable, then maybe that's telling you that there's very important changes that have happened in the last half million or a few hundred thousand years, specifically on our lineage, that were absent in Neanderthals and Denisovans. But what's significant is that 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- We don't know that. We just know that today we have it.
So it could have been only a couple of hundred thousand years ago or a hundred thousand years ago that these changes happened. But then we know all modern humans have them and modern human, different groups of modern humans have split up- Separate 200,000 years ago.
ago right so we know it's at least that old right right right although there is gene flow

between all groups of modern humans at least at low levels uh going to 100,000 years it's just

most of the separation between koisan and other groups happens 200,000 years ago let me just like

motivate for the audience why this is so fascinating first of all it's obviously so interesting like

what happens such that 70,000 years ago there are half a dozen different human species all

All right. the audience why this is so fascinating first of all it's obviously so interesting like what happens such that 70 000 years ago there are half a dozen different human species all around the world that are pretty different and then fast forward to now there's one um the fact that that happened is kind of wild but um so it's interesting and just from that but another reason that makes it interesting for me is because i talked to these people who talk about AI and what some of them have a very strong perspective that you just make the model bigger and the thing just wants to learn.
And so you just make it bigger, you give it more space and it'll just become intelligent. And one of the pieces of evidence they use for this is that something, something happened with human brains, dot, dot, dot, bigger, dot, dot, dot, dominating the entire earth.
And so that's the perspective that like you make these ai models bigger dot dot dot you know like something very powerful is going to come on the other end to the extent that um that story is accurate or inaccurate might actually have interesting implications for ai um which is sort of wild right like our anthropology or genetics about the ancient worlds maybe it has like some some bay Bayesian update on how well we think these AI models will do in the future? So one thing that what you're saying makes me think about is that it doesn't map on in a simple way as an analogy. So one of them is that the human brain is maybe only three times larger than that of a chimpanzee.
And that's not the kind of increase that computability has had since 40 years ago or something like that, which is many, many orders of magnitude increase. Not a factor of three, but many, many orders of magnitude increase.
And in fact, I'm aware of studies that have, for example, compared raw computability of chimpanzee babies to human babies. In fact, it's similar.
For example, ability to solve logic puzzles is pretty similar between chimpanzees and humans. And some people argue that humans are not even more intelligent than chimpanzees at some fundamental ability to compute.
And that what makes human distinctive is social learning abilities. And that that's where a lot of our ability has gone.
Our ability to see other people, to empathize with other people, to copy other people, to incorporate bodies of information that are learned by other people. And so I am not an expert in this topic, but it's a very appealing group of ideas that the adaptations that humans have are ones that allow us to access a rich amount of shared knowledge and not just to rely on figuring out each thing.
So that's not obviously the same as just add more computability, but maybe it has some similarities. Yeah.
So I guess I still don't understand is the answer to we just don't know what happened 60,000 years ago, such that before humans and other modern humans and other types of humans were interacting, but no one was in a dominant position, at least in Eurasia. And now humans, not only do we dominate, but like, in fact, we drive them to extinction.
There's none of them are around at all. Do we have any idea what would change between that time? So the model that, this is really outside my expertise, but ideas that have been floated, and I will summarize them possibly badly, are that 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 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-sized group of people that's not interacting with a 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 amount of shared knowledge becomes greater, and then you have a runaway process where an increasing body of shared knowledge of how to make particular tools, how to innovate, patterns of innovation, and so on, language, conceptual ideas run amok.
So an example that I've heard talked about in this context is what happened with, for example, indigenous Tasmanians. You probably know this story, but about 10,000 years ago, the ancestors of people in Tasmania, which is this large island south of Australia, were continuous with the aboriginal populations of Australia.
They had fire, they lost it. And they lost fire because it got forgotten somehow, and it's a cold place, and they just forgot it.
The cultural knowledge lost it. 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 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 happens 50 to 100,000 years ago.
And people who all have the capacity to do these things. 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 human population was 50,000, 100,000 years ago.
And 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 a cause, but there's many potential causes. But anyways, I think I remember somewhere that the ancestors of everybody in Eurasia was initially like 1,000 to 10,000 people.
Tell me, how small was the human population that was like the seed of this modern period? So the bottleneck occurred,

the bottleneck, by bottleneck we mean founder event, a relatively small number of people giving

rise to a large number of descendants today. It occurred well before the mixture with Neanderthals,

which is probably somewhere like 50,000 years ago, plus or minus 5,000 years or something like this. So 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 maybe thousands or even tens of thousands of years before the encounter with Neanderthal that pushed in some Neanderthal DNA into modern humans.
So one way to see this is, in fact, this was not an unusual thing, that this was not an unusual thing to have a group with low diversity. In fact, the great majority of African groups would have had very low diversity, and it's just 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.
And maybe it also expanded inside of Africa. So there's 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.
So 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 of some kind where it throws sparks into different parts of Eurasia and interacts with the local people. So for example, if you look at the first modern humans of African and Near Eastern origin who get to, for example, Europe, where we have the best data, we have a number in Western Siberia, where we have the best data so far.
We have a number of these very early ones from about 45,000 to 40,000 years ago, which are called initial Upper Paleolithic. And a good fraction of them have had Neanderthal ancestors in their last two to four to eight generations.
So that's a kind of crazy result. So we have only a couple of dozen or a dozen or so of these very early humans, and 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 in the Middle East or the Near East of humans expanding. 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 and they produce these mixed populations, like these initial upper Paleolithic groups we sample in the record, and they all go extinct, like including the modern human ones. 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, and that's what we happen to see. And so 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, but from a later wave from the core in the Near East after 39,000 years ago that re-people a place that's been sort of affected by these sparks coming out of the same region, and those groups too disappear.
Oh, that's so fascinating. So not only is a group that started 60,000 years ago and eventually makes it around, not only does that one not survive, but then the group that starts 39,000 years ago, that one is replaced.
And then obviously we'll later talk about the Amnaya and like, you can just keep going. Then the hunter-gatherers were replaced 8,500 years ago by the farmers coming from the Near East.
And then after that, by the Amnaya from the steppe. Okay.
So it is interesting that it just like group comes there, is replaced by the next group. That group stays there, is replaced by the next group.

I don't know if that model-

I think that that's probably right at some important level. I think it's not a triumphal march of superiority and inferiority with the group that now comes in having advantages, somehow establishing itself permanently.
I think that what you have is a very complicated situation of many people coming together and natural disasters or encounter with animals or encounter with other human groups resulting in an almost random process of who spreads or who ends up on top and other groups coming in afterward. And so it may be that from a big picture perspective, you end up having African lineages spreading into these different parts of groups, different parts of Eurasia.
That's certainly what happened. But at a local level, I think it would be very difficult to understand what's going on.
So, yeah, the big picture is interesting in two ways. First, that you're not thinking crudely in terms of the major species or the major subgroups of humanity, like Neanderthals, Denisovans, and modern humans.
It's like, no, in fact, even among these, there were so many subcategories of different groups in this archipelago. And then if you do a fine-grained analysis, that's even more fascinating than that.
And how much contingency and randomness there is in that process. I think that's right.
And, you know, there's lots of analogies that you have later. There's European farmers encountering step migrations.
There's Native Americans encountering Africans and Europeans as they come from the old world. There's various other groups encountering other groups.
And you have people who have cognitively or culturally all the capacity to thrive in other contexts. but just because of the nature of the interaction that happens, one group declines demographically and one group doesn't.
And it's just complicated. So I don't think you should conclude necessarily.
It's very tempting to think that at some level it's innate biological. I'm not trying to be politically correct, that it's innate some better biological hardware that makes it possible for these African lineages to spread into Eurasia.
I have no good insight into that topic. I don't think there's very good genetic evidence or any other kind of evidence to say that that contributed in a very strong way.
I think that it's just complicated, and we certainly have many modern examples where people with better or more competitive cultural complexes encountering each other and the ones that are more organized in a certain way sort of thrive somehow demographically more. Yeah.
Okay. So let's jump forward to then, since you mentioned this, the way in which after agriculture was developed in the Middle East, I don't know, 10,000, 12,000 years ago.
And then after that, the way the Native Americans, 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 the amniya with literally the bacteria that causes the bubonic plague, Yersenia pestis.
The question I'm trying to ask is the, going back a bit. So the James Scott, who I think just died a couple of 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, well, one thing I didn't realize until I read that book is just how new all the diseases or most of the diseases that afflict humans today are, everything from Colorado typhus to tuberculosis, if you just go down the list, because of agriculture, because of domestication of animals and because of the density that created.

And so the theory he says, talks about in the book is that potentially the reason the hunter gatherers, the quote unquote barbarians couldn't fight back against these early nation states was because they were getting killed off by the diseases. And I don't know how much evidence there is for this.
basically the question i 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 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. 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 it's an argument about the history. He's a historian, a Roman historian.
And it's a history of three major plagues in the Roman period, a couple of which, two of which are really not even very well known, and argues that the decline of the Roman empire is due to just weakening 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, 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. So 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 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 S.K. Willerslev and Christian Christensen and colleagues, looked at their DNA and they discovered in their sequence from the 100 or so humans they sequenced that there was also pathogen DNA.
And in 5 to 10% of the random people they sequenced around 4 or 5,000 years ago, there wasinia pestis which is the agent of the black death but actually without the uh without the plasmid that contributes to bubonic plague that's required for flea rat transmission so it must have been for example pneumonic plague the aerosolized transmission or something but five to ten percent of random deaths means that actually the percent of people who were dying must have been even higher because they weren't detecting everything that was there. So a study by another group, Johannes Krauss and colleagues, of people in plague pits in London from the 1300s epidemic found that 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, if they have bacteria in their teeth, they probably are almost certainly died of that agent.
So paper just came out a few weeks ago in Scandinavia, looking at these tombs from about 5,000 years ago of farmers who were just on the verge of encountering people from the steppe. and a huge fraction of them have black death when they die.
They're buried in tombs, a normal, even higher than 5 or 10%. So this whole pedigree with many, many generations, so it's not all at the same time, just like the parents, generation to generations, a very large fraction, like well more than 10%, have black death and 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 in this entire period are from this. It's so unbelievable, so ridiculous that such a high proportion of people over such a long period of time are dying from this one agent that people don't even say it.
They just publish one paper after the other, 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 bacteria in their blood and they're not dying of these things.
It doesn't seem that people are ignoring, 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 be coming from steppe rodents, probably. And so maybe the people on the steppe are somewhat more, I mean, they say it's still dying of it, but somewhat more protective 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, giving 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, you know, in the last few years, we had COVID-19. It killed a half percent of the world population or something like that, and it was so disruptive.
So if this thing is killing a third of people or half of people, randomly killing people with cultural knowledge, randomly ripping into structures like in, I don't know, was it Montezuma died or one of his parents, you know, resulting in civil wars in the Inca when the Europeans encountered them, just disrupting the cultures that were there. Maybe this would have 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 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. The last big stones of Stonehenge go up 4,500 years ago.
And then within 100 years, 90% of them are gone. And they're replaced by migrants from the continent bearing 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 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.
This is so crazy. So just for the audience, if you're keeping tally, this one bacteria, Yeah, you're sayingopestis, is responsible.
I mean, we learned in grade school that it's responsible for killing a third of Europeans, more recently causing the Black Death, right? And there's even theories that this helped with the Industrial Revolution because it drove wages up in Britain. And because of higher wages, they had to make machines and dot, dot, dot.
Robert Allen, the economist, has a theory about this. So, potentially, it caused the Industrial Revolution.
That one's more tended to- It causes inflation. So, it ends, I mean, in the medieval one, 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 seignorial responsibilities.
Yeah. So, that's one of my things.
The other is during the Bronze Age, it allows the steppe people basically to replace the existing hunter-gatherer or farmer population in Europe. Like literally all of Europe allows the population from the eastern steppes to like replace the existing people who build the Stonehenge 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 Festus.

Definitely.

That's documented with genetics.

So the fall of the Roman Empire,

the entire, like twice the sort of,

or at least once the replacement of the population in Europe,

and so the second time, you know,

basically like modernity happened afterwards. It's crazy, one disease, and potentially the the new world as well in terms of, I don't know how many people, what percentage of the deaths in the new world were.
It's estimated to be not the primary pathogen. Okay.
But who knows? And in any case, I mean, there's others too, right? So some of the other plagues in the Roman empire are definitely not Yersinia. So 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, there's anthropologists and 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? And this informs their political philosophy today, James Scott obviously being the main example here, right? Can you, 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? 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 our genomes would have 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. I don't think we know the answer yet to whether that's occurred, although there are beginning to be hints, and we could learn that from the DNA data.
Hints in which direction? 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 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 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, 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, which have hardly been connected with each other genetically over the time 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, I think the view amongst common trait geneticists is that we've been at a kind of steady state, where where the natural selection that does occur is just there pushing down slightly bad variants, 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 2 million years, we're clearly genetically quite different from our ancestors. 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 a kind of disconnect. It's tempting to think evolution has stopped from one perspective because there's so little fixed differences.
But on the other hand, somehow there are differences. 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.
Do you have a sense of what, because obviously 10,000 years we're talking about the beginning

of agriculture, do you have a sense of what those changes might look like, or is it too

early to tell?

So we, I mean, we're working right now on a study which is documenting changes over the

last 10,000 years in Europe and Western Eurasia, based on tracing changes in about 8,500 high

quality DNA sequences from people from this period that 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 there's been very strong change in frequency over time, where we're confident of, and we think there are many thousands that we can see traces of.
That is, the whole genome is seething with these changes in this period. Can you give us a sneak peek on, do we know what phenotype any particular ones correspond to? So it's very clear that there is extreme over-representation of change on variants that affect metabolism and immune traits.
And so if you look at traits that we know today affect immune disease or metabolic disease. these traits are highly overrepresented by a factor of maybe four in the collection of variants that are changing rapidly over time.
Whereas if you look at traits that are affecting cognition that we know in modern people modulate behavior, they're hardly affected at all. That is, selection in this last 10,000 years doesn't seem to be focusing, on average, on cognitive and behavioral traits.
It seems to be focusing on immune and cardiometabolic traits, on average, with exceptions. But on average, there's an extreme overrepresentation of cardiometabolic traits.
The immune thing makes sense, obviously, and more diseases. In direction is the metabolic? So one example of this is that there's a very clear downward selection against body fat and against predisposition to high body medicine decks, predisposition to what today manifests itself as type 2 diabetes.
So that genetic combination in West Eurasia has been pushed down again and again over the last 10,000 years under the pressure of natural selection, without a doubt, in its action on many, many independent genetic variants, all pushing in the same direction in an overwhelmingly statistically significant way. So one possible interpretation of this, and this is speculative, is that you're shifting from a mode of survival that's more feast and famine to one where food is more regular and it's not as advantageous to store fat.
And so there's selection against sort of fat storage. That story seems to point against the narrative that agriculture was terrible.
And if there had to be selection against storing fat, that seems to suggest that in fact, yeah, that things must have been pretty good. Well, on a timescale, I mean, selection acts, I don't know how you think selection acts, but at some level, it could be terrible on the individual level and good on the population level.
So I think that, you know, I'm not doubting the evidence that you're, I think, maybe referring to, which is that skeletally, there's a lot more sort of skeletally unwell people associated with the beginning of agriculture than there are in the hunter-gatherer period. And I think on an individual level, life could have been experienced more harshly.
But in terms of sort of survival, different animals have strategies of investing less in their young, but having many more young, or investing more in their young and having fewer young. And maybe the hunter-gatherer strategy might be the latter, and the farmer strategy might be having more young, and some of them survive longer or something.
More of them survive. And on average, over a lifetime, there might be a stable enough food that if you don't rely on such adaptations, it might be better.
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And now back to David Reich. 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 knowledge can we even tell that there was some other population here. We can see the trace of that.
And, 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 was or what the clashes look like but the the fact that for example the amnaya if you focus on that one example um replaces like it 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

everybody's um uh everybody's pledging fealty to this they're not organized in that way but like

Thank you. And there's the great Khan who's like, everybody's pledging fealty to.
They're not organized in that way, but they're still organized enough that they can go from place to place and we are the Amnaya and we're taking over. What did that concretely look like? Yeah.
So that's super interesting. And I'm going to back up a little bit because 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 archaeologists 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 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 archaeologists co-authors really just were very distressed by the implication.
because after the Second World War, there had been a reaction where people said this initial

idea that people had based on archaeology, where in the beginning of the 20th century, 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 coming through invasion or through movement into a region. And it's a very disruptive event, the arrival of the Corded Ware complex or the arrival of the Bell Beaker complex or something like this.
This is a very disruptive event mediated by invasion or so on. And that was used by, for example, the Nazis to argue that these were spreads of Aryans 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 material culture, pots, for example, or tools or ways of organizing life, what you might be seeing is more the spread of culture.
You might 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, or a new religion spreading, and it's not actually movement of people. 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, especially once, sorry, especially once when you have farmers who are densely settled, and how could these be pastoralists coming from somewhere else? They're not as dense on the ground. In India today, 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 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 archaeology who had made these arguments that change wasn't possible in this very—that 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 prehistory.
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, including many of mine, are likely to be wrong because we can see that, because when we have hard data, we're surprised.
I'm sorry for that long preamble. So what's happened in the last few years is there's been something of a reconciliation after the book.
Archaeology 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, 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 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 somehow kind of not what you think it is. Maybe it's not what happened 4,000 years later amongst the descendant of the Iberians in the Americas, where today in Colombia, 95% of the Y chromosomes are European, 95% of the mitochondrial DNAs are Native American.
We know what happened there. It wasn't friendly.
It wasn't peaceful. 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 at 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, 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 or 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 that earlier corded ware people are local, 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, very little continuity.
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. And do we know whether they were organized, because more modern versions of this, when Cortes goes over the new world, he's like serving fealty to the emperor of Spain and so forth, or like, I don't know, 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, I don't know if organization is the right word, but there was enough sort of like persistent invasion that like, we're going to keep going from 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, how organized was this basically is what I mean to ask.
So we don't know. And I think there's debates even about that.
I think one example I've heard archaeologists I work with think about is the Comanche in the U.S. Southwest, where, you know, it's another horse-based expanding group.
And they expand super dramatically, you know, in parallel to the Spanish expansion and alongside the U.S. expansion before encountering the U.S., sort of militarized United States 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.
The Beaker people and the Corded Ware people, they're contemporary to ancient Sumer and to a lot of the Egyptians that we actually have written history from. It's not so ancient.
They weren't writing, but they were contemporaries of these people, not so much far to their south. 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, males would band together and go on raiding parties and so on, and that would then maybe settle down later in life, you can imagine a process where built into the culture, you have a process of expansion, exploitation.
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 I wrote my book

was an understanding of the relationship

between the Yamnaya and groups

like the Corded Ware and the Beakers.

So the Yamnaya are these groups

that 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 they use the horse and the cart, which was newly invented, and the wheel to exploit the open steplands and be able to economically expand much more rapidly.
They're the 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 the forested parts of Europe, which is most of Europe. And somehow the ancestry of the Yamnaya gets absorbed by the Corded Ware group, 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 archaeologists think that they're so different they can't be the same. They 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.
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 of sort of male-centered mythologies and so on.
So this must be an extremely male-centered migration, a 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 step expansion from the East to West is very both sexes.
Both males and females expand. And people have found this confusing, and there's been a lot of 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. It expands to the edge of the range.
They encounter the Corded Ware complex people. And then what happens is the Corded Ware complex people interact with the Yamnaya people.
And in fact, the Yamnaya people actually lose out in that interaction. And in fact, the Cordedware males absorb and take Yamnaya females.
And they actually also take farmer females, because you actually see these sites in early Cordedware sites in Czechia where both things are happening. Females from farmers and females from Yamnaya are being absorbed into the corded wear 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 carried further through females being absorbed into the corded wear, 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 sort of parallel thing in another part of the world is what you see in remote Oceania in the Southwest Pacific.
So if you look at Vanuatu, which is the islands, some of the first islands that people got about 3,000 years ago in the Southwest Pacific, so 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 35,000 to 40,000 years ago, and they stop.
And the Pacific has all these fertile 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 Lapida 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.

And today, people in Vanuatu and Fiji and Tonga and New Caledonia

have only 10% of this DNA. 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 and the Bismarck Archipelago into Vanuatu of Papuans, people with overwhelmingly Papuan ancestry from New Guinea coming into Vanuatu, and that's the origin of the ancestry that's overwhelmingly there in Vanuatu, New Caledonia today.
So there's a two-step process. The initial step, which is East Asian ancestry and these people who invented outrigger canoe technology and long-distance sailing.
And then the technology becomes adopted by Papuins, 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, 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 that it should be.
So people think, oh, it should be the East Asian males kind of somehow dominating the local females or something. You see the reverse, and this is what's going on.
And it's not like, it's very complicated and subtle. So when you actually see evidence of males and females behaving differently, it proves that there's socially asymmetric behavior of two groups as they interact with.
What it means is confusing. It could be female mate choice, it could be violence, it could be be genocide.
It could be different patterns of male and female dispersal with groups who travel being of one sex or the other. And we can look for clues in the genetic data.
And certainly in concert with the archaeology, we can maybe figure out more. That's really interesting.
I guess speaking of this, we're going to a totally different era, but something I'm curious about. So going back to archaic humans, and 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 understand if like a new cultural technology is developed by this near east early tribe that like then they expand out through eurasia and i get like that might enable them to be so dominant. What I don't understand is how is it that none of the other ones survived? Like not even one tribe of Denisovans 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 everywhere. This one tribe of humans, one tribe of African humansan humans just dominated i don't know if i asked the question and uh yeah like how did none of them survive yeah i 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 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 In the Netherlands, for example, hunter-gatherers survived for several thousand 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 Wrangell Island north of Syberia until 4,000 years ago. So at some point, each of these places is encountered by the spread of modern humans at high densities.
The other thing is, it's not even clear to me what expansion means. So 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.
Like, who are the ancestors? So that might sound like a silly kind of philosophical statement, but genealogically, I don't know if this happened before or after my books. 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-African's ancestors who are Neanderthals is not 2%, which is the proportion of their DNA in our genomes today if you're a non-African person. It's more like 10 or 20% of your ancestors are Neanderthals.
And what actually happened was when Neanderthals and modern humans met and mixed, the Neanderthal DNA 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.
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%, 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 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're intact on the male line, and culturally they stay on the male line intact. And so I'm not trying to be politically correct.
I'm just saying that you can actually have scenarios where this happens. So for example, in elephants, if you look in 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 these 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.
And the interpretation of this is that you just have waves and waves of dominant male bulls from the savannah coming into populations and eventually just replacing all of the genome in waves and waves and waves of an intact forest population. And so all that's left is the mitochondrial sequence, which is passed on the maternal line.
It's not even obvious that non-Africans today are modern humans. They're just, maybe they're Neanderthals who became modernized by waves and waves of admixture.
One question you could have, because we were talking earlier about how small the initial population that populated all of Eurasia was, that was like, well, a couple thousand people um and one question you can have is like if and we were also talking about how random and contingent the the this whole history of humanity has been and one question you could have is like was there some chance if a couple of variables were different that modern like quote-unquotequote, civilization, basically greater population density,

greater development technology, so forth,

would not have happened except for some really,

really, like, lucky chances?

Or was it the case that even if that one tribe

didn't do it, some other tribe of humans

would have done it?

And even if some other tribe of humans from Africa

hadn't done it, then, like, Neanderthals

had enough cognitive sophistication

that they would have done it?

I know this is a very sort of, like,, but just like how, yeah, how random does primate to civilization feel? Does it feel like we had to go down the exact right path? Or was it like, this is kind of the trend across many different branches of the family that leads to humans? I don't know. I mean, it's very speculative, but like, I'm very tempted to think that there's so many of these groups that some of them would eventually have gone down this route.
And one example of this that's interesting to think about is the parallel development of agriculture in the Holocene in different parts of the world. So you have in the Americas what's almost certainly a completely independent development of agriculture 9,000, 8,000 years ago.
From that in Eurasia, you can argue whether the East Asian and Near Eastern developments are different. They probably are, but maybe you could argue they knew about each other somehow.
Or the Papuan one, maybe you could argue they somehow knew about what was going on in other parts of the world, but probably didn't. But certainly the Americas one was isolated.
And suddenly, for the first time, you have these independent evolutions of full-blown agriculture at the same time in many places in the world after the Ice Age. This makes you think that it's somehow deterministic, that somehow some kind of setup of characteristics at this time causes this to happen.
And why doesn't it happen at the previous period of stable climate before the last Ice Age? Some people say, well, maybe it was actually not as good as the last 10,000 years. But I find that confusing as a statement.
It seems that somehow some set of characteristics, it's tempting to think that some sort of cultural or biological, but more likely cultural characteristics are in place and seated already at the time of the last ice age, such as when the reemergence happens, this happens in multiple places simultaneously. Because it happens so fast, right? It's not like you had to wait for tens of thousands of years after the ice age.
No. It's like literally 2,000 years after the ice age.
That culture is very old in the Americas. Okay, so then the ice age, is it 100,000? Because before that, humans split off 200,000, or at least some branches of the human tree split off 200,000 years ago.
And Neanderthals split off even before that, but that's before the last Ice Age started, right? So to the extent that your earlier statement about a lot of cognitive sophistication was already evident 200,000 years ago or 300,000 years ago. Doesn't that imply that before the ice age we should have seen agriculture? Yeah, it's tempting to think that.
So I'm very confused about this personally. People say that the last 10,000 years are very unique on a scale of millions of years.
If that's true, maybe we're in a very special time. That is somehow a period of warmth and stability of climate that's unprecedented for two million years.
Maybe that's true. But the other way people often say it is that we're in these cyclical periods of a few tens of thousands of years.
And the Holocene, the last 12,000 years or so, is a period of warming. And then there's a period of a couple of tens of thousands of years, which is the last ice age.
And then there's before that, there's a few tens of thousands of years of warming. And that's when we sample the Neanderthals, the late Neanderthals from.
And then before that, there's another stage of cooling. And then before that, another stage of warming.
So this is marine isotope stage one, three, five, seven, nine are the warm periods. We're in one now.

And marine stage 2, 4, 6, 8, and so on are the ice ages.

So the last glacial maximum was marine isotope stage 2.

If there were, quote-unquote, lost civilizations, maybe not, obviously not as sophisticated as anywhere close to the last thousands of years, but maybe early Sumer or something like that, or like Comanche slash Yamnaya level or something. But that happened before the Ice Age or maybe in a part of the world during the Ice Age where climactic conditions were better.
Would we be able to tell based on modern techniques? I think we would. Okay.
And there's just not any evidence of them? I think nobody has found, I mean, there's very sophisticated human burials in Europe and Africa and East Asia and different parts of South Asia and Eurasia and so on. Australia in the last, you know, in the marine isotope stage three in the last period of warming.
And, you know, burials full of beads, burials full of symbolic behavior. Maybe you interpret this as civilization, but extensive settled societies you don't see.
I think we touched on this when we're talking about population size, but like one thing I'm sort of confused about when we talk about the lineage is like, in one sense, the lineage is very distributed because obviously many different archaic humans contributed to the human gene line. In another sense, it's also like, maybe the main one is a couple thousand people.
So like, I don't know if there's a question there, but like, I'm not even sure how to think about the, can they just like hang out in like the size of Montana? The entire human lineage is...

I think that the lesson from ancient DNA and the genome revolution has been that anyone in the world is the result of recurrent mixture.

Again and again in the past.

So you might think that the last 500 years are unusual periods of history with the people of African and European and Native American ancestry coming together in the Americas, and that this is unusual because of transatlantic travel, but almost every group in the world is the result of many mixture events as profound as these on many timescales. So South Asians are the result of mixture between groups very different from each other, as different as Europeans and East Asians, four to three to 2,000 years ago, coming together and then crystallizing into a relative lack of mixture.
Since that time, Europeans are the result of mixture of Yamnaya and farmers and hunter-gatherers. People in different Near Eastern groups are the mixture of kind of early Iranians and early Levantine people and Anatolians who are super different from each other.
There's huge differences amongst East Asians, huge differences amongst Papuans and East Asians, profound differences amongst different Native American groups that come together to form groups that we have data from later, and example after example we look for. So if you think about any one lineage today, any one group of people, and you want to trace people's ancestors back in time and think, where do our ancestors scatter in geography at different time points?

Almost everybody's ancestors are scattered into different geographic distributions that are not all in the same place. So the evidence that our lineage was mostly in Africa is based on an idea, I think, an assumption, a kind of inertial idea that our lineage must have always been in Africa, because Africa is the center of human history.
But if you look at the archaeological evidence, it's not incredibly clear. And if you look at the genetic evidence, we have many early branches from Eurasia and only one from Africa, and complexity and branching in Eurasia that's sampled in the DNA record.
DNA from Denisovans, DNA from unknown archaic lineages that contributed to Denisovans, Neanderthals, and all of those are represented in the Eurasian record, not in the African record. Part of that is the fact that ancient DNA is preserved in Eurasia.
But maybe actually there's a period when our lineage resides in Eurasia. It's not obviously wrong.
So I think that hypothesis is out there as a possibility. One thing I would love to see, I don't know if, I assume this will change over time as more data comes up, but some sort of like chart that is superimposed upon a world map and it evolves over time.
And then maybe you can like, you can just have sort of blobs representing different population groups. You can start off with the archaic humans and go back like 200,000 years ago.
Go back even before that. Because if this is a global event, it's not just an African event.
So you have, for hundreds of thousands of years, you can just see different populations splitting off, merging back together. And if somebody can make that sort of animation, I think that would be a very useful...
I think you can. People have tried to make animations like this in some way.
But one way to think about it, you know, I think there's a huge danger in being too interested in yourself. This comes across in my book, I think.
But it's very, very tempting to be interested in your own history and think it's important. It's obviously not important compared to other people's history.
However, if you think about one person's history and you think about where their ancestors lived two generations, four generations, eight generations back in the past, those are your great-grandparents. And great-great-grandparents, you may even know where they lived.
But then you can actually plot on a map a different number of generations back in the past where your ancestors lived. And it's interesting to do within your family because maybe you're from – I'm from – my ancestors going back a few generations are in Europe somewhere, different parts of Europe, for example.
So people do this. And when you get a test back from one of these personal ancestry testing companies like 23andMe, they'll say, oh, you are 20% Irish and 30% Chinese or whatever it is, and so on and so forth.
And what they're referring to is roll back 20 or 30 generations, where are your ancestors scattered in proportions? But then if you roll back 3,000 generations, there's some in East Africa and some Neanderthals, right? So what you can actually do is for any one group of people or any one person, there's different time slices that matter. 30 generations ago, you get the 23 and me output.
3,000 generations ago, you get the proportion of your ancestors who are Neanderthals or not Neanderthals or Denisovans or something like that, if you're from one of the many populations around the world that live in Denisovans. If you are any population, going back further in time, presumably there's something similar happening, where mostly in Africa, but possibly outside of Africa 300,000 years ago, people's ancestors will be coming from different places.
It's very plausible that people's ancestors are not all in Ethiopia at 200,000 years ago, that in fact, some of them are in North Africa, some of them are maybe in West Africa, some of them are in South Africa, some of them are in Eurasia, and that actually appreciable fractions are in each place, and that braid and that trellis is coming together again and again over time. As you move further back, they'll collapse.
Some will go extinct. Some will reappear.
Some will re-emerge. And at any one point, there's never a singularity.
I don't know if you're familiar with Nat Friedman's Vesuvius Challenge. I don't know if you saw that when it was going around.
So the scrolls in the library at Herculaneum, there's a volcano. I forgot.
It was like, was it basically during the Roman Empire? 79 AD. Okay, well, yeah.
And it buried the squirrels in that library. They all became literal ash or at least very burnt.
And so Nat Friedman found this professor who had done CT scans of these squirrels and that there was really no way to decipher them. We just had the CT scans.
But it felt like this is the kind of thing where somebody out there might be able to figure out a technique for how to do it. We know what the end results should look like.
We just don't know what the intermediate steps look like, but it feels plausible with modern technology. And so you offered a million dollar prize and a 22 year old

or 21 year old with um uh you know gpu and some coded up a you know uh uh i think a cnn model to

decipher these squirrels and anyways um is there something in your field which has this sort of

feeling where the uh there's something we need to figure out we don't know the exact right technique

but if you could put it out and just offer a million dollar bounty for it maybe somebody will

Thank you. where there's something we need to figure out.
We don't know the exact right technique, but if you could put it out and just offer a million dollar bounty for it, maybe somebody will come up with a cool new technique to figure it out. Yeah, I don't even, there's like many things in this area, but I'll give you the simple one and then I'll give you, I probably should give you a single answer.
But I think that the basic answer is what we need is DNA from Africa. So we need DNA, old DNA from 50,000 years ago, 100,000 years ago, 200,000 years ago from all over Africa, because it's super clear that our lineage is complicated within Africa.
There's archaic forms in the archaeological record, and modern human data is extremely substructured with evidence of having come together from many different lineages, which must have been different archaic forms in Africa, and contributing to people living today. So having that would crack our understanding of how modern human lineages braided together and relate to the other archaic lineages we have data from.
So that's obviously extremely valuable. And what does it need to get those samples? I think we need to, A, identify those skeletal remains or the sediments in old caves that are well-preserved or rock shelters that contain enough DNA to extract.
And I think we need extraction techniques that will allow us to get at that material. Maybe we even already have them, and we just need to wait until that begins to happen.

But it would be revolutionary,

because the experience in Eurasia has been

when we get DNA from old sites or new sites

for which there's been nothing, we find denisovins.

We find people like we completely didn't expect to see before

that break our understanding of the past.

I think the other area where I am super excited,

and I think it would be a thing to reward and to incentivize, would be to try to crack this body of information to try to understand how biological adaptation happened in the last hundreds of thousands of years. We simply don't know the answer to your question from a genetic point of view about how modern human cognitive and other types of propensities, how they develop, the biological underpinning of the differences that modern humans have from our closest living relatives.
We just don't know how they evolved. It's not even clear how biological they were.
But being able to interpret the genome in terms of how these changes occurred is we just don't know how. I was at a talk a few years ago that was really shocking to me, which was based on, there was a researcher at Caltech, and she was talking about being able to directly read the brains of macaque monkeys.
A monkey would be shown 2,000 photographs, and her student would be recording from different neurons in its visual cortex and learning the neurons' response to different images. And so what they would do is they would decompose the images of faces, human faces, into eigenvectors with the principal component analysis.
And then the neurons, specific neurons, were responding to particular eigenvectors. And they learned the language of how the photographs and the decomposition of them computationally mapped on to the neurons.
And they actually learned a language for how that's the case. And then what they did is they showed a 2001st photograph to the monkey.
They recorded from its neurons, and then tried to use the neurons to reassemble a photograph. And it was a perfect reassembly, the photograph.
They had actually completely learned how the brain, this cac's brain, represents the photograph going through the brain representation. So in that case, they were able to completely figure out the language of appreciation of a photograph through the biological representation of it.
And if you look at the parallel problem of the genome, how does the genome code for development and how we get to how we are today, how do we have our capacities, and so on, to me it sounds like at first principles, if ask me, what's a simpler problem, figuring out how to represent the natural world in our brain or figuring out how to code for development? I think my cognitive bias, if you were presented ab initio, this problem would be to say it's easier to code for development than to represent the outside world in a brain. But this group and other groups are figuring out how to do this nearly perfectly with a readout from the brain.
And we really can't read a genome and tell you how a person looks, how a person develops. We can begin to say what terrible diseases they have, but not even predict that so well.
And so that's very depressing that we can't actually read the genome enough to actually see how that occurs. And we actually don't even know how evolution happens.
Like, for example,

does evolution happen by lots of little changes pushing in some direction? Like, for example, if we want to move toward a different positive set point for height or for, you know, some

cognitive capacity or propensity or something, you know, is this by infinitesimal change of

polygenicity, many genetic variants pushing in the same direction? That's the mathematician's

bias. Or is it like the example I told you about before with David Gokman and Liran Carmel, with the voice box, where everything pushes in the same direction and goes up to 100% and shifts all in the same direction in an incredibly simple and simplistic way? If you talk to neuroscientists and molecular biologists, their brain tends toward the latter, and this few examples suggest that maybe that's occurred.
And so maybe this kind of polygenic paradigm of adaptation, when adaptation really matters, is that really what happens when important adaptation matters, happens? Or is it instead something simple and simplistic and reliant on a small number of genes? So what I would really like to know is, can we mine the genetic data we have from modern genomes and archaic genomes? We now have Neanderthals and Denisovans. We now have some early modern humans who are far enough back in time that appreciable change may have occurred.
And can we actually learn the patterns of biological adaptation well enough to actually read the code of how we change and how we adapt to new pressures? And I think that that's something that's not impossible to imagine. We can learn how to do, but it takes a different way of thinking.
One thing that would also be interesting there is one big debate in trying to forecast AI is how big is the information content that describes the human brain? Because with AI models, we know, obviously we can tell very easily, like here's how difficult it is to, here's how many bits it takes to encode the parameters. But if you want to go back to, like, how many bits is it to encode the training paradigm itself, there's obviously the training code, then there's the hyperparameters, and here's how many kilobytes that is.
And the question is, we know that the human genome is three gigabytes, but we know only a small fraction is protein coding. And then also, how do you count the percentage that is responsible for regulation and so forth? But if you could only get the part that is responsible for the brain, how big would that be? Because can we compare how big that is with respect to how big the trading code for a model is?

And then we'll give interesting insights into how similar those two processes are.

So what we're beginning to be able to do, I mean, I don't know how important the particular class of work I'm involved in right now, but we're engaging with this in some way right now because we have incredible data from Europe in the last 10,000 years with huge numbers of samples where we can watch very small changes in frequency over 10,000 years. So in this period of time, which is not a particularly important time in human evolution, it's well after the important stuff happened, it's the last 10,000 years, but it's an eventful time.
The environments became very different, the lifestyles became very different. And so this is a period of time where we've done an experiment of nature.
A push has happened against the human genome. There's agriculture.
There's people living more densely. There's infectious disease happening in a different way, in a different type than before.
And how does the genome respond to this traumatic set of conditions? And so you can actually watch all these little variables, all these little gene frequencies, tens of millions of them shifting up and and down in coordination. And what can you learn from that? Because we now have all the measurements, right? We have a selection coefficient measured at 10 million positions across the genome.
And we know what the effect of those are on traits today, because they've been measured in large numbers on the order of a million people today. So what can you do with this data set? How relevant is this to important evolution? So I think that that's the type of rich data that could potentially be mined to learn something sort of qualitatively interesting beyond the storytelling that's characterized molecular biology, beyond the FOXP2 where you say, oh, maybe it's this, maybe this is the holy grail, or maybe that, maybe that's the holy grail.
Maybe, maybe you learn something about the process that's, that's, that's, that's, that's, that's deep and profound. And so I think that my million dollars goes to someone who can actually come up with a way of thinking about the process.
That's really kind of qualitatively profound. Interesting.
All right. Well, you're here to, I guess we need to find the million dollars first, but somebody, if you've got a million dollars and somebody else, if you've got the idea,

well, we can make a market here.

One of the interesting things, we were talking about the contingency of human history and human evolution. And one of the really interesting things is not only is it contingent, but it seems to be persistent, at least across the last few thousand years and the way that genetics have changed, culture has changed.
So when the Indo-Europeans, the Amnaya, disrupt, you know, whatever, the Indus Valley civilization 4,000 years ago or something like that, not only does that mean that the languages, which are spoken in India today, or at least many of them, are descended from this group, but literally the actual core of hinduism are descended from this is this initial group how is it possible that for 4 000 years the things like caste things like basic mythology can be preserved with such high fidelity especially in an era for half of that you don't have writing um not half of that for for at least a couple or two thousand years, you don't even have writing. How is that sort of persistent cultural heritability preserved? Well, you're asking me a cultural question, not a genetic one.
So what you see in the genetic data from South Asia is an amazing process. So today in South Asia, almost everybody is on a gradient of ancestry with two poles, what we call the ancestral North Indians and the ancestral South Indians, with very few exceptions.
The exceptions are people with your last name, Patel. Oh, yeah? And as a minor exception, but it's interesting that that's your last name.
But also people from Munda who speak Austro-Asiatic languages or are unmixed with them, or people who are Tibetan-Burman speakers. But most people are on a mixture between two poles, ancestral North Indians and ancestral South Indians.
And when you look at genetic data from India, it looks like what you see today in African Americans with people with relatively higher or lower proportions of, say, European and West African ancestry. And so it looks like a population in the process of mixture, like African Americans, who are the result of mixture in the last 10 or so generations between mostly two very different populations, mixing in different proportions.
But what happened in India is it froze. So the mixing started and and then it froze.
And the freezing happened 2,000 to 3,000 years ago, and it froze because of cultural change. So what happens in India is you have a three-part change.
You have an arrival of three source populations. It's actually parallel to what you see in Europe.
There's a local hunter-gatherer population. There's what's probably a farming population, maybe also a hunter-gatherer population initially.
And then there are these people descended at some level from steppe pastoralists. These are the three primary ancestral populations.
They come together at the end of the decline of the Harappan civilization, which ends about 3,800 years ago. And groups from this Harappan group, which we actually have sampled, and they're all on a different gradient.
They mix with the steppe groups and with the local hunter-gatherer groups to form and coalesce to these two later groups, which we call the ancestral North Indians and ancestral South Indians. And then mixtures of these two mixed populations form in the gangetic plane, form people all along this gradient.
And it's really a very simple mixture of two sources. And then the cultural change happens, which locks in the caste system, and people freeze, and they stop mixing very much.
And so what you see is instead of people collapsing to a point, which is what you see in Europe after this type of mixing process of these three sources happen in any one region, you see this gradient forming, and it's stable. And because of the enduringness of the caste system, you actually have a snapshot going back a couple of thousand years without this continuing change.
And so it's kind of an amazing system genetically to look at because of people's reluctance to mix with people from very different groups in traditional communities. And so the three steps are coming together of very

different populations, and then convulsive profound mixing of groups that had previously not mixed,

and then locking into this static system as the caste system sets in, which is documented in the

early texts like the Rig Veda. And you can actually see the change in that discussion during the course of the Rig Veda.
I know you warned about being too interested in yourself, but what was it about the Patels? Why are they an exception? So the first good genomic data from South Asians is embarrassingly from Houston, Texas. So in the Human Haplotype Map Project, there was a sample from Houston, Texas of Gujaratis in Houston, Texas.
A lot of Patels in Houston.

G-I-H.

And if you look at them, people are actually not on this gradient, but they're in a few

different places.

They're clustered into groups.

And there's the main gradient, and there's an off-gradient group.

And I forgot how we figured this out, but someone figured out that these people are

all Patels.

And Patels have their own distinctive history with different relationships to people in

Central Asia. And it's probably some additional ancestry from Central Asia, pushing them off the main gradient.

Interesting. We've obviously talked about so many different types of fields.
I'm not sure where exactly, in what field you started your research, but obviously now your lab is doing stuff in like genetics and you have to touch on how does your research um combine with archaeological record what are the inferences you can make from that or obviously with different kinds of history there's so many different disciplines here and how does one sort of like how you start your field at a certain researching a certain topic how do you you just keep expanding? Now I'm going to master archaeology. Now I'm going to master anthropology.
Now I'm going to, like, how does that process work through your career? It's very unstable life. So I think that in some areas, like in archaeology, a lot of my colleagues who I respect tremendously, the career trajectory is you learn to become an archaeologist, you dig, and you have a set of digs that you're doing for dozens and dozens of years with similar or slowly evolving techniques.
And my work has just changed so radically. When I started doing this work, one could not sequence a whole genome.
The genome was not yet sequenced. We had very little genetic variation accessible.
The amount of data has increased by orders and orders of magnitude every few years. The types of data that we collect, the ability to collect ancient DNA beginning 14 years ago, the ability to generate the volumes of it we have.
We had no ancient DNA in 2009. And then in 2014, we only had a few hundred individuals with genome-scale data.
We have tens of thousands of individuals with genome-scale data. We had data from places we didn't have data before.
So it's such a destabilizing process. And so someone like me wanders into areas that I'm not expert in.
I'm not South Asia, and I get to be part of trying to learn about history of South Asia. I get to interact with archaeologists at the cutting edge of learning about ancient Southwest Pacific or ancient China or ancient Southern Europe.
It's like an incredible privilege, but also I'm a kind of rank amateur in terms of a lot of the work I do. So one wanders from one area one's an amateur into another area one's an amateur in and tries to learn a lot.
Maybe this is a little bit like what it's like in Silicon Valley right now, constantly doing new things and bringing some skills to bear that are useful and hopefully trying to be respectful of the people one works with and the tremendous knowledge people, and to learn as much as one can and to work with other people to try to produce some joint research product that makes progress. Yeah.
How do people, so like, I don't know, somebody's doing archaeology for their entire career on a certain group in some mountain somewhere, and then you come in you were like you know here here's a paper we figured out what what the exact right exact genetic combination that explains all your research is is it like is there a reaction usually um uh i i don't know how much of this you can say but like basically are people sometimes disappointed that you've been able to figure out the things in their field with a different technique? I think that a lot of people we work with are incredibly excited about being able to do this. Prehistory is a period of time we know so little about.
We have such poor clues. True archaeologists who are truly dedicated to understanding the past are super thirsty for knowledge about the time periods.
And if a new scientific technique becomes available that can probe these times, the true archaeologists who are truly interested in the past get incredibly excited. And they embrace it as they've embraced previous scientific techniques, such as scientific archaeology, such as isotopic analysis, such as radiocarbon dating.
And that's been my experience with people again and again in archaeology, with people who really want to know about the time periods before writing when at some point one didn't even imagine one could learn anything, being excited about this new type of information. I think sometimes people are dug into particular views of the past that are challenged by the new findings that come from scientific research, such as ancient DNA.
And when the DNA is strictly in opposition to some of these models, that becomes an area of tension. And I think I have found myself to be proven wrong in a number of cases, including by my own work or by other work amongst my colleagues.
And I hope to be someone who can welcome that. One of my idols in this field is the archaeologist Colin Renfrew, who is a British archaeologist who is responsible for the Anatolian theory of Indo-European origins, The idea that farmers spread Indo-European languages, the language spoken in Armenia and in Iran and in northern India and in much of Europe today, spread with farming after 8,500 years ago from Anatolia in all different directions, and that the demographic expansion and economic transformation associated with that spread farming.
It's very plausible, and there was a debate with Maria Gimbutas and others who argued that these languages spread from the steppe north of the Black and Caspian seas, and one of the main arguments for the Anatolian hypothesis was that steppe expansions could not have been demographically significant because they were much thinner on the ground than farming expansions, and that this is why the steppe could not explain it, even though other linguistic arguments made the steppe seem more plausible. And so when the genetic revolution happened with regarding to our understanding of Yamnai expansions and Indo-European origins in 2015, Colin Renfrew at some point said, I was wrong.
I was wrong about this topic. In fact, the weight of evidence now suggests that in in fact, demographic transformation did come from the step.

It's kind of amazing it did.

Maybe it's from disease. Maybe it's from something else.
Who knows what it is? That's a very interesting topic. But we adapt.
We learn. So I think that this is incredibly inspiring to be able to change one's opinion.
Final question. So you mentioned these different revolutions in our ability to understand the past.
radi like radiocarbon dating to obviously now with ancient dna and genomic sequencing um is there something that feels like the next thing along the spectrum because one would hope in the very future like a thousand years from now the future ais are looking back on human history and um hopefully there's like no loss period hopefully

like literally they they know what kind of gods the the tribe in the near east that basically uh settled eurasia worship like they would know everything right and then uh along that spectrum we're making progress but is what is the next thing after uh advances in more more genomic sequencing or more samples from different parts of the world?

I think I don't know. So the discovery of the ability to extract DNA from ancient human remains was such a shock that we could even do this.
We just didn't think we could do this. There's a section in the introduction of my book, which was sort of my impression of what it was like.
I had a conversation with my PhD supervisor about what it would be like if one somehow could open a cave or a room that was echoing still with languages that don't exist anymore, that are not yet spoken. And you could hear the words still echoing somehow after thousands and thousands of years and record that down.
That's what ancient DNA is like. It's an unexpected gift from the past that what we thought was an incredibly delicate biological molecule, in fact, is intact.
And there must be other such things. Just it's hard to imagine what they are.
In ancient DNA, there is an extraordinary amount still to do. So there is systematic sampling from many, many places in the world where there has not yet been sampling.
There is systematic sampling and the ability to sample from deep, deep into the past, up to the point where we can begin to decouple these lineages from each other. I think that will reveal incredible richness.
And I think that that's something that we should all look forward to, the insights that come from that, both in terms of the understanding of individual places, including places like many parts of Africa and South Asia and Australia and New Guinea and so on, where we have essentially no data currently in terms of ancient DNA, but also in terms of deep, deep, deep time and the deep lineages that mix together to form us

where we really have no sampling

except for the Denisovans and Neanderthals right now.

I think that's a great place to close.

David, thank you so much for coming on the podcast.

I highly, highly recommend your book,

Who We Are and How We Got Here.

I mean, yeah, it's just so wild.

Just basically a lot of the stuff you learned in grade school

at least needs a lot more

clarification.

Some of it is wrong.

And the fact that that's the case is crazy.

And I hope that, I don't know, in five years, 10 years, there's a new edition of the book

or a new future book you write that all the questions that we talked about today, which

we don't have the answers to.

It seems like there's a bunch of progress happening here.

And I'm very eager to see what the future results look like.

Great.

Yeah.