Sean Carroll's Mindscape: Science, Society, Philosophy, Culture, Arts, and Ideas - 179 | David Reich on Genetics and Ancient Humanity

Episode Date: January 10, 2022

Human beings like to divide themselves into groups, and then cooperate, socialize, and reproduce with members of their own group. But they're not very absolutist about it; groups tend to gradually (or suddenly) intermingle, as people explore, intermarry, or conquer each other. David Reich has pioneered the use of genetic data in uncovering the history of ancient humanity: what groups existed where and when, and how they interacted. The result is a picture of churning populations in constant flux, including "ghost populations" that no longer exist today. Support Mindscape on Patreon. David Reich received his Ph.D. in zoology from the University of Oxford. He is currently a professor of genetics at Harvard Medical School. Among his awards are the Dan David Prize, the National Academy of Sciences Award in Molecular Biology, the Wiley Prize, the Darwin-Wallace Medal, and the Massry Prize. He is the author of Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past. Lab web page Harvard faculty page Publications Wikipedia

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Visit vital proteins.com to learn more and where to buy. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Hello, everyone. Welcome to the Mindscape podcast. I'm your host, Sean Carroll. We talked about DNA and genomes here on the podcast quite a bit. There's a lot of information locked up in our DNA, both information about how we as organisms or other organisms function right in the here and now, but also there's information about our history. Remember, a few months ago we talked with Betuel Kachar about paleogenomics, learning about very, very early life.

by looking at the current genomes of different organisms and seeing what they have in common. But if you want to look at human history, you're talking about thousands or hundreds of thousands of years ago, not billions of years ago. And that gives you a different thing that you can do. You can look at the actual DNA of the actual humans from tens of thousands or hundreds of thousands of years ago. Can't do that if you're looking a billion years ago. There's no DNA left from a billion years ago. So this is a new field of science that has really exploded in the past decade or two, learning about ancient human history by looking at the actual genomes from skeletons and fossils of real human beings who lived back then. And today's guest, David Reich, is one of the world's

leaders in this field. He's made many discoveries along these lines in his lab. Perhaps most provocatively, he's one of the people who discovered the fact that there is Neanderthal DNA in most human beings today. Other than Africans, other than people whose ancestors have been in Africa the whole time, most other human beings have a tiny amount of Neanderthal DNA mixed in. So this tells you something, since Neanderthals were Europeans and Asians predominantly, and human beings, Homo sapiens, came out of Africa. What this means is that when the humans came out of Africa, they got it on with some Neanderthals, right? They mixed up the gene pools a little bit. And that kind of analysis can be done at many layers in many different times,

and we learn a lot. A lot of surprising discoveries have been made. I don't want to give away all the surprising discoveries. David will tell us about them. But there's this idea that has come about of ghost populations. You know, you find some archaeological find, and look, so there's some civilization or some settling. Maybe it's not really civilization, but some group of human beings. They use certain tools or they hunted or they gathered or whatever. But how are they related to other groups of human beings? The DNA is telling us this.

And so the ghost populations are populations of people who no longer exist, but there's little genomic remnants, little trace signals in the DNA of other human beings that lets us indicate that they were there and think about, how they related historically to other populations. We can use this DNA evidence in conjunction with evidence from archaeology and language and even written history to put together a much more nuanced and complex view of how human beings have developed ever since human beings came on the scene roughly 160,000 years ago. So again, this is one of those areas which we like to do on Minescape where it's kind of brand new. You know, anything that didn't exist when I was a graduate,

student, as far as I'm concerned, is completely brand new. And that means that we're just at the beginning of figuring out what we're learning. But David has interesting things to say about human behavior and history in Africa, Europe, Asia, the Americas, the whole bit. Lots of surprises along the way. So let's go. David Reich, welcome to the Mindscape podcast. Hi. I have to start with one amusing anecdote because I get many people on the podcast in many different areas. And without trying, I keep noticing that a lot of people, even though they're doing, you know, neuroscience or philosophy, they had some physics in their background. And I promise I'm not trying to do that. But indeed, I noticed on Wikipedia somewhere that you have a little bit of physics in your

background. Yeah, I got an undergraduate degree. And so that means that I studied physics formally for a few years. But I didn't go on to get a graduate degree or do develop a real expertise in any particular area. Ah, but you actually not just took courses, but got a degree in Sure, my bachelor's degree is in physics. Did you have hopes to become particle physicist or astrophysicist? I applied to grad school in atomic, optical, and molecular physics, and I was going to go to Berkeley, but I deferred it, and I ended up getting distracted. Distracted in a good way, clearly. Does the physics education color how you do your DNA research these days? I think so. I think that a lot of my work relies on trying to be sophisticated about quantitative thinking about the data that we produce.

And that the laboratory is a kind of hybrid laboratory that both generates data, but very much also develops methodology that's data-driven in order to be able to analyze the data in new ways. And that's always been central to our laboratory. In fact, even the primary thing. And I think I'm able to do that because I have this quantitative background. That's very nice. And there's a revolution going on. But let's explain because I think we've talked on the podcast before about paleogenomics, right, about learning about early life from modern genomes, given that there's a certain diversity of life and they have certain things in common. You can say things about the past. But that's not exactly what you're doing. You're actually taking advantage of digging up bones of human beings and looking at their DNA. Yeah, that's right. And so if your interest is in deep, deep time and trying to understand the history of life, the time scales you're interested in are millions, tens of millions, and hundreds of millions, maybe even in some cases thousands of millions years. But ancient DNA, DNA doesn't really preserve more than a million years and mostly doesn't preserve more than hundreds of thousands of years. So if your question is to try to understand events, for example, associated with mammalian radiations and dispersal,

it's not going to help you very much. But what it does do is it actively and powerfully interrogates the last tens of thousands and maybe hundreds of thousands of years. And so modern humans descend largely from a common ancestral population in the last 200,000 years. And so what we can do with ancient DNA is sample DNA from all around the world from known times and places and archaeological sites and see how the people whose DNA we obtain is related to each other, the other sites and also to people today.

And that allows us to understand how the diversity of the world got to be the way it is today. And what's actually quite interesting is that when one does this, when you look at the DNA from ancient archaeological sites and see, test how it's related to other archaeological sites and people today, it's consistently surprising. And so the reconstructions one makes from populations living today are often quite different from the reconstructions you make when you actually looked at DNA from known times and places. and that's telling you that people have moved around too much, such that the present-day diversity around the world as obscured what was there before. And is your team actually going out there and digging up bones, or do you partner with the archaeologists who are digging up the bones?

It's almost all partnership with hundreds of archaeologists around the world we work with on a variety of projects. So they'll throw you a bone, quite literally, and you will take out its DNA. Is this something that we've known how to do for a long time, or is that actually part of what is new and fun? So the ancient DNA field has been around for maybe 40 years, but it's only become really serious in the last 10 years. And it's been around 40 years because people for the first time were convincingly getting snippets of DNA out of old samples. But really until 10 years ago, the only substantial amounts of DNA people were getting were from the mitochondrial sequences.

Mitochondrial sequences are the energy factories of cells. It's about 16,000 DNA units, DNA letters long. and that's about one, two hundred thousandth of the human genome in size. It's only a small fraction of our genome, and it contains the record of our mothers, mothers, mothers, mothers, mother, an entirely maternal lineage, which is, of course, fascinating, important, but it's limited in terms of its statistical information

it provides about the past because it's only a single statistical instantiation of the evolutionary process recorded in mitochondrial DNA. But, of course, the whole genome records not just one's mother's mother's mother, but one's mother's mother's father and mother's father's mothers. All of one's genealogical ancestors, at least possibly, in practice, tens of, or hundreds of thousands. And with all that data, one could obtain precise information about how people are related to each other.

And, I mean, maybe good background distinguishing between mitochondrial DNA and nuclear DNA? What do we call the rest of the DNA? So your DNA is packaged into 47 units in your cells, in the great majority of your cells, 23 pairs of chromosomes and your mitochondrial sequence. Your mitochondrial sequence is carried in the eggs that your mother's egg is fertilized, and it occurs in several thousand copies typically per cell. But each cell also has one copy each of those 40-scent six DNA packets, the chromosomes that come in 23 pairs.

So the copy number of them is much lower than that of the mitochondrial DNA, but there's much, much more information in them. 200,000 times more of your DNA is in them, and that's where almost all the genetic information is encoded. It used to be that people studied mitochondrial DNA because it occurred in thousands of times more copies. I was going to ask, good. Given how degraded the material it is,

maybe that's where you start because you have a better shot of pulling it out if you have several orders of magnitude more material to start with. But with the new techniques, we can actually work with everything. And are the new techniques just cheaper? Is that the real, well, I mean, effectively for your purposes, Is the reason why there's been a revolutionary last 10 years just because it's easier to reliably sequence the ancient DNA? Or have we learned something sort of qualitatively different about how to analyze it?

I think it's all of those things, but it's technically driven, as many of these things are. And we're probably getting DNA out with maybe eight or nine orders of magnitude more efficiency than we were 13 years ago. Efficiency in this case means... Cost, say. Okay. And the reasons are number one, short-read sequencing, which is the DNA sequencing revolution, which made sequencing literally 10 to the 5th to 10 to the 6th times more cheaper than it used to be. And that happened in the late 2000s. And another reason is that the only way it was possible before to study DNA was to use polymerase chain reaction, PCR, which required pulling out segments of DNA by

putting unique primers down on top of the DNA, and that would end up sacrificing maybe 30 or 40 DNA letters just in order to pull out the sequence. But the typical fragments of ancient DNA are only about 40 or 50 bases. So if you sacrifice over almost everything just in order to pull it out, there's almost nothing left. And so what was happening is it was incredibly expensive to sequence, and the only way people were able to pull it out was by polymerase chain reaction, which lost almost all of the target. And so there was almost nothing left.

There have been major technical biochemical improvements that have massively increased the efficiency of the extraction, the purity of the extraction. So all of those are improvements as well. And the result has been was that beginning in 2009, 2010, it began possible to generate whole genome sequence data from human remains thousands, tens of thousands, in some case hundreds of thousands of years old. But when you say whole genome, are we only getting 30 or 40 base pairs? Well, the DNA is naturally fragmented by the degradation process. into fragments that are rarely more than 70 or 60 basis long. And the typical length that we get from sequencing is maybe 30 or 40 or 50 basis. But there are billions and billions of molecules even in a degraded DNA extract.

And by brute forcing it or using various tricks, we can get many hundreds of millions of independent DNA fragments and puzzle together genome scales worth of data. Good. That's what we need for background for the DNA. How about background for the evolution of human beings? Because it's always very confusing to me. There's a lot of names of different species and subspecies. What is the big picture story, let's say, from when humans and other primates split, which was a few million years ago?

So our closest living relatives are the chimpanzees and bonobos, and slightly more distant to that is gorillas. the chimpanzees and bonobo lineage split from hours probably somewhere between five to seven million, maybe eight million years ago. It's not even completely clear where that occurred. You mean where geographically? Where geographically that occurred. And the guerrillas a couple of million years earlier than that. In the intervening time, the lineage leading to modern humans was certainly at some times. almost certainly was in parts of Africa, although not necessarily during the whole time,

leaving skeletons like the Australopithecines and the artapithecines, and the early homo-human lineages like Homo Habilis and Homo erectus, which also dispersed to diverse places in Eurasia already about 2 million years ago. At which point the threat is not clear anymore where the ancestors of people, of humans, primary humans, lived. But beginning maybe five or 400,000 years ago, it's absolutely clear that the modern human lineage, giving rise to the great majority of people today, is again in Africa. And the earliest skeletal remains of people whose skeletons look anatomically modern are two to 300,000 years ago in different parts of Africa. And then there is an explosion of anatomically modern humans out of Africa in the Near East between 50,000 and 100,000 years ago that disperses around Eurasia and eventually very quickly also to Australia. in New Guinea, and then after 15,000 years ago, into the Americas, and then in the last

couple of thousand years to the last habitable places on Earth. And, I mean, that means we're very young, right? Species-wise, you know, we split off from Jimson Bonobo's millions of years ago, but modern humans only came into existence maybe 200,000 years ago. Or maybe three or 400,000 years ago. Not clear. And there's, and only 70,000 years ago, there were at least five groups living around the world that we know about today through a combination of skeletal analysis and DNA data that were

each as different from each other, much more different from each other than any groups living today, the most diverse living groups living today. But within a few tens of thousands of years, we were alone on the planet. These groups kind of were subsumed within, mixed with, or displaced by the expansion of modern humans or at the same time as modern humans were expanding. So it's not clear how young we are comparatively. To me, it's not, I think it's sometimes exaggerated how young we are or how non-diverse we are. If you look, for example, compared to chimpanzees. Today, chimpanzees, western chimpanzees, if I remember right, have about as much diversity as humans do.

But there are also some other chimpanzee groups like central chimpanzees and eastern chimpanzees and Nigerian chimpanzees. And together, all of those chimpanzees are more diverse than modern humans are. But perhaps they would not have been more diverse than humans were 70,000 years ago. My best skin ever at 45? Give me a theme song and a best skin care award because it feels like this. Right there. That's farmhouse fresh skin all right. I'm blowing and everyone asks how.

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See nutrition info on hero.com for sodium and sugar content. Well, so it is an interesting thing. We are taught about evolution of various species, and you see speciation in different branches and so forth. But this human story, you know, reminds us that it's hard to draw fast lines between species and what counts as species. Like you say, there was a lot of, I should put words in your mouth. A hundred thousand years ago, there were organisms that were sort of close to human that were all over the the place, but were they different species or were they sort of different subspecies? How would you describe it? I think that's a philosophical question. And as a biologist and as a geneticist,

I'm actually not so interested in philosophy and what is a species. It's not particularly important to me. I've thought about that topic a lot. A lot of people are interested in that topic. And in fact, our collaborative team has thought about that topic and we punted on it. We did not, we refused to engage with it. So in 2008, in 10, I was lucky to be part of a team that was able to sequence DNA from a tip of a finger bone from a cave in Siberia. And we obtained incredibly high-quality DNA from this tip of a fingerbone. And it turned out not to be from any known human group.

It was from a group that was almost as different from Neanderthals as Neanderthals were from human. It was separated by many hundreds of thousands of years, maybe 400,000 years from anything we had seen before. So it was incredibly exciting. We didn't have any archaeology that had pointed there to being a different group living in this region. And so it was incredibly exciting to have this data. And some of the people on our team wanted to call this a new species name. That's one of the ambitions of many paleoanthropologists to have a paper which coins a new species name. The name was going to be homo-altiensis, you know, human from the Altai. And we had a long debate and discussion in our team. And we decided to just call it by a common colloquial. name Denisovans. And the reason we decided to do this was that it was completely unclear to us what a species is. The classic biological definition philosophically is determined by maybe the biological species concept originally suggested by the biologist Ernst Meyer who argued that species are groups of organisms that do not in practice interbreed with each other. And what we had seen in our

genetic data is that these groups were interbreeding with each other. And so, and we're able to form successful mixed groups, like all of non-Africans today who are the descendants of mixtures between Neanderthals and modern humans, or like people in New Guinea today who are descendants of mixtures between Neanderthals-Denisavans and modern humans. So if the definition is that mixtures of groups do not incur and do not lead to successful populations, if they do occur, then our situation did not qualify. On the other hand, the physical anthropologists have argues to us quite often and said, well, the Neanderthals must be a different species, not by the biological species concept, but by the morphological species concept.

They're so different from each other in terms of their forms. They need to be considered a different species. They're very specialized. I don't have an opinion. It feels very philosophical to me, and I'm not so interested in philosophy. I love philosophy, but I get that there are good philosophical questions and less useful ones, right? But is there a way of, from a more scientific perspective, at least saying, that there was greater diversity of human forms 100,000 years ago than there is today?

Or is even that a bit of exaggeration? That's absolutely correct. So if you actually, you can imagine some kind of measure of variation amongst populations living around the world. Maybe that measure of variation is that if you take two random individuals from, you know, geographically around the world, what is their average genetic differentiation from each other? and how much larger is that compared to the differentiation within the population? That's known as FST. It's a measure of how the variation is partitioned,

and it would have been larger 70,000 years ago or 100,000 years ago than it is today. Today, that number is maybe 0.1, and it depends on how you compute it. But on average, the amount of genetic difference between two individuals or two genomes within a population is six times greater than the average across two populations. So only a small amount of the variation existing within humans today is a cross-population variation. But amongst chimpanzees today and amongst humans many tens of thousands of years ago, that would have been a relatively higher proportion.

Okay. That makes sense. And let's zoom in on this because there's a famous and incredibly fascinating example of, you know, the humans, the predecessors of monsters. human fan out from Africa 50,000 years ago. And one of the places they go is to Europe or Eurasia, maybe, and they meet the Neanderthals. So one question is, is it Neanderthals or Neanderthals? Is there an age in there? I've seen it either way. They're both correct in the English language.

Oh, that's too bad. So in our papers, we've used both depending on journal style. Fair enough. I grew up with Neanderthals, so that's probably what I'm going to try to stick with. But Neanderthals, again, they can clearly interbreed because that's what they did, right? And And so how recent is this understanding and what exactly did we learn? Yeah. So Neanderthals from archaeology are an incredibly impressive group of humans that lived in Western Eurasia, not just Europe, but also places like Central Asia and even to parts of East Asia, probably.

So there's a debate about how far east they ranged. Neanderthals appear clearly in the skeletal record a few hundred thousand years ago, and late Neanderthals persist in Europe until about 40,000 years ago. at which point they seem to disappear from the skeletal record as a distinct group of humans. Neanderthals in Europe, late Neanderthals, as well as earlier Neanderthals, were large. They were as big or bigger than modern humans. Their brain capacities were as large or larger than those of modern humans, even though their skulls were differently distributed. So you said as large or larger.

So if you do allometrically scaled brain capacity, it's just as large or larger. And they're also larger individuals. And they made complex tools that must require a rich amount of learning and skill in order to create and had been doing so for hundreds of thousands of years prior to the time they disappeared. So these humans occupied Western Eurasia, and we know they encountered modern humans as modern humans expanded out of Africa in the Near East. We know that because we can see the archaeological and skeletal remains of modern humans as they expand out of Africa in the Near East between 150,000

years ago. And we can also see sites, for example, in parts of France where there are Neanderthals and modern humans living side by side and alternating occupations in the same caves. We can see that less sharply in the Near East in places like present-day Israel in the Mount Carmel region, where you can again also see Neanderthals and modern humans, but not clearly at the same time the way you can see them in Europe, but in fact, it seems likely they were living there at the same time and interbred, because that's the most likely location where the interbreeding probably occurred. And so a question has always been, as modern humans expanded out of Africa and the Near East, to how did they interact with these

other impressive humans who they encountered, who were using at least initially similarly complex and sophisticated economic and life history strategies. And the genetic data, beginning in the 80s seemed to be suggesting that modern humans, people living all around the world today, shared no DNA that was plausibly derived from a group like Neanderthals. All modern humans, for example, on the mitochondrial DNA, the maternally inherited DNA, or also on the paternal DNA, on the Y chromosome, all seemed to descend from a common ancestor in the last couple of hundred thousand years, which was highly unlikely to come from a group like Neanderthals, which would minimally have diverged many hundreds of thousands of years or even a million years ago,

people thought at the time. And so the- Sorry, had that been true, would we have thought that that's because the modern humans wiped them out or just couldn't interbreed with them? Or was there even a theory? Multiple possible explanations. Some population geneticists, people in my field, argued that there could not have been very much interbreeding because we know from studies in ecology of lots of different species that if there's even a little bit of interbreeding when a small pioneer group, even with an advantage, spreads into a region, occupying, by a group that's more well established and larger in population, as you might imagine,

modern humans might do as they expand it into a territory previously occupied by Neanderthals. If this occurs, even with a low rate of interbreeding, they'll eventually be swamping of the nuclear genome by the genetic material coming in from the larger group if the process is not instantaneous. And so again and again in different types of animals, you see this nuclear swamping effect during a range expansion, such that even though the population that migrates out is successful ecologically, genetically, the existing population somehow leaves a major imprint. There was no evidence of that, and so it suggested that maybe there was very, very little interbreeding, or if there was interbreeding, there were some kinds of incompatibilities,

as they call, you know, either socially or biologically in causing these groups to interbreed. So when I went to grad school, that was the orthodoxy. That was the situation. We thought there was no evidence amongst modern humans of divergent ancestry that might be consistent with substantial interbreeding with a group like Neanderthals. Okay, but. So, I mean, I think that one of the questions that we were interested in once we knew that we could obtain genome-scale data from Neanderthals, once Svante Pabeo's group working in Leipzig, made it clear that they would be able to get genome-scale data from Neanderthals, which happened in the last year.

of the 2000s, and they pulled together a consortium, which I was lucky to be involved with, once it became clear that that was going to be possible, there were a number of questions, and one of them is, what is the relationship of Neanderthals to modern humans? One of those questions is, did they interbreed with people they encountered like non-Africans, the top candidate would have been the ancestors of Europeans? And there were other questions like, when did this lineage split from that of modern humans? would it have been a few hundred thousand years ago, would it have been a million years ago?

So these were the types of questions we were after. And so when we looked at the data, that was my job to study the relationship amongst these groups along with other people we were working with. One of the questions we asked, of course, was are they more closely related to some humans than to others living today? Because that would be perhaps a sign of interbreeding.

If there was interbreeding with some humans more than others, then you might expect the descendants of those humans to be more closely related. When we looked at the data, we saw such a signal, and we were actually quite incredulous at that signal because I certainly came from a background which thought there was no such signal. And so I was very skeptical. And also, some features of the signal were surprising. There was no excess signal in Europeans. In fact, the signal in East Asians was just as strong.

And so that was surprising, too, because East Asians, there were no Neanderthals in East Asia. And so this was really seemed to be a sharp divide between sub-Saharan Asian. Africa and Eurasia and the rest of the non-African populations. So that was surprising too. So what we did over the next couple of years is we really wrestled with this observation. We thought maybe this is a artifact of our dirty data, of various problems with our data that might occur. Contamination, which had afflicted our field.

And so we tested the data in various ways and stratified the data in various ways, looked at various types of this type data, looked at various types of. of modern human data, looked at various technical processes of the data computationally, and developed several different statistical techniques, which would independently look at different types of information that is recorded in genetic data about how populations are related to each other. And they were all pointing very clearly and consistently at a history of interpreting. So essentially, every population of people here on Earth does have some Neanderthal

DNA in them other than the ones who stayed in Africa. That's close to true, although sub-Saharan Africans today, to many sub-Saharan Africans today have some degree of Neanderthal ancestry, possibly due to small amounts of back-to-Africa gene flows over the last tens of thousands of years. And we can't currently rule out some persistent Neanderthal exchange, but it's at a much, much lower level, if at all. So what does this teach us? I mean, what are we then thinking about the relationship between the early humans and the Neanderthals? Were they friends? Or did they go to war and capture some slaves? Is that something that we can even hope to answer?

I think we could probably hope or we should try to learn more than we currently know. But currently there's a number of alternative scenarios that are consistent with the data. I'm not trying to mince words at all. But what is clearly the case based on the archaeology, not based on the geneal. is that when Neanderthals and modern humans encountered each other, there was a period of coexistence, but that the coexistence in any one geographic region was limited in time, you know, at most a couple of thousand years in the parts of Western Europe where it's been looked at most carefully carefully disappear. However, it's also true that a lot of the modern human groups that

they coexisted with also disappear. So it's not obvious that the Neanderthals were out-competed by modern humans. In fact, there's some archaeological evidence that some of the Neanderthals even picked up modern human technology and learned from them, and especially in France, where there is some archaeological evidence from this. But after about 39,000 years ago, many of the early modern human archaeological cultures that are evident in Europe before that time, as well as Neanderthals, disappear and are replaced by a relatively homogeneous archaeological culture that potentially re-expans from one place, maybe even the Near East. So it's not just that the Neanderthals disappear, it's that there's many human groups disappearing, one of them,

or multiple of them are Neanderthals. And whether that's because modern humans displaced Neanderthals because they had a more different psychology that sort of was incompatible with coexistence or a different life history strategy or whether there was active conflict over limited resources or whether they were just more modern humans because they had a different economic strategy

that it allowed them to exploit the environment more intensively and the Neanderthals just got absorbed through mixture. We don't know. Okay, okay, because I was going to ask, but I think you just answered it, should we broadly think of the, if we were drawing these diagrams of species coming up on a tree through time, should we think of Neanderthal's ending or being absorbed back into the rest of humanity? And maybe the answer is we just don't know. Well, I think that's a philosophical question, which is probably interesting.

Okay, good. Well, you could at least delineate which philosophical questions are interesting or not. Okay, so, I mean, that was, and this was, you know, a revolutionary discovery, this is not what people really thought, like you just said. I mean, this is something that we've learned very recently about the genomic history of modern humans. That's right. It's this and many, many other findings from the application of this technology have been shocking and surprising. And again, all within the last 10 years, it's safe to say, roughly speaking, you have a graph at some point of the number of genomes that have been sequenced

that is sort of like single digits for a long time and then just explodes. Yeah, that's right. And And so, for example, in 2009, it was zero, and in 2010 it was five, and maybe in 2014, it was maybe 40 or so. And now it's well over 7,000. Okay. So we're learning a lot. So, and now we're at the point where it's just so many interesting things going on that I want to hit some of the highlights and then feel free to chime in if I'm missing a highlight.

But clearly there was a lot of action in Europe post the intermingling of modern humans and Neanderthals. Right? So, I mean, I hear my notes roughly between four and 10,000 years ago. There were different populations in Europe. And so one question is, is it just, do we think that there's all this diversity that we know about in Europe just because we've studied Europe more? Or do you think that it's a reflection of the actual migration patterns and so forth? I think that everyone who thinks about this data should think about the data from Europe, not as indicating that Europe is in any way. a special place, but just as a reflection of what's possible to do when one has a lot of data.

And the reason we have so much data from Europe is for several reasons. One is that it's in European laboratories that the technology for doing this work developed first. And it's also European archaeologists who have been pretty consistent over the last century and a half at assembling skeletal material and keeping it in museums and other collections. And there's also resources in Europe and in the United States and for doing this kind of work. And the field's so young that people are now only reporting the DNA that they started working on, even though it's rapidly expanding to other parts of the world. So Europe's not more important than other places of the world, but it's a place that you can use almost as a kind of laboratory to understand what's

possible with applying this type of technology to learn about the past. And what we now have is many, many thousands of whole genome data sets published and many, many more unpublished, more than we have published, that provide a nearly gapless record of European population history and space and time over the last 10,000 years with a more gapy record between about 50,000 and 10,000 years. And that's an incredibly powerful resource. We are locked in two dimensions today in terms of our understanding of human variation. But we now have a third, and in Europe, it goes back deep in time, and it's dense, and it's possible to do things that we couldn't do before. And there's a sense in which, if you apply this sort of logic to, or this sort of way of

thinking to Europeans today, they're all one population in a way that there were different populations in the near past. That's right. I mean, I think that Europeans today are not one population, there's substantial variation amongst Europeans and more broadly Western Eurasians or people who are called, quote, Caucasians or white on the U.S. census. But there's actually pretty substantial differences amongst these groups, but they're genetically much more similar to each other, everybody in that category usually, than, for example, Europeans are to East Asians today. And it wasn't always so.

So if you look in the places where 400 years ago, people of that U.S. census category would have lived, the level of differentiation amongst those groups. a few hundred years ago would have been similar to what it is now, but roll back 8,000 years and there would be many groups in that region as different from each other as Europeans and East Asians are. So this was one of the many senses in which people's intuitions, including mine, were wrong about the past. If you had asked me in, I don't know, 2002, what would be the level of differentiation within

Europe 10,000 years ago? I would have guessed that it would have been relatively modest, that this region, like today, would have been a region of relative genetic homogeneity. But in fact, you look at the data, that's completely not the case. And if you were to assign census categories in the past, they would have broken down along completely different fault lines from the way you break them down today. So the past is not really well described by the present. It's very important to keep that in mind.

And one of the things we see again and again with ancient DNA data is when one goes and collects DNA from archaeological context that have never been interrogated before with this technology and makes a guess beforehand about how these people would be related to people living afterward and people living today, almost always that guess is wrong and often profoundly wrong. And we know this now not just in Europe, but in many, many places in East Africa, multiple places in East Africa, in South Central Africa, places like Malawi, in Cameroon, in India, in Pakistan, in Central Asia, in East Asia, in Japan, in multiple parts of the Americas,

Essentially, everywhere we look almost, we see patterns like this, where again and again, you see that groups in the South Pacific that are not directly ancestral or primarily ancestral to people living today. Love bread, bake goods and pasta, but not the way they make you feel? What if I told you there are macro-friendly options that don't taste like sawdust and sadness? Satisfying sandwiches, fully loaded bagels, noodles noodles, noodles that can stand up to your favorite chunky sauces, all delicious. Craveworthy and smart.

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sodium and sugar content. I like the way you put it that there were populations in what we think of as Europe 8,000 years ago that were as different from each other as modern europeans are different from East Asians. That's something we can visualize a little bit. How much do we know about these populations? And in particular, there's this wonderful idea of a ghost population, like a whole group of people that we wouldn't even think existed just on the basis of looking at who exists today and moving backwards. Only through looking at DNA, do we realize, oh, there's this whole kind of person that we didn't know existed? Right. So ghost populations are groups that emerge out of models. So when you write down a statistical model, a mathematical model, for trying to understand

how you derive the ancestry of present-day populations from previous populations, models are never perfect, but you write down a model and you can do a goodness of fit test to the data. And you could say, oh, this population is a mixture of two, three, four ancestral populations at different times in the past. And when you do such a model, for example, for present-day European populations, to first approximation, in many European populations today can be relatively well described as a mixture of three ancestral populations. We knew that already in 2014.

And you can ask, what are those three ancestral populations look like in order to fit the data? And in fact, none of those populations exist today. The one that's closest to existing today are groups like present-day Sardinians, but even Sardinians are not a perfect proxy for one of those three ancestral populations.

What happened is that these three ingredients source populations for present-day Europeans are predicted based on the genetic data of people today, but they're not present anymore because they got mixed together and they exist in different mixed proportions in people living today. So what often happens is when you look at modern data or a mixture of modern and ancient data, you can see that there probably was, or there must have been, or there parsimoniously was, a earlier population that now no longer exists in mixed form, but contributed substantially to one or more populations living today. That's what we call a ghost population, a population we

reconstruct statistically from groups living today, but we don't have samples from today. And what we find again and again with DNA is we predict these populations and then we see them once we obtain DNA from the right time in place. So we see them in the archaeological data, in the bones, in the DNA from the bones that we were actually decayed. Correct. Yeah. Okay. So and are the Yomnia one of these? Am I pronouncing you correctly? That's right. That's a ghost population where they live? So the Yamaya are a group of people who are archaeologically were expand about 5,000 years ago across the steps north of the Black and Caspian Cs. The earliest Yamniya are probably north of the Black and Caspian Sea somewhere, maybe in the Don or the Denea River valleys or in the Volga River valleys. but they develop a new economy that is based on at least two major inventions that didn't exist prior to the time that they expanded that weren't used in those regions before.

One of them is the invention of the wheel, which was invented around that time, and we don't even know who invented at first, because once it was invented, it spread like wildfire. But plausibly it was the amniah? It's not obviously not the amniya, but it's not obviously them either. And they certainly picked it up pretty fast. And the other was the domestication of the horse, and it was probably not fully domesticated yet, but they were using domesticated horse and domesticated herds. And they took their wagons, and they hitched them to domesticated animals, and they moved them out into the open steplands, which had been previously uninhabitable

because they were far away from water, but they were able to take their supplies out into these regions, and graze much larger herds than was possible before on the large biological resources, for example, the grasslands of the steppe. And these people expanded very dramatically. So, instead of prior to this time, there were many diverse archaeological cultures making different types of ceramics and pots. But after the Yamnai expand, there's a monoculture in homogeneity across a vast region all the way from Central Europe, Hungary, all the way to Mongolia, as these people expand very rapidly. It was not known how this impacted the populations of places that had been previously and continued to be densely settled, like Europe. but we now know from the genetic data that this was a massive impact.

So in Northern Europe, more than half the ancestry of many northern parts of Europe today descends from this expansion. And it's not necessarily like an empire, right, like with a central rule or anything like that, but the culture of these people, like you said, spread from Europe to Central Asia? That's right, yeah. So I think that the, and the DNA from these people spreads from all of these places. And so I think that many archaeologists would tell you, do not confuse this with an invasion, and they'd be right to emphasize that. I mean, maybe this is about the time of the earliest Egyptian civilization, so maybe there there would be capacity for an invasion and for organized state society. But up on the steps, there would not have been that type of organized state society in the same way.

We do not know to what extent this was systematic rating or exploitation. Some people argue it might have been. Some people argue it might not have been. But in any case, this group was very effective in terms of expanding. And there's a number of arguments about whether this was related to economic exploitations of niches that were not previously exploited related to grazing or use of new technologies or to what extent this was active and involuntary displacement. I mean, since there's so much that we don't know, is it a responsible speculation to wonder whether they might have been more centrally organized than we give them credit for that, I mean, there might have been a 5,000 or 8,000-year-old empire that really did answer to a single government?

So this is not my expertise, but I'm almost certain the answer is very powerfully that that would be extremely unlikely. Okay. So these people were highly decentralized. There's no evidence of, and there's a lot of archaeology, there's no evidence of centers of power or central settlements. There's no evidence of large congregations of individuals. There's no evidence of systematic, you know, warlike events, even though there is evidence, perhaps, or there could be evidence maybe of raiding and certainly kinds of evidence of violence.

I think people, if they were interested in violence as a kind of mechanism for these groups expanding, would be more interested in ideas like cattle raiding or resource rating or, you know, raids on other communities to take, you know, by men after women or various things like that. but not organized in a large way. People argue that mythology spoke in Europe and India today, associated with Indo-European language-speaking cultures, has shared traits that might descend from a population that spread these languages and some of that shared mythology. And people try to reconstruct some of the shared values in these ancestral Indo-European language speakers.

And people argue, based on this, that this would have been a society that was focused, focused on practices like cattle writing or various male-centered expansion practices that are seen in kind of distorted, reflected form in Indic mythology and Nordic mythology and Greek mythology and so on. Well, and the application of gender to these questions is not purely hypothetical, right? Because we can separately look at the DNA from the petriline line and the metriline line by looking at Y chromosomes and mitochondrial DNA. And X chromosomes in the rest of the genome because X chromosome is a kind of female colored chromosome because two-thirds of the X-cromosomes running around in the world today

are in women compared to only one-third in men, whereas for the rest of the nuclear DNA, it's half and half. And so you can kind of, the X-chromosome is huge compared to the other parts of mitochondrial DNA and Y-chromosome, which are only one instantiation of the evolutionary process, whereas the X-chromosome is thousands. And so it actually contains quite a lot of information. So even though the information is in some sense not as crisp as that of the Y chromosome and mitochondrial DNA, it's arguably in some cases may be usually balanced out by the many, many independent flips of the evolutionary coin,

allowing one to learn more precisely how groups are related to each other. And so by looking at the X chromosome and solving the system of equations, where in one case it's two-thirds, one-third, and for the rest of the genome, it's one-half, one-half, one can extrapolate out what the male and female contribution to different events would have been. And so we have that tool available to us, too, to understand the process of sex bias in mixture processes of different populations. And presumably, unsurprisingly, one's guess is correct that there are certain men out there who are very good at spreading their seed all over the world. That we have our most recent common male ancestor is probably more recent than our most recent common female ancestor. The opposite is true, actually.

So, no, I think your argument is correct. The argument was one thing. The truth is the oldest common male ancestor is about twice as deep as that of the most recent common female ancestor. So we estimate based on counting mutations since a common ancestor, which serves as a molecular clock, and knowing what the mutation accumulation rate is approximately, we estimate that the common female ancestor is roughly 150 to 180,000 years ago, approximately that. And until recently, we thought that that was about the same for the Y chromosome. But then a personal ancestry group studying African-Americans found a Y-chromosome type that was twice as deep in African-Americans. And then surveys in Africa found a higher frequency of this type, specifically in Cameroon, where there's a number of groups that have a substantial frequency of this Y-chromosome type, which has persisted.

But in studies of many, many, many, many, tens or even hundreds of thousands of people, nobody has found a mitochondrial sequence older than this one. So it's actually quite interesting, the Y chromosome and mitochondrial dynamics spread around the world. Mitochondrial differentiation across groups tends to be lower on a short geographic scale and potentially higher on a large geographic scale, whereas the Y chromosomes are the opposite. So females and males migrate at different scales. So in a lot of communities, but this is not true exclusively, women are the ones who are the ones who, move between communities. So many communities, but not all communities, are patrilocal or very local. And so you'll have, typically, men will range less far from their homesteads than women. So you might have, I don't know, in a model, in a diffusion model, you might have in this community, women,

on average, diffusing 10 kilometers or 20 kilometers from their homestead and men maybe not as much. And so that type of process will result in mixing on a scale of tens or hundreds of kilometers. But long, long range migratory, and so that on the mitochondrial sequences, but long, long range mitochondrial movements seem to be, human movements seem often to be propelled to a larger extent by male movement. And so that would produce more homogenization, Y-chromosomily, on a large scale. So there's a scaling factor geographically that is not obvious. So women diffuse a little bit more quickly, but men either stay at home or go way far away.

Maybe, yeah, on a scale of thousands of years, with exceptions. There's always going to be exceptions. I think we can take that for granted here. I was going to ask about how we should envision what is actually going on in these migrations. Is it like hundreds or thousands of people picking up stakes and moving from one place to another? Or is it, you know, a few explorers going out? Or do we have any idea about something like that? I think we have an increasing amount of information about that process.

But that is one of the big open, interesting questions that can be addressed in principle and that we have a lot of, um, possible technical ways to make progress and addressing through looking at this type of data. But I think that in different instances, there are different processes that one might imagine are occurring. This June, the world comes to Los Angeles. Kick off FIFA World Cup 2026 at the FIFA Fan Festival at the iconic Los Angeles Memorial Coliseum. Watch matches live on Giant Screen. Feel every goal with thousands of fans

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Can we help understand that by understanding how these populations moved around? Because it was always surprising to me that India, which seems pretty isolated from Europe in ancient history in my brain, shared this sort of language family with us. That's been surprising to people for more than 200 years ever since it was noticed by people. who learned Sanskrit and also had learned Latin and Greek and noticed that these ancient languages were pretty similar to each other as well as the languages derived from them or related to them. It's been a big mystery about why people across such a broad region all the way from India to the Atlantic shores of Europe spoke similar languages and places also like Armenia. And actually, archaeological work has since uncovered Indo-European-speaking peoples

and places where Indo-European languages are not commonly spoken today, including early divergent forms of them, for example, in Anatolia, spoken by the Hittites several thousand years ago, and even in the eastern deserts of the present-day territory of China. And so it's been a big mystery how these languages got to spread so far. And we know from ethnographic studies that languages tend to spread through movements of people. And so large-scale movements of people, although there are exceptions to that, where there's elite population conversions of the languages, as occurred, for example, in Hungary.

But usually it's through large-scale movements of people. So one is tempted to argue that there were movements of people that were vectors for spreads of these languages. And since the timescale over which languages are discernibly related to each other is really only shallow. At some level, it's really not more than 10,000 years. And some people even think that's a stretch in terms of reconstruction of shared languages. that provides us information or hypotheses that we can test with genetic data about how particular

language groups spread. Indo-European languages are an amazing instance, but by no means the only instance of amazing language spreads. One of the most amazing is the spread of Austronesian languages. These are the languages spoken throughout the Pacific and with the most diversity in Aboriginal peoples of Taiwan, but spoken throughout the Pacific from Easter Island and New Zealand in Hawaii through many, many of the islands of the Pacific, through Indonesia, almost all Indonesian languages are Austronesian, and even Madagascar off the eastern coast of Africa,

speak these languages. This expansion occurred just in the last three or four thousand years and is geographically and humanly, at least as dramatic as that of Indo-European. And there, too, there's a question of the movements of people that spread these languages. But there's others as well, everywhere in many, many places in the world. But Indo-European is an amazing example. and one of several where we have strong insights from the genetic data. Well, and for the Austronesians, what do we know, is the genetic data helpful there? Or is it just not there yet?

We haven't figured it out? Yes, it's super helpful, maybe even more clarifying in some ways than the Indo-European language data at this point. So linguistically, the deepest roots of Austronesian languages are in Taiwan, as I mentioned. If you look at Aboriginal groups in Taiwan and study their languages and build a tree of how those languages are related to each other and to the ones spoken in this much vaster region. The deepest roots of that language tree, the earliest splitting ones, are all in Taiwan. There's like eight or nine or so deep splits, and all but one of them are in Taiwan. In Taiwan, okay.

And then there is a cascade of splits that are reconstructed that suggest a possible expansion route through the Philippines and then in different branches, both toward western parts of Indonesia and also eastward jumping into places like the Open Pacific for the first time, like Vanuatu, Fiji, Samoa, Tonga, and then bursting even further out into Polynesia. And so there's this whole cascade of linguistic bifurcations that has been used to argue for possible spreads of people. The genetic data creates a tracer die. If you look at Taiwanese Aboriginal-related ancestry, that is present in essentially all groups that speak these languages, including in Madagascar, who are admixed with mostly sub-Saharan African ancestry,

but about a third Austronesian ancestry that's hardly present or not present in the rest of mainland Africa and is consistent with being spread by these movements of people. So what we see in the Indo-European case and also in the Austronesian case are these tracer dyes, which are distinctive genetic frequency distributions that are characteristic or of the ghost population that you think may have originally spread these languages, and the genetics adds to the linguistic data in allowing one to trace the possible and likely movements of it. One particular case that's very interesting about this

in the case of the Austronesian language spread is that first spreads that brought people out into the previously uninhabited islands of the Pacific, the last major uninhabited habitable places for humans on Earth, which really only began to be inhabited about 3,000 years. ago. So humans get to the Solomon Islands and Bismarck Islands archipelagos off the coast of New Guinea and Australia about 40 or 30, 40, 50,000 years ago, very anciently, very close soon after the original spread of modern humans out of Africa and stop. So there's other islands not so distant,

well, they are pretty distant, but not so distant from those that are empty of humans. And then after 3,000 years ago, they begin to be peopleed by humans. And that's they're peopled by people using ocean-faring technology that was invented by these, almost certainly by these Austronesian speakers who use it to spread from the Philippines and ultimately from Taiwan, skirting the coast of New Guinea, and then they make it to places like Vanuatu and New Caledonia and eventually, and quickly Fiji and Tonga and so on. And these people are able to go over large ocean spaces and navigate, much earlier than other Chinese and European navigators were able to cross these large ocean spaces.

If you look at the genetics of the first people who break this ancestry, they look almost entirely East Asian, like Taiwanese, aboriginals. But the people who live in Vanuatu today have almost no ancestry from this group. And what you see is there's this initial expansion 3,000 years ago or so

that gets to Vanuatu with almost entirely Taiwanese-related ancestry. Today it's maybe only 10%. And then maybe 500 years later, there's a massive wave from New Guinea, from people related to present-day New Guinea, that almost completely displaces the local population, sporadically in different ways in different parts of the islands,

and leaves only ultimately maybe 10% from this initial wave that may not even be originally, mostly from Vanuatu itself, but from other groups that were picked up along the way. It's a very sex-biased process where primarily male people from New Guinea mix with these seagoing ocean people. You might have thought that these seagoing technologists from, ultimately from Taiwan might be your bias, might be to think that those are the expanding males, but it's not true. It's the Papuans who are

the male biased expanding population in that case. So there's many instances like this that we're learning about from DNA and that are very surprising. This was a surprise. One might have thought that the mixed populations of the Pacifics with both large amounts of Papuan ancestry and all of them and large amounts of Taiwanese ancestry and at least some of them, including Polynesians, where it's maybe 75%, might be evidence of a long drawn-out process of expansion. But when you look at the data, it's a nearly unadmixed initial Taiwanese-related expansion, followed by massive movements from New Guinea, a place where most people in their prejudiced picture of the world might not expect large-scale movements of people, but in fact, it's the primary

ancestry of many of these groups. So it's an extremely dynamic picture of human beings. It's not like a population goes, takes over a land, and just dominates it for the next 10,000 years, right? Like there's waves of people coming all over the place in Europe and everywhere else. Yeah, and I think we forget that in our cultural memory. I mean, I, you know, come from a Jewish background, right? From Europe, that's where my second degree and third degree ancestors are from. And, you know, the history of Jews in Europe in northern Europe is one where people kind of get comfortable, become integrated in the community and are valuable, valued

members of their community, or at least tolerated members of their community. for a couple of generations. And then there's a pogrom and the community's wiped out. And it's a disaster. Everybody's worried. And then they kind of come back. You know, the next generation, they get comfortable. They think they kind of forget that there was a problem.

And then a few generations later, there's another pogrom. So I think on a deeper, longer time scale, I think human history is often like this. People often have a picture in their mind that they descend from people who have been there forever and that there hasn't been migration and mixture between groups that. that are quite different and maybe even alien is not an important part of their past. But in fact, looking at genetic data makes it absolutely clear that there are very few, if maybe no groups in the world that don't have major mixture as part of their history. So I think that's a bigger sort of pattern that becomes very clear when one studies this type of data,

which is that migration and mixture are very central and integral to us when we think at the temporal scale of many thousands or tens of thousands of years. Yeah, and it's longer than a human lifetime. So we just think that anything longer than 100 years is permanent as far as most people are concerned. But there are two examples. You have many, many wonderful examples in the book, and I encourage people to read it. But there are two that I don't want to let pass by without talking about them. India and the Americas.

I mean, we already mentioned India a little bit. But what was remarkable is the idea that these sort of population flows can be relevant to the caste system in India today. There's sort of different populations that have a rough car. respondents to how Indians live today. Is that a fair thing to say? I think that there is correlation to the caste hierarchies and variation. The caste system in India is, I think, very hard to understand for everybody, including for me, and certainly hard to understand for people outside of the system, but has multiple aspects to it, including a kind of organization into strata of people who are in a sort of hierarchy in maybe five or four strata of priests and, you know, warriors or rulers or

merchants and commoners and people who are outside the caste system and people who are, you know, even lower on the scale. So that's one stratification. But there's also stratification into groups that have different local economic and tribal identities. And there's at least 5,000 of those groups. And many of them are endogamous. They really don't mix with other groups. Even those they live amongst. And the genetic data that we've studied, this has been probably the single biggest focus of my laboratory over the last 12 years, the genetic data shows that many of these groups really have not mixed with very much, with other groups they've lived amongst for thousands of years. And you can see the signatures of that in the genetic data, where groups that

may today have population of a couple of million people, in fact, have lived cheek by gel next to other groups, and in fact have, through social norms, made it almost impossible for members of their groups to mix with neighbors. So what you have in India is to some extent a frozen picture of the structure that existed thousands or a couple of thousand years ago after the system locked in, at least in the ancestry of some of these groups. So it maybe makes it possible to use present-day groups to go back a little bit deeper in time and to record a little bit about what the population structure might have been like a couple

of thousand years ago, where there's more mixture and there's less sort of barriers amongst groups, although there are still barriers amongst groups in some groups in Europe. So what you see in India is that there are gradients. People in India today have a very complicated mix of ancestry, but many groups in India today can be relatively well approximated statistically as a mixture, as a mathematical mixture, of two ancestral populations. One could be called the ancestral North Indians,

which are a group that are relatively more closely related to Central Asians and Middle Easterners and Europe. Europeans, ultimately. And another is called the ancestral South Indians who bear much less relationship to those groups. And we've known that since 2009. And we now know through analysis. That's still this crazy reason. I'm sorry.

We now know this through analysis of ancient. And these groups are quite different from each other, kind of as different from each other as Europeans and East Asians. And people range all across the spectrum of proportions from these groups, although everybody is mixed. There are exceptions to this, but most people fall in this category. and they follow the two major language groups in India, Dravidian, and Indo-European. But those two groups we now know from co-analysis with ancient DNA are themselves mixed, more

anciently, of three more ancient groups. One is an Iranian-related group. One is a step-pastoralist-related group. And a third is an ancient Southeast Asian-related group related to indigenous Southeast Asians, New Guineans, Andamanese Islanders, and more tribal groups from some people. Southeast Asia. Would all of these qualify as ghost populations or some of them? So these would all be ghost populations that no longer exist in a mixed form. But we have them all sampled in DNA. And in fact, the endomones we have directly sampled with relatively little mixture. And we're not arguing these

people come from the endaman Islanders, but rather than the endomenees descend from a group that had little mixture of compared to the group that's actually the true source population. So this is a model for South Asians today. And if you look at the proportion of ancestry from step pastoralists. It's not very high in India today. It's never more than about 20 percent, and it goes down to almost zero percent in some groups. But we can tell that that step ancestry pulsed into India between 4,000 and 3,500 years ago by comparison to diverse genetic data we have from Central Asians and people from the northern parts of South Asia. We know exactly when it comes in. It's relatively much higher in frequency in people who are of

traditionally higher social status in the Indian caste system. It's especially high frequency adjusting for other factors in people who are from the traditionally custodians of the Indo-European texts like the Brahmans and Bumihars in the Indo-European texts. And so this suggests that this step ancestry is associated with Indo-European associated culture in South Asia, adding another line of evidence to this pulse of ancestry from the step about 4,000 to 3,500 years ago, with earlier ancestry from the Amnaya we talked about before, being responsible for spreading these languages into India and militating against other explanations for the spread of these languages. There's very strong evidence now from genetic data

that cross-Iraimian plateau spread is very unlikely. And I was going to say that the timescale is about that of the Rig Veda, right, the classic Indian text that is still very important to modern India. Yeah, critical, as well as many. the other texts, but people reconstruct the writing of the Rig Veda to maybe three to four thousand years ago, and that would be consistent with arising something around this time of this spread. You could probably do a whole podcast just on that, but I know that you have to go. So let me just ask a little bit about the Americas, because I was fascinated to learn there is one of the populations that settled in the Americas is called the first Americans, but it turns

out they were not the first Americans. Yeah, it's a kind of tautological kind of name. name first Americans. And, you know, in some ways, maybe it's not an ideal name because it's a statement about priority or about history in the name itself. And maybe we should try to avoid such names in the future. But almost all the ancestry of Native Americans prior to today that doesn't come from European and African migrants in the last 500 years comes from a single ancestral group that we call First Americans that likely spread into the Americas maybe around 15,000 years ago or before, almost certainly across the Bering Land Bridge from Eurasia. We know a lot about

that group. We have many early skeletons from that group, although not from the earliest times. And we actually know about their earlier formation from different strands of ancestry present in Eurasia and represented by different ghost populations represented from DNA data. So that's very interesting. However, there are hints in the genetic data, maybe strong hints, that actually the earliest spreads in the Americas were not homogeneous, and in fact, there might have been a substructured spread of modern humans into the Americas after 15,000 years ago. The Americas was uninhabited by humans before that time, or much before that time, although there's now increasing hints in the data, and the archaeological data, maybe people argue there might have been earlier presence,

but large-scale occupation really begins only after. 15 or even somewhat after that time. And in Brazil, in particular, there are some groups that bear relatively more close relatedness to people in Southeast Asia and Australia and New Guinea than do the early spreads than the major population. So that suggests the possibility that the early spread is substructured and a later expansion, maybe 12,000 or 13,000 years ago of people that are the primary first American population created small pockets of remaining groups

that exist in mixed form in some parts of the Americas today, like Amazonia, and bear greater relationship attesting to this earlier population spread. It would be really exciting to get ancient DNA that has a large proportion of ancestry from this group. I am not 100% sure this group even really existed.

These are four sigma effects. I don't know. I don't think they rise to Higgs boson level of significance. And so maybe we should treat them as not yet competitive. Well, this is a perfect segue into the final question, which is, where are we going? I mean, this whole field is so young. Like, I have this slight resistance to the existence of any field that came into existence

after I got my PhD. But still, okay, new sciences come along all the time. What are the next steps? Where are we going in the decades to come? What should people be looking out for in the front page of the New York Times? Well, I think it's been a bit of a wild west field. It's been really exciting. Every new sample has been, every new sample.

from a previously unexcavated archaeological context has been incredibly exciting. So we're a little bit in the kind of prospecting phase where there's been an emphasis on getting even one or a few samples from previously unknown archaeological context. And the field is also in the hands of the technicians. So the people who know how to generate this data reliably and convincingly. and also people who are custodians of the set of tools, both laboratory tools and technical tools to convincingly analyze the data. Now, this creates a sort of kind of like a bit of a problem because the actual people who know the most about the topics that are being interrogated are not the people who are doing the primary research. And so someone like myself can have the great privilege of, you know, the great privilege of, analyzing data on the one hand from Thanoatu, data on the other hand from Europe,

and learning about all these places. But I'm not really deeply trained to know the context. We have to collaborate with archaeologists, and we try to do that, and we aim to do that. But really the future is one where the people who really know about these topics will be able to learn enough about these fields to use the technology to address the questions that they know are most interesting. And I think that transition is now happening in the last couple of years. And for example, we have a paper coming out in two weeks, which is a really good example. of that. It's a case, a paper where we were writing a paper on the history of Britain in the last

few thousand years, and we were reporting data from 793 individuals in one paper. That paper is going to be published in December 22nd. And dumped in the back of the paper was a family tree we had reconstructed from a single tomb in Britain from about 100 years after farmers got there almost 6,000 years ago. So in this figure, we had found a big family that we've now reconcrucreated. constructed 27 people all buried in the same tomb, all we know exactly how they're related to each other. And this co-author of our paper, which had 223 authors, almost all of them archaeologists, who had contributed samples to this large paper. He had contributed some samples not relevant to that story, but something else. But he noticed this data, and he said, that's my expertise.

I spent almost all my time looking at Cotswold Severn Long Cairns in Southern England dating from this time. And there's all these archaeological questions about their meaning and what they meant to their communities. And I'd like to look harder at that. So he talked to one of our geneticist co-authors. And they looked really hard at this family. And what we can tell is that the tomb was one where it was entirely patrilineal. Everybody buried in the tomb descends from the same patriarch who reproduced with four women. And we can see that. And it wasn't a normal patrilocality, though. and because kinship and fatherhood was defined not just biologically, but we actually see multiple cases of adoption.

So this man and his descendants adopted other sons from that their wives previously had with other men and incorporated them into the pedigree. So what we're seeing is a community where there were different rules of kinship and you can learn about that and answers questions for these people and these archaeologists about what these tombs meant to the people who use them, and it's the kind of thing that I think archaeologists are particularly excited about. It's the kind of study that archaeologists can lead. And so that's, I think, sort of the future, one of the futures of this field,

which is really taking this technology and midwifing it and handing it and translating it and transferring it to the people who really can pose the questions the best. I've seen genomics field after genomics field doing this. I remember genome-wide association analysis, which I've done a lot of work in, especially in the 2000s, trying to see associations to diseases like diabetes or prostate cancer. I remember those studies originally were carried out in studies of hundreds or thousands of individuals by geneticists who were technicians who really knew how to carry out these studies.

But quickly, these were transferred to the hands of the epidemiologists, the people who really knew the questions best and were able to assemble cohorts of hundreds of thousands of people. And that's where that field is now and that's where genetics and ancient DNA are going. I foresee specialization in the future, but it sure is exciting to be there on the ground floor when you can do the Americas in Africa and Europe and Asia all at the same time. And so David Reich, thanks very much for giving us this introduction to a really exciting time. Good. Thank you.