Dwarkesh Podcast
David Reich – Why the Bronze Age was an inflection point in human evolution
08 May 2026
Chapter 1: What is the main topic discussed in this episode?
I am back with David Reich, who is a professor of ancient DNA at Harvard. How do you describe what it is that you study?
I'm a geneticist, and I work on human history and how ancient people relate to each other and people living today.
Great. And so we did an interview, was it two years ago at this point? Which ended up being one of the most popular interviews I've ever done. I think people just found really compelling that there's so much about human history we don't know and are just learning about now as a result of the kinds of techniques that your lab is using.
And you have a new preprint that's very exciting and I wanted to talk to you about it. So let's begin. Can you give me a little bit of context on what we're talking about today?
Well, the dream was that when this field started, this ancient DNA field started more than 16 or 17 years ago, that we were going to learn a lot about biology, learn about how people's biology changed over time by getting DNA out of ancient human remains and tracking changes over time. And that dream has really not been realized since the beginning of this field.
So while the field's been a big success with regard to learning about human history, it's resulted in surprising findings about human migrations, people not being descended from
from the people who lived in the same place hundreds or thousands or tens of thousands of years before, and mixture being common in human history, sex bias processes being common in human history, and things that were not expected from archaeology. And so the field's been a big success from that perspective.
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Chapter 2: How has natural selection changed in human evolution?
But what's not been successful is learning about biology and biological change. And one big reason for that has been that the sample sizes have been too small. So when you have a single person's DNA, it provides a tremendous amount of information about history. And that's because when you look at one person's DNA, it's not a single person. It's many people. It's your two parents.
It's your four grandparents. It's your eight great-grandparents and 16 great-great-grandparents and so on. And going back in time, thousands, tens of thousands, even hundreds of thousands of ancestors going back in time contributed to people today.
So when you look at the DNA of a single person's genome or a Neanderthal genome, you have effectively tens of thousands of ancestors all represented in your data. And you can position that individual exquisitely with respect to other people from whom you have data.
But when you are interested in how a particular genetic variant that affects something like your skin pigmentation or affects your ability to digest cow's milk into adulthood or affects a behavioral trait, when you want to see how that changes over time, a single person gives you only one sample or maybe two samples, the one that is in their mother and the one that's in their father.
And so to get a high-resolution picture of how the frequency changes over time, you need to have very big sample sizes of truly very large numbers of people. And we just didn't have that until the last few years.
So what motivates this study that we're, I think, talking about today and the work that hopefully another number of groups will be doing in the coming years is the fact that we now finally have those numbers and we can do something with the data to see how frequency changes over time.
Can I ask a question? I'll be asking a lot of naive questions through the next few hours, but why are frequency changes especially interesting?
So what we're interested in is using the experiment of nature that's occurred in our history over the last tens of thousands of years to understand what's biologically significant in our DNA. And if there has been a change in environment that...
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Chapter 3: What evidence suggests selection intensified during the Bronze Age?
a population has experienced, for example, people have shifted to agriculture or begun living close to domesticated animals or moved to a new environment from a cold place to a warm place or a low place to a high place, then there's pressure on the population to adapt to these new stresses, these new needs.
And the way you're going to detect that is you're going to see that the frequency of a genetic variant that, for example, might allow you to live at higher altitude, for example, or that might
sort of nudge you to have a different behavioral pattern that might be advantageous in the new situation, that genetic variant might push systematically in some direction in a way that is enough that you can detect it. Now, it's very hard to detect slight shifts in frequency by a few percent or a 10% unless you have a very, very big sample size.
And so what we're looking for are those changes in frequency that are too extreme to be due to chance, and that will tell us that there have been pushes against the biology as a result of the changes in environment that people have experienced.
Interesting. Okay. So what did you guys find?
So seven years ago, Ali Akbari, who at the time was a postdoctoral scientist in my laboratory and a few years later became a permanent staff scientist in my laboratory, set out to use the data that we were producing to learn about biological change over time.
And I think the reason he was interested in our laboratory rather than other places was that a focus of our laboratory has been generating truly large amounts of data from ancient humans. We've been really trying to industrialize the process, make it very inexpensive, make it high quality, and generate large numbers of samples with lots of good data for this purpose.
So there's been this large amount of data that we've generated, and it made it possible to conceive again of asking the question about whether there's been frequency changes over time. So the mainstream view in human evolution in the last several decades has been that natural selection has been pretty quiescent over the last several hundred thousands of years of human history.
And there's several lines of evidence that have been deployed to document this.
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Chapter 4: Why didn't evolution maximize intelligence in humans?
But however, if there has been natural selection, for example, to help people in one place digest digest alcohol better, or, for example, digest milk better, or do something else better, what you might expect is that there would be some mutation that would have rocketed up to very high frequency. And 40 or 50,000 years is a lot of time. It's maybe 1,500 or 2,000 generations.
And so that might be enough time, easily, to see 100% different in frequency. And yet you don't see any more compared to what you'd expect by chance. So this made it seem that just selection has been quiescent. Maybe a few hundred thousand years ago, the ancestral human population got to some kind of optimum. And after that, there hasn't been much genetic change in one way or the other.
And there's been small amounts of natural selection or there's been selection to remove bad mutations that are constantly raining down on the genome, but not what we call directional selection, which is newly arising mutations or mutations being pushed in a systematic direction to help the population.
get to a different adaptive set point that's more favorable for the conditions that population is living in. So we were able to partition how much of the changes in frequencies of all the mutations that we're seeing in the DNA, we're looking at about 10 million positions that vary, is due to directional selection, adaptation, versus other factors, especially genetic drift.
Chapter 5: What factors limited evolution before the Ice Age?
And 98% of it is other factors, especially genetic drift. So it's overwhelmingly migrations in population structure causing fluctuations in frequency. And as a result, it's super hard to actually detect the signals of natural selection, in adaptive natural selection, because they're a tiny fraction of the total frequency change. The vast majority of it are these migrations and mixtures.
Nevertheless, there's so much natural selection, as our study has shown, that in fact it's been rampant in the genome.
Can I ask a clarifying question here? So why are we discounting population admixture or replacement as selection? Because if you think about it at a group level, if one population replaces another population, isn't that selection?
Chapter 6: What is the Neanderthal puzzle that David Reich explores?
I remember from the last episode, you were explaining how there's been huge changes in what kinds of people are in a specific area. One population came in and kind of replaced the previous one. And then a new population came in and replaced the previous one. And to the extent that the genetics are relevant to why that population replaced the other one,
Why should that not count towards what we understand to be selection over the last 10,000 years?
It could count and may count and probably should count in some respects. But it could also be that this population replacement is due to some cultural phenomenon, technology held by one of these groups, not others. And maybe there's some genetic mutations that are contributing to this. Who knows? It's possible. But what you're seeing is a whole genome shift.
And so what we're looking to see is whether there's one place in the DNA that is driving the change in a way that's different from the rest of the genome. And really from a statistical point of view, what happens at these times of migration is there's just huge fluctuations in frequencies. And these are extremely uninformative times for looking and detecting natural selection.
The best moments to detect natural selection is when migrations and population admixtures are not happening for a few hundred years. And during these times, you can actually see the mutation slowly blowing in one direction as a result.
Really, the way we think about the history of Europe and the Middle East, and the way we think about it for the purpose of this study, is as an archipelago of little populations in space and time, each of which are pretty isolated from each other.
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Chapter 7: How does David Reich's methodology differ from previous studies?
A little population in Britain, isolated for a few hundred years. A little population in Hungary, isolated for a few hundred years, between big events of migration and mixture. And in each of those little experiments of nature, we can ask, does this mutation slightly increase in frequency? Does that same mutation slightly increase in frequency?
And if all the arrows point in the same direction, we win. And they're telling us that natural selection is occurring. So for example, 4,500 years ago in Europe, almost all mutations go through huge frequency changes. And that's not because of natural selection. It's because of the steppe migration from the steppe north of the Black and Caspian Sea.
40%, 50%, 80% of the DNA becomes Yamnaya from steppe pastoralists. And their frequencies of mutations were different, not because of selection necessarily, but just because they had evolved in different places for thousands and tens of thousands of years. And then if you look at the descendant populations, there's huge changes in frequency. Yeah.
And it's very, what you need to do is see, oh, is natural selection explaining a shift more than you would expect by chance?
Okay. In this next section, David explains the nitty gritty of the methodology of this paper.
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Chapter 8: What implications does this research have for understanding human history?
It's honestly a bit technical and I wanted you to get a sense of the results first. So I've moved that section to the end. If you want to understand the methodology, just stick around for the full episode. Okay. You found these locations that seem to be under selection. Oh, another clarifying question. So you have
You say 3,800 locations, which we were 50% confident have been under selection in the last 10,000 years.
7,200, where we're 50% confident. Oh, sorry. So I think we're getting about 7,200 positions in the DNA that have 50% confidence of being real. Yeah. So only half of those are real. We don't know which ones, so 3,600 of them are real.
Does that also mean that outside of those 7,200, you're confident the other location the genome are not under selection? No.
If you look at the 25 percent probability cutoff, there will be tens of thousands, and there will be many real ones there too. In fact, multiple analyses we do suggest that the genome is vibrating with natural selection, And there's all sorts of weaker effects that are there that would be picked up in larger studies even than we've done.
And that, in fact, almost every position in the DNA is correlated to a position and being dragged in one way or the other by natural selection. Instead of being quiescent, natural selection is everywhere. Even though it's only 2% of the frequency change, it's tugging the positions in one direction or the other everywhere.
So we analyzed these positions that we had identified, these hundreds of positions, the ones we were super confident about. And we looked to see whether they were randomly distributed in the DNA or whether they had patterns.
And what we did is we looked at maybe a hundred or so traits where there had been genome-wide association studies for all sorts of different traits, like ones associated with immunity or autoimmunity or behavior or metabolism and basically other things.
And for each of these, we could ask, are the genetic variations that are known to affect these traits from genome-wide association studies, do they have an unusual number of genetic selection signals? And what we found is there was a vast enrichment by about a four or five-fold for immune traits. That is, there was a super concentration of selected signals in immune traits.
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