David Reich – Why the Bronze Age was an inflection point in human evolution

I am back with David Reich, who is a professor of ancient DNA at Harvard. How do you describe what it is that you, you study?

I'm a geneticist, and I work on human history and how people, relate, 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 that's very exciting, and I wanted to talk to you about it. So let's begin. Can you can you give me a little bit of context and what we're talking about today?

Well well, the dream was that when this field started, this ancient DNA field started, more than sixteen or seventeen years ago, that we were gonna 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 the people who lived in the same place

And can can I ask you 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 in our history over the last tens of thousands of years to understand what's biologically, significant, in our in our DNA. And if there has been a change in environment that 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 a low place to a high place, then there's pressure on the population to adapt to these new stresses, these new these new needs. And the way you're gonn

Interesting. Okay. So what did you guys find?

So seven years ago, Ali Akbarri, who is a, 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, wit

I 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? I 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 why wh