David Reich

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.

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...

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.

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.

One is that if you compare diverse populations from different continents around the world, for example, Europeans and East Asians, and you look at mutations that differ in frequency between these groups, all mutations differ a little bit in frequency, sometimes a lot, you can say, what are the most different mutations in terms of frequency between Europeans and East Asians?

And there's almost no genetic changes that are 100% different

and frequency between Europeans and East Asians.