David Reich

It's not a more important part of the world than other places, but it's the place where maybe 70 or 80% of the data in the ancient DNA literature so far comes from due to historical reasons.

And it provides us with a natural laboratory where we can see what happens over one place over time as environments change to the genome.

It's really interesting to imagine doing this type of analysis in other parts of the world.

And the comparative analyses are super important and interesting.

But this study right now is about this one place in the world where we have particularly fantastic data.

The other thing we did is we developed an entirely new methodology that hadn't been used in this area before.

And the methodology is based on a technique that had been developed for finding risk factors for disease in medical studies.

And a simple way to explain it is we ask how to predict the genetic type a person has based on its pattern of relatedness to other people.

include the ancient and modern people.

And then we look at how closely related each of these 22,000 people are to each other.

And we predict the genetic type at each position in the DNA at 10 million positions based on the pattern of relatedness to all of the other 22,000 people.

And then we ask if natural selection blowing the frequency of the mutation in the same direction in all the geographic places and at all times predicts the data a little bit better than just knowing the relatedness to all the other samples in the database.

So we're simply asking...

The alternative hypothesis is that selection has been blowing in the same direction at all times.

And we simply ask if that explains the data better.

And that's a dumb assumption because, of course, the truth is that natural selection is going to have changed in frequency over time.

But we're just asking the simplest of questions, whether assuming a constant rate of selection explains the data more than not doing so.

So when we analyzed the data this way, we looked at 10 million positions in the DNA that in these 22,000 people, 16,000 of them were ancient.

And we looked to see if there was more change in this consistent direction over time than you would expect by chance.

And when we analyzed the data, we found many, many hundreds of places in the DNA that were changing too much over time in too consistent a way to be explained by chance.