David Reich

And in any case, I mean, there's others too, right?

So some of the other plagues in the Roman Empire are definitely not Yersinia.

We have indirect information about some of these things.

So one thing that you might hope to learn about is whether our genomes reacted to the innovation of agriculture in a disrupted way.

So you might think that our genomes would have been in some kind of steady state, sort of natural selection had adapted us to the previous environments we were in.

And you might expect that in reaction to a change so economically...

dietarily, cognitively transformative as agriculture, the genome might shift in terms of how it adapts, and so you might actually see that in terms of adaptation of the genome.

You might expect to see a quickening of natural selection or a change.

I don't think we know the answer yet to whether that's occurred, although there are beginning to be hints, and we could learn that from the DNA data.

So one question is, so there's an increasing view amongst geneticists and that natural selection is...

a process where there's relatively little directional selection to adapt to new environments.

One piece of evidence connected to this is the finding that there's very few genetic changes that are 100% different in frequency between, say, Europeans and East Asians, or West Africans and Europeans, or West Africans and East Asians.

If there had been genetic variants that had had modest selective advantages, 1%, half a percent, 2%, that's actually a lot, but year by year, that had arisen, and then that's...

In a few hundred generations, they would have risen from very rare to very common and, in fact, gone to 100%.

There's thousands of generations separating Europeans and East Asians and West Africans and Europeans and so on.

So if that was a common process in evolution, we would expect many genetic changes to be 100% different in frequency between Europeans and East Asians or West Africans and Europeans.

We see almost none.

So what that suggests at some level is that there's not strong adaptation over the last 50,000 years, because if there was, we would have seen genetic variants driving to 100% frequency difference across different groups around the world, which have hardly been connected with each other genetically over the timeframe that we're talking about.

We don't see those variants, so maybe selection hasn't been important.

But maybe over a shorter period of time, selection has quickened and variants have started rising in frequency in the last maybe few hundred generations or something like that.