David Reich

Remember, the previous observation is that natural selection seems to have been quiescent over a timescale of hundreds of thousands or many tens of thousands of years.

Reason?

That you don't see 100% different in frequency variants across Europeans and East Asians.

Yes.

So now we're seeing hundreds of positions that are rocketing up in frequency with selection rates 1% or more in a lot of cases.

So 1% or more selection rates will mean that there'll be a rapid doubling over periods of dozens of generations.

And so over 1,500, 2,000 generations like you see separating Europeans and East Asians, shouldn't you see many genetic variants that are 100% different in frequency across populations?

We were able to show that this is explained by at least two factors.

One of them is that we actually, in this part of the world, Europe and the Middle East, are in a period of accelerated natural selection.

One way to see this is to look at this enrichment pattern that we're observing, where immune traits are unusually associated with these selection signals.

And we could compare the last 5,000 years of our time period, what's called the Bronze Age and further onward, to the previous 5,000 years.

And what we see is that this intensification of selection around immune traits, similarly the intensification around metabolic traits, has accelerated over this time period.

So it's not like natural selection has been at the same rate over all places and times.

In fact, it's increasing over the time period we're analyzing.

And so plausibly, the whole time period has increased compared to previous periods.

So we're in a period of intensified selection.

That's not implausible because this is a population that went through a huge shock in terms of the way people live and the culture.

So this is a population that almost everybody we're analyzing are farmers or food producers in one way or another.

Farming was invented for the first time anywhere in the world in the Middle East 11,000 or 12,000 years ago.

The people who invented farming exploded into Europe after 8,500 years ago and spread across Europe and expanded rapidly.