David Reich

This is like a crazy result because it looks like there's no key selective sweeps that have occurred in this period that is ancestral to everyone living today.

We talked before about no selective sweeps between Europeans and East Asians, but there don't even seem to be any selective sweeps shared between all humans in this really important period when a lot of...

evidence in the material culture record appears.

And so it could be that there's biological adaptation in this period, but it's polygenic.

There's lots of mutations that all shift in the same direction to help the population to move to a new set point, but there's no key biological change that rises to high frequency in this time.

So this is 100,000 to 50,000 years ago, and this is the population that's ancestral to West Africans, to most East Africans, to all non-Africans.

And there's a couple of populations in Africa that have substantial ancestry that comes from more divergent groups.

For example, Khoisan from Southern Africa or Central African rainforest hunter-gatherers.

have substantial fractions of their ancestry from groups that diverged maybe 200,000 years ago from the other lineages.

But all of these groups today are able to go to college, do everything everybody else does, and so there is no evidence that there is any key mutation lacking in some groups that are not present in the others.

I think a lot of us in human genetics think that our population contains within it the clay that's needed to make almost any trait.

And that depending on environmental conditions or selection conditions, the mean value of these traits will move in different directions.

There's an empirical question, a real question about how much selection there's been in different human populations over time.

One of the things this new work that we're involved in is doing is showing that at least in the last 18,000 years, 10,000 years, 5,000 years in this part of the world, there actually has been significant movement, at least for a handful of important traits.

Exactly.

We looked at more than 500 traits, about 100 of them, complex traits, showed significant movement in systematic direction over this time period.

So it really does seem that there is a response to the environments people are living in that has occurred over this period and is potentially stronger than in previous periods.

That is such an interesting question, right?

Genetically, we're there.

The common ancestral population has all of the ingredients for farming 50,000 years ago.