David Reich

And then after that, it's much less.

And so this seems to be a very impactful, eventful, important period where a lot of the processes that we are seeing become very powerful.

And it's surprising on first principles.

You might think, before you walked into this genetic data, that the big change is going to be starting to grow plants.

Yeah.

and maybe farm animals.

That happens in the Neolithic, beginning 11 or 12,000 years ago and spreads into Europe after 8,500 years ago.

But actually the intensification happens like 5,000 years ago, 4,000 years ago.

And so it's really interesting, this observation of that being a key point, that being an inflection point, tells us something about when humans, at least in this part of the world, were wrenched into a way of living

that was so different from how the hunter-gatherer ancestors lived that the organism had to adapt very strongly.

And that maybe the degree of that wrenching process moving into the Bronze Age was qualitatively greater than the degree of the wrenching process that happened from the initial transition to growing plants.

So, which is surprising because our cartoon picture is that the big transition is farming, but the genetic data, the biological readout is saying,

our genome is reacting much more strongly to these events that happened 5,000 years ago.

That may be the case.

It also may be the case that that period is just too short to see much effect.

So what you're looking for in the Bhatia et al paper, where we looked at about 30,000 African-Americans and looked to see whether there is, instead of the average percentage of maybe around 80% West African ancestry,

In some places in the DNA, more than 80%.

In some places in the DNA, less than 80%.

Significantly, as you would expect if there was natural selection from some genetic variant from Europeans or from Africans, we didn't see any place in the DNA that was significantly different from what you would expect by chance.

And so one possible explanation for that is just that there's only a handful of generations, maybe five, over which the natural selection would operate.