David Reich

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.

And we might be able to appreciate that.

So maybe we could see whether there's been a quickening of natural selection over that time period.

There's a question about, I think the view amongst common trait geneticists is that we've been at a kind of steady state where almost where the natural selection that does occur is just there pushing down slightly bad variants, not...

not adapting to new situation.

We're at a kind of stable point.

So it's not clear how that works because over a scale of 2 million years, we're clearly genetically quite different from our ancestors.

Our brains are bigger.

We do some things differently.

Our proportions are different.

And yet over the last 200,000 years, we are...

not profoundly different.

There's not genetic changes that differ dramatically across populations.

So there's like a kind of disconnect.