The Bronze Age radically altered our DNA – David Reich

Added: 2026-05-11

[00:00:00]the Bronze Age is not 300 years, it's 3,000 years. It's the power of compound interest, and you have enough time to begin to see a strong effect. But, this really, really, really does seem to be a very impactful time in terms of human history, and you can see it in our complex traits. So, for example, if you look at pigmentation, for example, uh which is the uh strongest signal of selection for a complex trait in uh our data set. So, you look at genetic mutations that are known to affect pigmentations, you add up their effect across all of the DNA, so there's dozens or hundreds of them, and you look to see in what time are is the natural selection strongest, and the time period is really uh 2,000 to 4,000 years ago.

[00:00:45]So, and for some of these other traits as well, you see again the time period over which the selection is strongest uh is 2,000 4,000 years ago. So, for example, if you look at uh genetic variants that affect uh measures of cognitive performance, for example, uh such as performance on uh intelligence tests uh in uh people in white British people today. Uh so, this is of course a very strange trait to measure in the past, because there were no intelligence tests, and there was no school, but it is a predictor today, and you could look at how it's changed in the past. And we see very strong natural selection for this combination of genetic variants that predicts people's performance on IQ tests, and also is highly correlated to the predictor that predicts the number of years of school, or the household wealth of people, all crazy traits in the past, because there was no wealth in the past, there was no school in the past. But, if you look at the predictors today, there is a strong uh there is a strong movement in a systematic direction, a large effect, about a standard deviation on the scale of modern variation. Then, we can do this trick of looking to see whether there's periods of time when this natural selection has occurred more intensely or less intensely. What we do is we drag a 2,000-year window through our data and we repeat our whole analysis not on 18,000 years but just on a short 2,000 year window. And we can measure the strength of selection in each of these 2,000 year windows. And what you see when you look at intelligence is you see that this maxes out on the Bronze Age between 5,000, 4,000, 3,000, 2,000 years ago and the impact in the last 2,000 years is almost nothing. There's no evidence of natural selection at all.

[00:02:22]You might think your bias coming into this, my bias perhaps, if there's any natural selection on this trait at all, might be that it would be unusually strong in the last 2,000 years. Maybe this is a time of industrialization.

[00:02:33]Maybe this is a time of greater need for this particular trait. But in fact, there's no evidence of natural selection at all in the last 2,000 years but there's very strong evidence in between 2,000 and 4,000 years ago where instead of a one standard deviation strength of selection, it's a two standard deviation strength uh sort of averaged over this time period. And the the standard deviation here is how how much the polygenic score for the trait itself moves or How much the polygenic score trait moves uh over a 10,000 year period.

[00:03:05]>> Got it. Uh within a population that is held constant in terms of its ancestry.

[00:03:10]>> Got it. Because what's actually we're doing is we're looking in our data set at a kind of heterogeneous group of people. There's, you know, Southern Europeans and Northern Europeans and hunter-gatherers and farmers and at different kinds in the past those groups are more or less represented.

[00:03:24]>> Yeah. So, the whole strength of the methodology Ali Akbari developed is it corrects for that uh changing ancestry over time. And as I mentioned before, really what's being asked here is we've divided up our whole data set into an archipelago of little populations in different places in space and time. And we're asking in each place in space and time, a little pocket of people in Britain from 4,000 years ago to 3,500 years ago, a little pocket of people in Hungary, a little pocket of people in in Italy from 2,000 years ago to 1,500 years ago. In each of these places uh where the ancestry is relatively similar without being too disrupted in that short period by uh migrations, we watch to see if the genetic changes blow in the same direction. And what we're doing here is we're measuring the strength of selection at each point in time after correcting for the big population changes that have occurred.

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