Audio · 2026-05-08 · 6 moments

David Reich – Why the Bronze Age was an inflection point in human evolution

"Instead of being quiescent, natural selection is everywhere." ✦ AI generated

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Data◆

Selection on the genetic predictors of cognitive/IQ-test performance peaked between roughly 5,000 and 2,000 years ago during the Bronze Age, with almost no detectable selection on this trait in the last 2,000 years.

Reich's team found that selection intensity on the genetic predictor of IQ-test performance spiked sharply between 4,000 and 2,000 years ago, pushing the trait up by roughly a standard deviation, while the last 2,000 years show essentially no selection on it at all.

transcript

David Reich: What you see when you look at intelligence is that this maxes out in the Bronze Age, between 5,000 and 2,000 years ago. The impact in the last 2,000 years is almost nothing. There’s no evidence of natural selection at all.

supports · 1

Claim

The genetic predictor commonly labeled 'years of schooling' or intelligence actually reflects a broader life-history trade-off — such as the toggle between having more children with less investment versus fewer children with more investment — which is why its selection direction reverses across different times and populations rather than moving consistently toward 'more intelligence.'

Reich speculates that the genetic score linked to years of schooling is entangled with traits like age at first childbirth, obesity, and walking pace, and may really capture a life-history toggle between quantity and quality of offspring — explaining why its selection direction flips depending on the environment.

transcript

David Reich: There’s a toggle between having more kids and investing less in them, and having fewer kids and investing more in excelling in various ways. You can imagine that at different times and in different places… In ecology, there are different ways. Mammals often invest a lot with a pregnancy and a small number of children, whereas fish will spawn huge numbers of offspring into the river, the great majority of whom will be eaten.

Mechanism

For the strong selection coefficients (roughly 0.5-1% or more) found in this study, natural selection operates effectively whether a population is 1,000 or many millions; what actually limits how fast such selection can produce large trait shifts is elapsed time, not population size.

Reich argues that once selection coefficients are as strong as those found in the Bronze Age data (~0.5-1%), population size stops mattering — even a population of 1,000-10,000 people is plenty large for selection to work, so the real constraint on rapid evolutionary change is simply time elapsed.

transcript

David Reich: We’re talking about strong, measurable selection coefficients on the order of half a percent or more in this study. All of those are going to work in small populations or large populations. It’s not going to be affected by the population size.

Mechanism

Agriculture did not emerge earlier than roughly 11,000-12,000 years ago—despite humans worldwide having had the necessary cognitive and cultural toolkit for tens of thousands of years—because the pre-Holocene climate was far less stable, and only the unusual climate stability of the last 12,000 years, unique on a roughly two-million-year timescale, allowed multiple populations to independently develop farming.

Reich notes that genetically and cognitively, dispersed human populations had everything needed for farming tens of thousands of years earlier, yet agriculture only appeared globally after roughly 12,000 years ago — coinciding with the Holocene's unusual climate stability, which he says climatologists confirm is unique on a two-million-year timescale.

transcript

David Reich: 12,000 years ago we switched into this period of not just warmth, but climate stability. It’s hard to believe that we’re living in such a special time. But if you look at data from the bottoms of ponds where you can measure the fluctuations of temperatures using isotopic signatures, apparently we’re in a period where it’s fluctuating a lot less year to year, 10 years to 10 years, and 100 years to 100 years.

Claim◆

Neanderthals and modern humans both descend from a single population that invented Middle Stone Age (Levallois) technology roughly 300,000 years ago and expanded outward — the branch that moved into Europe was genetically swamped by local archaic humans (becoming Neanderthals, ~95% archaic), while the branch that expanded within Africa mixed with more diverged archaic Africans in smaller proportion (becoming modern humans, ~20% archaic) — so Neanderthals and modern humans share the same cultural ancestry and differ mainly in which archaic population absorbed them.

In an off-the-cuff whiteboard sketch, Reich proposes that Neanderthals aren't simply a separate archaic lineage modern humans later interbred with, but are descendants of a Middle-Stone-Age-inventing population related to modern humans that expanded into Europe and got genetically swamped by local archaics — while the same expansion into Africa, diluted less by more divergent archaic Africans, produced modern humans.

transcript

David Reich: So you actually have this key population that makes the Middle Stone Age or Levallois technology. It appears and expands in all directions—into Europe and into Africa 200,000 to 300,000 years ago—bringing this technology, new ideas, and perhaps some genetic adaptations. It expands into archaic humans in Europe, mixes with the local population, and gets 95% replaced but still retains its cultural features and maybe some genetic features.

Mechanism

A novel statistical method — modeling genome-wide relatedness among 22,000 ancient and modern individuals and testing whether constant directional selection better predicts allele frequencies — combined with independent validation against UK Biobank trait-association data, shows that selection signals with high statistic values (above ~5) are overwhelmingly real rather than artifacts.

Reich explains how the team validated their selection statistic by showing it predicts enrichment for known trait-associated mutations from the UK Biobank — enrichment rises from 15% to roughly 60-70% as the statistic crosses a threshold of five, giving a calibrated way to know which signals are real.

transcript

David Reich: As we did that, the enrichment for genetic mutations that affect traits got higher and higher. Whereas it was only 15% when we didn’t use our selection statistic, when we required the selection statistic to be above about five, there was about a five-fold enrichment for mutations that cause traits.

Highlight slides

IQ Selection Peaked in the Bronze Age✦ from: Selection on the genetic predictors of cognitive/IQ-test performance peaked between roughly 5,000 and 2,000 years ago during the Bronze Age, with almost no detectable selection on this trait in the last 2,000 years.Selection Timeline: Then vs. Now✦ from: Selection on the genetic predictors of cognitive/IQ-test performance peaked between roughly 5,000 and 2,000 years ago during the Bronze Age, with almost no detectable selection on this trait in the last 2,000 years.One Ancestral Population, Two Fates✦ from: Neanderthals and modern humans both descend from a single population that invented Middle Stone Age (Levallois) technology roughly 300,000 years ago and expanded outward — the branch that moved into Europe was genetically swamped by local archaic humans (becoming Neanderthals, ~95% archaic), while the branch that expanded within Africa mixed with more diverged archaic Africans in smaller proportion (becoming modern humans, ~20% archaic) — so Neanderthals and modern humans share the same cultural ancestry and differ mainly in which archaic population absorbed them.Archaic Ancestry: Neanderthals vs. Modern Humans✦ from: Neanderthals and modern humans both descend from a single population that invented Middle Stone Age (Levallois) technology roughly 300,000 years ago and expanded outward — the branch that moved into Europe was genetically swamped by local archaic humans (becoming Neanderthals, ~95% archaic), while the branch that expanded within Africa mixed with more diverged archaic Africans in smaller proportion (becoming modern humans, ~20% archaic) — so Neanderthals and modern humans share the same cultural ancestry and differ mainly in which archaic population absorbed them.Shared Cultural Root, Different Archaic Mix✦ from: Neanderthals and modern humans both descend from a single population that invented Middle Stone Age (Levallois) technology roughly 300,000 years ago and expanded outward — the branch that moved into Europe was genetically swamped by local archaic humans (becoming Neanderthals, ~95% archaic), while the branch that expanded within Africa mixed with more diverged archaic Africans in smaller proportion (becoming modern humans, ~20% archaic) — so Neanderthals and modern humans share the same cultural ancestry and differ mainly in which archaic population absorbed them.

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