David Reich – Why the Bronze Age was an inflection point in human evolution
In a Nutshell
Ancient DNA analysis of ~16,000 Europeans/Middle Easterners over 18,000 years reveals rampant natural selection, with ~3,600 genomic positions (99% confidence) showing directional shifts, especially intensifying in the Bronze Age (~5,000-2,000 ya) across immune, metabolic, pigmentation, and polygenic traits like predictors of cognitive performance, years of schooling, and reduced obesity—far stronger than the Neolithic farming transition or recent centuries. This "evolutionary mismatch" from hunter-gatherer genetics clashing with high-density pastoral/urban life drove adaptations like lactase persistence, depigmentation, and ~2 standard deviation gains in executive function proxies, countering expectations of quiescent selection post-Out-of-Africa. A new model posits Neanderthals as culturally modern hybrids (~95% archaic + 5% early modern input ~300,000 ya), explaining shared Levallois tools, mtDNA/Y lineages, and wavefront expansions, simplifying convoluted admixture histories.
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Humans in this part of the world were wrenched into a way of living so different from their hunter-gatherer ancestors that the organism had to adapt very strongly. The degree of this wrenching process moving into the Bronze Age was qualitatively greater than the initial transition to growing plants, despite the cartoon picture emphasizing farming as the big transition. Genetic data shows the genome reacting much more strongly to events 5,000 years ago.
David Reich is a professor of ancient DNA at Harvard. He is a geneticist working on human history and how ancient people relate to each other and people living today. They did an interview two years ago that was one of the most popular.
People found the previous interview compelling because ancient DNA techniques reveal much unknown human history. New preprint is exciting. The dream 16-17 years ago when the ancient DNA field started was to learn about biology and how people's biology changed over time by tracking changes in DNA from ancient human remains. This dream has not been realized; the field succeeded in human history with surprising findings on migrations, mixture being common, sex-biased processes, and results unexpected from archaeology.
Success in history but not biology due to small sample sizes. One person's DNA provides information about history because it represents two parents, four grandparents, etc., effectively tens of thousands of ancestors, positioning the individual exquisitely relative to others. For genetic variants affecting skin pigmentation, lactase persistence, or behavioral traits, one person gives only one or two samples. High-resolution frequency changes over time require very large sample sizes, only available in the last few years.
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