David Reich discusses ancient DNA findings that challenge prior assumptions about human evolution. His lab's new study reveals the Bronze Age as a period of intense natural selection across immune, metabolic, and even cognitive traits—contrary to the long-held view that selection was quiescent in recent millennia.
Summarized by Podsumo
The Bronze Age (5,000–3,000 years ago) was a major inflection point for natural selection, with immune and metabolic traits showing accelerated selection due to population density, agriculture, and urbanization.
The study identifies hundreds of genetic positions under selection, including surprising selection on polygenic scores for cognitive performance and years of schooling, peaking in the Bronze Age rather than in recent centuries.
Reich and his team developed a new statistical method that corrects for migration and drift, allowing them to detect selection signals that previous methods missed—revealing that natural selection is 'rampant' across the genome.
The research suggests that modern humans' cognitive and behavioral capacities were likely in place 300,000–400,000 years ago, yet agriculture only emerged after the Ice Age due to climate stability, not genetic change.
"What we've found is that instead of being quiescent, natural selection is everywhere, even though it's only 2% of the frequency change, it's tugging the positions in one direction or the other everywhere."
"The genome is vibrating with natural selection. Almost every position in the DNA is correlated to a position that is dragged in one way or the other by natural selection."
"I've really been almost traumatized by this. Again and again, I've come into a project with some kind of guess about what the data was showing. Then the data doesn't show that."