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

David Reich, a professor of ancient DNA at Harvard, discusses a major new preprint from his lab led by postdoctoral scientist Ali Akbari. The study uses ancient DNA from approximately 16,000 individuals spanning 18,000 years in Europe and the Middle East — a roughly 14-fold increase over previous datasets — to detect signals of natural selection across 10 million positions in the genome. A key methodological innovation was borrowing a technique from medical genetics: using the correlation between selection signals and known trait-associated variants from genome-wide association studies (GWAS) as an independent calibration tool to validate which selection signals are real.

The central finding challenges the long-held view that human natural selection has been largely quiescent over the last few hundred thousand years. While 98% of allele frequency changes are attributable to genetic drift and migration, the remaining 2% attributed to directional selection is nonetheless pervasive — tugging at positions across the genome. The team identified 479 positions with 99% confidence of being under real selection, and approximately 3,800 with 50% confidence. The genome-wide association with immune and metabolic traits showed a four-to-five-fold enrichment among high-confidence selection signals, while behavioral/cognitive traits showed weaker enrichment due to their highly polygenic architecture.

A striking and unexpected finding is that the intensification of natural selection peaked during the Bronze Age (roughly 5,000–2,000 years ago), rather than at the Neolithic transition to farming (~10,000–8,500 years ago). This is visible across multiple traits: skin depigmentation in Europeans, lactase persistence, metabolic traits related to obesity and Type 2 diabetes, immune traits like the TYK2 tuberculosis risk variant, and even the polygenic score for cognitive performance/years of schooling. Reich argues this reflects a qualitative wrenching of human biology caused by the dramatic shift to high-density, urban, agriculturally intensive, and zoonotic-disease-rich Bronze Age environments.

On cognitive traits specifically, Reich reports that the polygenic score predicting performance on IQ tests and years of schooling in modern white British people shows a roughly one standard deviation increase over the last 10,000 years, with the strongest selection signal between 5,000 and 2,000 years ago and virtually no detectable selection in the last 2,000 years. The validity of this signal is bolstered by a cross-population replication: the same genetic variants that predict years of schooling in Chinese people in China today show a five-to-six standard deviation correlation with the ancient European trajectory, making chance an implausible explanation.

Reich also discusses how European hunter-gatherers score approximately three standard deviations below the modern mean on this polygenic predictor, which he notes is primarily a migration artifact rather than selection. He reflects on the 'collective intelligence hypothesis' — the idea that ancient people may have been smarter — noting the data point in the opposite direction, though he cautions that what's being measured may be a broader executive function or 'deferred gratification' trait correlated with many outcomes, not raw intelligence per se.

On the question of why farming wasn't invented before the Holocene despite genetic readiness, Reich endorses the climate stability hypothesis: the last 12,000 years represent an unusually stable climatic period on a two-million-year timescale, which enabled independent agricultural invention across multiple continents.

In a post-recording whiteboard session, Reich presents a speculative new model of Neanderthal origins. He argues that the standard model — in which Denisovans and Neanderthals are genomic sisters diverging from modern humans ~700,000–800,000 years ago — is increasingly strained by the fact that Neanderthal mitochondrial DNA and Y chromosomes cluster with modern humans rather than Denisovans, pointing to a ~200,000–300,000-year-old interbreeding event. He proposes that a population originating in the Caucasus or Northeast Africa invented Levallois/Middle Stone Age technology and expanded both into Europe (creating Neanderthals via ~95% genomic replacement with local archaics while culturally persisting) and into Africa (contributing ~20% to anatomically modern humans). This model would explain the shared cultural toolkit and uniparental marker clustering of Neanderthals and modern humans, and draws an analogy to the Ptolemaic epicycle problem — a standard model patched with increasing complications rather than replaced by a simpler alternative.