The Biological Clock that Doomed the Neanderthals - David Reich

Why It Matters

This framing shifts part of the explanation for Neanderthal disappearance from competition or violence to fundamental biological limits on gene flow, reshaping interpretations of human migration, admixture, and the mechanics of species replacement.

Summary

Geneticist David Reich argues that increasing evolutionary separation created biological incompatibilities that contributed to Neanderthal extinction. Early encounters around 300,000 years ago allowed cultural exchange and some gene flow, but by 70,000 years ago divergence between lineages—on the order of hundreds of thousands to over a million years—meant hybrids were often unfit or infertile. As modern humans expanded into territories occupied by archaic populations, interbreeding produced offspring with low survival or reproductive success, limiting genetic permeability. Reich contends these reproductive barriers help explain why Neanderthals did not persist despite contact with our ancestors.