New research shows that recurrent episodes of gene flow, beginning 250,000 to 200,000 years ago, affected the genomes and biology of both modern humans and Neanderthals, and estimates that Neanderthals have 2.5 to 3.7% human ancestry. Li et al. provide insights into the history of admixture between modern humans and Neanderthals, show that gene flow had substantial impacts on patterns of modern human and Neanderthal genomic variation, and show that...