HYDERABAD, INDIA—By studying the genomes of hundreds of South Asians, Harvard geneticist David Reich and his colleagues and have discovered that two different ancient populations, from north and south, began to mix some 4,200 years ago. "From genetic data, remarkably, you see this picture emerging of cultural change,” says Reich. But around 2,000 years ago, the large scale mixing of DNA abruptly stopped, probably because of the introduction of the caste system, which restricts marriage outside of strictly defined groups. Reich is now interested in studying the modern-day health implications of this ancient event, which has resulted in genetically isolated groups.