A genome study of over 8,000 South Asian samples suggests that the ability to digest milk in adulthood, known as lactase persistence, is largely linked to ancient Steppe herder migrations. While most South Asians inherited this trait from these migrations, specific groups like the Todas and Gujjars exhibit unusually high milk tolerance due to their pastoralist lifestyles. This genetic variant for lactase persistence is believed to have entered South Asia between 3,000 and 4,000 years ago.