A new study of South Asian genomes from the University of California, Berkeley analyzed approximately 8,000 modern and ancient genomes to understand the evolutionary history of adult humans' ability to digest lactose.[1][2] The research focused on the −13910T genetic variant, which is associated with persistent production of lactase, the enzyme needed to digest the lactose in milk.[1][2] The findings show that this gene was introduced to South Asia during the historical and medieval periods by the migration of steppe herders, but did not spread widely among most populations as it did in Europe.[2] Despite South Asia being the largest producer and consumer of milk in the world, most adults in India, Pakistan, and Bangladesh do not produce enough lactase and consume traditional dairy products in fermented or processed form, such as yogurt, ghee, and paneer.[2] The exceptions are two traditional pastoralist communities—the Toda of southern India and the Gujjar of northwestern India and Pakistan—where the ability to digest lactose occurs in about 90 percent of the population.[1][2] This high prevalence in these groups suggests that lifestyle and long-term dependence on fresh dairy products exerted a strong evolutionary pressure to maintain the ability to digest milk into adulthood.[2] Research thus demonstrates that the evolution of the ability to digest lactose is not a unified story of natural selection, but a mosaic of demographic and cultural history.[1]