This paper argues that a genetic adaptation to the Neolithic Revolution led to differential levels of development in the precolonial era. The ability to digest milk, or to be lactase persistent, is conferred by a gene variant that is unequally distributed across the Old World. Milk provided qualitative and quantitative advantages to the diet that led to differences in the carrying capacities of respective countries. It is shown through a number of specifications that country-level variation in the frequency of lactase persistence is positively and significantly related to population density in 1,500 CE; specifically, a one standard deviation increase in the frequency of lactase persistent individuals (roughly 24 percentage points) is associated with roughly a 40 % increase in precolonial population density. This relationship is robust to a large number of sample specifications and potentially omitted variables.