Adaptations of Humans to Adult Milk Intake

In most individuals, the ability to digest lactose, the sugar present in milk, declines rapidly after weaning because of decreasing levels of the enzyme lactase in the small intestine. However, there are individuals who maintain the ability to digest milk into adulthood due to a genetic adaptation in populations that have a history of pastoralism. Here I will discuss our studies of the genetic basis of lactose tolerance in African pastoralist populations.  We have identified several mutations that arose independently in East African pastoralist populations.  We demonstrate a striking footprint of natural selection in the genomes of individuals with these mutations.  We show that the age of the mutations associated with lactose tolerance in Europeans and Africans is correlated with the archeological evidence for origins of cattle domestication.  Thus, the genetic adaption for lactose tolerance is an excellent example of gene-culture co-evolution.