Thousands of years ago, a mutation in the human genome allowed many adults to digest lactose and drink milk. iStockphoto.com hide caption itoggle caption iStockphoto.com The Salt An Evolutionary Whodunit: How Did Humans Develop Lactose Tolerance? December 28, 2012 Thousands of years ago, ancient farmers gained the ability to consume milk as adults without getting an upset stomach. A remarkable mutation let some of them digest lactose sugar. But scientists still puzzle over why that mutation persisted and became prevalent in modern humans.