Benjamin Phelan in Slate:
To repurpose a handy metaphor, let's call two of the first Homo sapiens Adam and Eve. By the time they welcomed their firstborn, that rascal Cain, into the world, 2 million centuries of evolution had established how his infancy would play out. For the first few years of his life, he would take his nourishment from Eve's breast. Once he reached about 4 or 5 years old, his body would begin to slow its production of lactase, the enzyme that allows mammals to digest the lactose in milk. Thereafter, nursing or drinking another animal's milk would have given the little hell-raiser stomach cramps and potentially life-threatening diarrhea; in the absence of lactase, lactose simply rots in the guts. With Cain weaned, Abel could claim more of his mother's attention and all of her milk. This kept a lid on sibling rivalry—though it didn't quell the animus between these particular sibs—while allowing women to bear more young. The pattern was the same for all mammals: At the end of infancy, we became lactose-intolerant for life.
Two hundred thousand years later, around 10,000 B.C., this began to change. A genetic mutation appeared, somewhere near modern-day Turkey, that jammed the lactase-production gene permanently in the “on” position. The original mutant was probably a male who passed the gene on to his children. People carrying the mutation could drink milk their entire lives. Genomic analyses have shown that within a few thousand years, at a rate that evolutionary biologists had thought impossibly rapid, this mutation spread throughout Eurasia, to Great Britain, Scandinavia, the Mediterranean, India and all points in between, stopping only at the Himalayas. Independently, other mutations for lactose tolerance arose in Africa and the Middle East, though not in the Americas, Australia, or the Far East.
In an evolutionary eye-blink, 80 percent of Europeans became milk-drinkers; in somepopulations, the proportion is close to 100 percent. (Though globally, lactose intolerance is the norm; around two-thirds of humans cannot drink milk in adulthood.) The speed of this transformation is one of the weirder mysteries in the story of human evolution, more so because it's not clear why anybody needed the mutation to begin with.