We know that most adults in the world are lactose-intolerant. That means they’ve lost the ability to produce lactase, an enzyme that helps babies digest milk. Those who continue to produce lactase and digest milk into adulthood are overwhelmingly northern Europeans. The conventional wisdom was that Europeans evolved this trait to survive, particularly those in the far north where the growing season is short and people rely on animal fats in milk. There are a couple of problems with this theory, however. First off, there are plenty of people in northern Asia who drink milk all their lives even though they do not produce lactase beyond infancy. It doesn’t bother them nearly as much as you’d think from reading about the subject, and certainly not enough to affect group survival. The second problem is that scientists have determined that European adults drank milk as far back at 9,000 years ago, but the switch to lactose-tolerance only occurred about 5,000 years ago, and quite suddenly for an evolutionary trait.
The answer is that there must have been an environmental stressor that made lactose-intolerant people die off in droves about 5,000 years ago, so that the genes responsible for continued lactase production could reach critical mass in the population. That would be a stressor on top of the inability to digest milk, because people do not die of lactose intolerance. The candidates are famine, drought, and pathogens that would disproportionally affect those who could not digest milk. Read what the research says so far about the evolution of lactose-tolerance at Smithsonian.
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Source: neatorama