The ability to spoon down ice cream or chug a milkshake might not seem like an evolutionary advantage in our weight-conscious society. But scientists say that 5,000 to 10,000 years ago, around the time dairy farming began in Northern Europe, natural selection encouraged the spread of a genetic mutation that enabled adults to digest the sugars in milk. Those with the new gene-lactase-had a nutritional advantage over those who lacked it, so they proliferated, along with the mutation.