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Milk, as the popular slogan goes, does a body good. It contains essential nutrients including fat, protein, sugar, as well as calcium , other minerals, and vitamin D needed for bones. Most people in the world lose the ability to digest lactose, the main sugar in milk, shortly after weaning. For these people, drinking fresh milk can lead to unpleasant bloating, flatulence, and cramps. However, about one-third of people in the world— mostly those whose ancestors originate in Europe, the Middle East, Africa, and southern Asia—continue to produce the enzyme lactase, which is responsible for lactose digestion, throughout adulthood. This trait is called lactase persistence, and recent genetic evidence has shown that it evolved independently in different parts of the world over the last 10,000 years as a result of strong natural selection. Why lactase persistence has evolved under such strong natural selection remains something of a mystery. The most widely cited explanation is that in the absence of dietary sources for vitamin D and with insufficient sunlight to make vitamin D in the skin, early northern European farmers were at risk of bone disease. Milk is an excellent source of calcium and an adequate source of vitamin D. So, as this " calcium assimilation hypothesis " proposes, having the ability to drink fresh milk into adulthood could have led to a major survival advantage. In a new article in Molecular Biology and Evolution journal, Sverrisdóttir et al. (2014) looked for the mutation that causes lactase persistence in Europeans (referred to as À13,910*T) in the bones of early farmers from sunny Spain. They didn't find it! They then used computer simulations to estimate how much natural selection would be needed to push the frequency of À13,910*T up to what is seen in Iberia today (about one-third have the mutation). To their surprise, the answer was " a lot! " What does this tell us about the calcium assimilation hypothesis? Well in Iberia, there is plenty enough sunlight to produce vitamin D in the skin, so calcium deficiency shouldn't have been a problem for those early farmers. As Sverrisdóttir et al. (2014) reason, if selection was a necessary drive up for lactase persistence frequency in people for whom calcium deficiency was not an issue, then the calcium assimilation hypothesis could not be the main explanation for the observed frequencies of lactase persistence in the Iberian Peninsula today, and so not the …

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