Milk, as the popular slogan goes, does a body good. It contains essential nutrients including fats, proteins, sugars, as well as the 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 them 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 paper in the journal Molecular Biology and Evolution
, Oddný Sverrisdóttir, and colleagues looked for the mutation that causes lactase persistence in Europeans (referred to as -13,910*T) in the bones of early farmers from sunny Spain. They didn't find the mutation in any of the ancient DNA samples! They then used computer simulations to estimate how much natural selection would be needed to push the frequency of -13,910*T up to what is seen is 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 make vitamin D in the skin, so calcium deficiency shouldn't have been a problem for those early farmers. As Sverrisdóttir and colleagues reason, if selection was necessary drive up 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 only explanation for the evolution of lactase persistence in Europe. They conclude that other evolutionary selective pressures must have been at work to explain the presence of this trait in modern Europeans. "Using ancient DNA and computer simulations, we show that strong natural selection has acted on lactase persistence in Iberia over the last 7,000 years. Sunlight in Iberia is sufficient to allow the synthesis of vitamin D in the skin for most of the year. It is therefore unlikely that the risk of calcium deficiency was the main driver for the evolution of lactase persistence (the 'calcium assimilation hypothesis') in this region. Additional evolutionary forces need to be identified to explain this example of strong natural selection in the genome of Europeans."