How Humans Evolved the Ability to Digest Milk

It turns out that calcium absorption wasn't the only reason that Europeans evolved milk tolerance. Scientists have uncovered ancient DNA from early Iberian farmers and have found another reason why Europeans may have evolved milk tolerance.

Most people of European decent take milk drinking for granted. We're able to produce the enzyme lactase in adulthood, which allows us to digest the milk sugar lactose. Yet this isn't the norm in much of the world, and wasn't the norm for our ancestors from the Stone Age. In fact, the ability to digest lactose has only evolved within the last ten thousand years under strong natural selection.

Why exactly this trait evolved so quickly has been a bit of a mystery to researchers. It first appeared in pastoralist populations. But why was it such an advantage to our ancestors? The scientists now may be able to answer that question.

The researchers examined DNA from ancient bones. The DNA in these particular samples was extremely well-preserved. The scientists then used computer simulation to see how much natural selection would be needed to drive lactase persistence up to today's frequencies in that Iberia.

"But here's the thing," said Oddny Sverrisdottir, one of the researchers, in a news release. "If natural selection is driving lactase persistence evolution in a place where people have no problems making vitamin D in their skin, then clearly the vitamin D and calcium explanation (known as the calcium assimilation hypothesis) isn't cutting it. So while the calcium assimilation hypothesis may have some relevance in Northern Europe it's clearly not the whole story."

So then why did this trait evolve? While many early European farmers would not have been lactase persistent, they would still have been able to consume fermented milk products, such as yoghurt and cheese. In famine conditions, though, it's likely that the farmers consumed most of the fermented products and left only the more high-lactose products. This would have caused high selection for those who could stomach milk products.

The findings reveal a bit more about the evolution of milk tolerance. In addition, they show how and possibly why this trait evolved in certain populations so rapidly. This sheds a bit more light on our ancient ancestors.

The findings are published in the journal Molecular Biology and Evolution.