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A group of people living in Chile are evolving the ability to digest milk as adults, as most Europeans did thousands of years ago. The finding shows evolution is still changing us even now. It also questions our ideas about why milk digestion evolved.

Nicol谩s Montalva of the Universidad Mayor in Santiago, Chile and his colleagues have studied the people living in Chile鈥檚 Coquimbo region, between the famously dry Atacama desert and country鈥檚 central valleys. It is dry so arable farming is hard.

To survive, the people have become 鈥渁gropastoralists鈥�. In winter they grow food on small plots. But they also keep goats and make cheese from their milk, which they sell in summer. The people drink a lot of goat鈥檚 milk 鈥� which is surprising because native Americans cannot digest milk properly.

Like all mammals, humans have an enzyme called lactase that breaks down the lactose sugar in milk. This enzyme normally gets turned off once a baby is weaned. But within the last 10,000 years some populations 鈥� particularly in northern Europe 鈥� have acquired mutations that keep the lactase active, allowing adults to digest milk. This is called lactase persistence.

Milk drinkers

Many non-Europeans lack these mutations and suffer symptoms like diarrhoea and flatulence if they drink milk. Because the Americas were first settled over 15,000 years ago by Asian people, native Americans could not digest lactose.

鈥淚n South America there has been no history of milking of mammals,鈥� says co-author Dallas Swallow of University College London in the UK. Only when Europeans arrived 500 years ago did this change. The colonisers 鈥� who brought domestic animals like goats with them 鈥� interbred with local people.

In 2015 Montalva鈥檚 team showed that . Now they have taken a closer look.

They collected DNA samples and other data from 451 individuals. Like most Chilean populations, their DNA was about 50:50 European and native American. All those with lactase persistence had the same聽鈥楨uropean鈥� mutation. It was presumably brought over during colonisation.

The lactase persistence mutation carried by the people in Coquimbo has been favoured by natural selection: the DNA around it is almost identical in everyone that carries it.

Dairy good

鈥淭hat really adds something to the story,鈥� says Laure S茅gurel of the Museum of Humankind in Paris, France. The question is .

The obvious explanation is that it opens up milk as a , so lactase-persistent people ought to be better nourished. Montalva鈥檚 team found evidence for that. On average, lactase-persistent Coquimbo men are heavier than non-lactase-persistent men 鈥� suggesting they have a more nutritious diet.

However, there was no trend among women. What鈥檚 more, the effect in men was independent of how much milk they drank. In fact, all the Coquimbo people drank about the same amount, regardless of genes.

It may not be about drinking extra calories, says Swallow. 鈥淢ilk has a lot of micronutrients.鈥�

鈥淚鈥檓 not really convinced it has so much to do with diet,鈥� says Segurel. Instead, it might be that some people live in close contact with animals, increasing the risk of disease, and drinking their milk protects against this, she says.

Or perhaps the microbes in people鈥檚 guts are the key. 鈥淲e know that individuals that are and aren鈥檛 lactase-persistent get different microbiota,鈥� says Segurel. Drinking milk might seed extra microbes in people鈥檚 guts that help them digest their food better.