But some of the mysterious absences of lactase-persistence could be down to sheer chance: whether anyone in a group of pastoralists happened to get the right mutation. Until fairly recently there were a lot fewer people on Earth and local populations were smaller, so some groups would miss out by plain bad luck.

“I think the most coherent part of the picture is that there’s a correlation with the way of life, with pastoralism,” says Swallow. “But you have to have the mutation first.” Only then could natural selection go to work.

In the case of Mongolian herders, Swallow points out that they typically drink fermented milk, which again has a lower lactose content. Arguably, the ease with which milk can be processed to be more edible makes the rise of lactase persistence even more puzzling. “Because we were so good at adapting culturally to processing and fermenting the milk, I’m struggling with why we ever adapted genetically,” says Swallow’s PhD student Catherine Walker.

There may have been several factors promoting lactase persistence, not just one. Swallow suspects that the key may have been milk’s nutritional benefits, such as that it is rich in fat, protein, sugar and micronutrients like calcium and vitamin D.

It is also a source of clean water. Depending on where your community lived, you may have evolved to tolerate it for one reason over another.

It’s unclear whether lactase persistence is still being actively favoured by evolution, and thus whether it will become more widespread, says Swallow. In 2018 she co-authored a study of a group of pastoralists in the Coquimbo region of Chile, who acquired the lactase-persistence mutation when their ancestors interbred with newly-arrived Europeans 500 years ago. The trait is now spreading through the population: it is being favoured by evolution, as it was in northern Europeans 5,000 years ago.