Thanks a whole lot, Neanderthals!

And just because something is bad for modern humans does not necessarily mean it was bad for their hunting-and-gathering ancestors. Some genes might put their bearers at risk of obesity in the modern world of fatty, sugary snacks. But in a world where food is scarce (as it presumably was in the northern latitudes where modern humans and Neanderthals mixed), those same genes might help their owners through lean periods.

Neanderthal DNA seems to put modern humans at risk of a specific sort of malnutrition caused by a lack of thiamine, a B vitamin that is vital for carbohydrate metabolism. But, says Dr Simonti, that same genetic variant may also make it easier to digest fats. Millennia ago, when people obtained less of their energy from refined carbohydrates, the trade-off may have been worthwhile. In a world where grain crops have become a staple food, it may not be.