Why can most adults in the world not digest milk?

Lactase, the enzyme that splits milk sugar, is made by every human infant and then normally switched off after weaning - because for almost all of our history there was no milk after weaning. Roughly two thirds of the world's adults are lactose intolerant, and that is the default human condition rather than the disorder.

The exception is the interesting part. In a few populations a mutation keeps lactase switched on for life, and it spread with extraordinary speed - within a few thousand years of the domestication of cattle, and independently at least four separate times, in northern Europe and in three different pastoralist populations in Africa and the Middle East. It is one of the strongest signals of recent natural selection in the human genome.

Quite why the advantage was so large is still argued about. Milk as a famine food, as a safe drink where water was not, and as a source of vitamin D and calcium in low-sunlight northern latitudes have all been proposed. Meanwhile cultures without the mutation solved the problem the other way, with bacteria: fermenting milk into yoghurt and cheese removes most of the lactose, which is why lactose intolerance and a great cheese tradition can coexist perfectly happily.