Milk contains a type of sugar called lactose. When we are babies, our bodies make a special enzyme called lactase that allows us to digest the lactose in our mother‟s milk. But after we are weaned in early childhood, for many people this stops. Without lactase, we cannot properly digest the lactose in milk. But then evolution kicked in: some people began to keep their lactase enzymes active into adulthood. This “lactase persistence” allowed them to drink milk without side effects. It is the result of mutations in a section of DNA that controls the activity of the lactase gene. But in many populations, such as those in Africa, in Asia and South America, the trait is uncommon. Even people who are lactase-non-persistent exploit the option of processing milk into butter, yoghurt, cream or cheese – all of which have reduced amount of lactose. There is clearly a pattern behind which populations evolved high levels of lactase persistence and which didn‟t, says a genetics professor Dallas Swallow of University College London. Those with the trait are pastoralists: people who raise livestock. Hunter-gatherers, who do not keep animals, did not acquire the mutations. Neither did “forest gardeners” who cultivated plants. But milk consumption is going down, says a study. Statistics tell a different story. While milk consumption has fallen in the US, in Asia demand is growing, where most people are non-lactase-persistent. Whatever advantages the people there see in milk, they outweigh the potential digestive issues or the need to process the milk.