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Lactose, or milk sugar, is a disaccharide made of two simple sugars – glucose and galactose – in a 1:1 ratio.

Lactase enables us to digest the milk sugar lactose, so lactase persistence is useful for a diet involving dairy products.

This ability, known as lactase persistence, comes from an enzyme that breaks down milk sugar and usually shuts down after young children are weaned.

Over the past 10,000 years, populations living far apart in Europe, Africa, South Asia, and the Middle East separately acquired a key genetic change: the ability to digest the milk sugar lactose as adults.

Early Europeans may have also lessened the painful effects of milk sugar by fermenting milk into cheese or turning it into butter.