Most every dairy product contains lactose, which is a type of sugar. Specifically, lactose is a disaccharide made up of one galactose molecule and one glucose molecule. Lactase is an enzyme that breaks the bond that joins the galactose sugar to the glucose sugar to form this “double sugar.” Lactose-intolerant people lack the enzyme lactase, so they can’t metabolize this particular molecule.

All mammals can digest lactose when they are first born, but it is very rare for adult mammals to continue to be able to absorb the sugars found in dairy products. Recent studies have suggested that humans only obtained this ability around the same time they began domesticating animals. This makes it possibly the most recent example of evolution in our species.