Milk is full of a sugar called lactose, which needs a special enzyme called lactase to break it down. But children used to stop producing the enzyme around the age of 4 or 5—and those of us who still make it are, in fact, mutants.
We're talking mutants in the scientific, genetic sense of the word here, and in particular about the lactase persistence trait, which popped up about 7,500 years ago somewhere in central Europe. It's that, and that alone, which means you get to enjoy lactose-laden shakes, ice cream and, well, milk—otherwise, we'd all be lactose intolerant. [Sci Show]