Today, however, 35 percent of the global population — mostly people with European ancestry — can digest lactose in adulthood without a hitch.

So, how we did we transition from milk-a-phobics to milk-a-holics? "The first and most correct answer is, we don't know," says Mark Thomas, an evolutionary geneticist at University College London in the UK.

Most babies can digest milk without getting an upset stomach thanks to an enzyme called lactase. Up until several thousand years ago, that enzyme turned off once a person grew into adulthood — meaning most adults were lactose intolerant (or "lactase non-persistent" as scientists call it).

But now that doesn't happen for most people of Northern and Central European descent and in certain African and Middle Eastern populations. This development of lactose tolerance took only about 20,000 years – the evolutionary equivalent of a hot minute – but it would have required extremely strong selective pressure.

"Something happened when we started drinking milk that reduced mortality," saysLoren Cordain, an exercise physiologist at Colorado State University and an expert on Paleolithic nutrition. That something, though, is a bit of a mystery.