Then, the team used computer simulations to see what kind of genetic shifts were necessary to get from a population where no one could digest milk past childhood to one where about a third of adults could drink it over the course of just 5,000 years.
Those simulations ruled out the possibility that the mutation reached its current level just by chance and instead showed that there was strong selection for it: Something gave people who had the milk-drinking gene a big advantage over people who didn’t.
But if it wasn’t the vitamin D that made milk so beneficial in sunny Spain, then what was it?
The new findings can’t answer that question, but Sverrisdóttir has a favorite theory. Early farmers were eating cheese and yogurt long before they could drink milk because fermented dairy products are easier to digest. But in times of famine, when crops failed and all of the processed dairy foods had been consumed, people would have turned to milk out of desperation.
Those who happened to have a lurking mutation that helped them digest it would’ve thrived while those who were lactose intolerant would’ve ended up with life-threatening diarrhea.
“During normal times, if you were well-fed and you had diarrhea for days, it wouldn’t matter much,” Sverrisdóttir said. “But if you were already starving, this would mean the difference between life and death. People would have not lived long enough to get their genes into the next generation. This was the new super-food for people who could tolerate it.”
It’s still possible that milk got its value from vitamin D’s calcium-absorbing powers in some places, said Pascale Gerbault, an evolutionary geneticist at University College London.
But the new study suggests that there may have been multiple reasons why milk was so pivotal in changing the course of human evolution and that those reasons varied through location and time.
“It makes sense that milk was a good food resource at different points in our evolution,” Gerbault said. “But what were the situations that triggered these pressures? They’re not quite known yet.”