Ice cream. Butter. Yogurt. Cheese. A tall, cold glass of milk. Dairy is an essential part of the modern American diet. But, as the rise of coconut ice cream, cashew butter, and oat yogurt suggests, while some people may choose not to drink cow’s milk, many more simply can’t digest it well.

In the ancient past, our ancestors, like all mammals, could not digest milk after infancy—and even today, an estimated 68 percent of the global human population is lactose intolerant. The real mystery is why some people do drink milk.

It’s not mysterious why we might want to. Drinking milk has a lot of benefits, especially when food is scarce. Herds of sheep, goats, and cattle are a mobile and renewable source of nutrition and clean, drinkable liquid, able to thrive in environments where humans otherwise do not.

Milk can be consumed fresh or processed to keep for months, if not years (make that 3,500 years in the case of bog butter). Also, if modern human experience is any guide, it tastes pretty good.

(Millions of Americans drink raw milk—but is it safe?)

Still, drinking milk into adulthood, let alone other animals’ milk, is a weird behavior in the animal kingdom, and it has had a lot of big, weird effects. Scientists are still getting to the bottom of why the practice began, and why it persisted. This research could unlock new understandings of our food cultures, our microbiomes, and even our DNA.

When humans first turned to animal milk

The earliest evidence of animal milk drinking dates back almost 9,000 years to modern-day Turkey near the sea of Marmara, where milk fats have been found on ancient shards of pottery. Richard Evershed, a biogeochemist at the University of Bristol, says his team found evidence of milk in even the oldest pots. “They were probably milking before the invention of pots,” Evershed says.

In early settled communities, like the proto-city of Çatalhöyük in modern Turkey, milk would have been part of a diverse diet. Jessica Hendy, an archaeological scientist at the University of York, says that one bowl she analyzed from that site from the late Neolithic had evidence of dairy mixed with residue from pulses like barley. “They seemed to be using milk as part of a meal like we might do today,” she says.

(Here’s how your favorite plant-based milk impacts the planet.)

Milk appears to have been a major staple for ancient pastoralists, a mobile way of life built around herds of sheep, goats, and cattle. Researchers analyzing ancient dental plaque have identified individuals who consumed goat’s milk dating back 6,000 years in East Africa, where pastoralism offered real advantages.

“The Sahara was drying and the less rain you have the more unpredictable it is, so it makes much better sense to move animals where the food is then wait for it to come to any one place,” says Fiona Marshall, an archaeologist and professor emerita at the University of Washington St. Louis. In modern pastoral societies, milk remains essential; in northern Kenya, the Maasai’s traditional dietary staples are milk, cow’s blood, and meat.

Milk drinking spreads across the world

From its origins in what is now modern-day Turkey, the technology of dairying, as well as the herders themselves, spread into the Caucuses and then across Europe. “Milk is following the spread of agriculture—it’s part of the package,” Evershed says. In central Poland, some of the earliest evidence of cheesemaking appears on a sieve-like piece of pottery which dates back to the sixth millennium B.C.

By the Bronze Age, about 3,000 years ago, people may have been using cow’s milk to wean their babies. When Julie Dunne, an archaeologist at the University of Bristol, tested a set of fanciful, animal-shaped pots with spouts found in children’s graves in modern-day Germany, she found evidence of cow’s milk. Dunne was especially enamored of the playful designs. “They obviously wanted to make [their] babies laugh and smile.”

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Meanwhile, in the vast Eurasian steppe, nomadic pastoralism with sheep, goats, horses, camels, yak, and even reindeer became the backbone of a succession of pastoral empires, from the Xiongnu to the Mongols. Researchers have found evidence that dairy was the fuel of these societies.

“The steppe is a highway that connects Europe to East Asia, just a continuous stretch of grass, if you can survive there,” says Christina Warinner, a Harvard anthropologist who researches early human foods and the microbiome. The short growing season made it difficult to farm, but herds of sheep and other ruminants could feast on the grass and convert it into food for people—including milk.

But why can only some people digest milk?

For a long time, researchers believed that milk drinking evolved as a cultural practice hand in hand with the spread of genetic mutations that allowed people to tolerate milk into adulthood. But recent findings suggest milk drinking preceded these mutations, and might not even require them.

 In Europe and East Africa, several different genetic mutations allowing adults to break down lactose, the sugar present in milk, became some of the most strongly selected traits in the human genome. Sarah Tishkoff, a geneticist at the University of Pennsylvania, says the genetic trait correlates with pastoralism, but scientists are still nailing down the mechanism—or mechanisms—for its spread. “It has to offer a huge advantage to people who have the mutation,” she says.

In Europe, people appear to have been drinking milk for thousands of years before any genetic ability to drink milk became common. The ancient cheesemaking equipment might offer part of the solution: Fermenting milk into yogurt, cheese, or other products reduces the amount of lactose.

In Mongolia, researchers have not yet found a genetic mutation that allows people to digest lactose, despite the major role of dairy in that culture. Some scientists hypothesize that other microbes could be helping: In the research Warriner is conducting in Mongolia, her team has found that people who live in the countryside seem to process lactose more easily (i.e. with less gas) than those who live in cities, even though their genetics are identical. “The gut microbiome bacteria may be assisting,” she says.

What we do know about the history of milk reveals how wrong-headed one-size-fits-all nutritional guidance can be. In modern America, milk-drinking has been presented as both a universal good and, by fads like the paleo diet, downright unnatural. In reality, how milk is prepared can change the nutritional picture, and how our bodies process it depends, at least in part, on our own ancient history. “If you can digest milk, there’s a good chance you have a pastoral heritage,” Marshall says. “People find that fascinating.”