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.

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.”