Craving a stiff drink after the holiday weekend? Your desire to consume alcohol, as well as your body’s ability to break down the ethanol that makes you tipsy, dates back about 10 million years, researchers have discovered. The new finding not only helps shed light on the behavior of our primate ancestors, but also might explain why alcoholism—or even the craving for a single drink—exists in the first place.

“The fact that they could put together all this evolutionary history was really fascinating,” says Brenda Benefit, an anthropologist at New Mexico State University, Las Cruces, who was not involved in the study.

Scientists knew that the human ability to metabolize ethanol—allowing people to consume moderate amounts of alcohol without getting sick—relies on a set of proteins including the alcohol dehydrogenase enzyme ADH4. Although all primates have ADH4, which performs the crucial first step in breaking down ethanol, not all can metabolize alcohol; lemurs and baboons, for instance, have a version of ADH4 that’s less effective than the human one. Researchers didn’t know how long ago people evolved the more active form of the enzyme. Some scientists suspected it didn’t arise until humans started fermenting foods about 9000 years ago.

Matthew Carrigan and Steven Benner—biologists at the Foundation for Applied Molecular Evolution in Gainesville, Florida—along with colleagues there, sequenced ADH4 proteins from 19 modern primates and then worked backward to determine the sequence of the protein at different points in primate history. Then they created copies of the ancient proteins coded for by the different gene versions to test how efficiently each metabolized ethanol. They showed that the most ancient forms of ADH4—found in primates as far back as 50 million years ago—only broke down small amounts of ethanol very slowly. But about 10 million years ago, the team reports online today in the Proceedings of the National Academy of Sciences, a common ancestor of humans, chimpanzees, and gorillas evolved a version of the protein that was 40 times more efficient at ethanol metabolism.

“Around this same time, the Earth cooled off, food sources changed, and this primate ancestor started to explore life on the ground,” says Carrigan, who now works at Santa Fe College in Gainesville. And this new way of life meant that, for the first time, primates started eating not only fruit picked from trees, but also the fallen fruits below. And fallen fruits, when they’re exposed to bacteria in the environment that convert sugars to alcohols, will begin to accumulate ethanol. “If you were the ancestor without this new mutation in ADH4, the ethanol would quickly build up in your blood and you’d get inebriated much faster,” Carrigan says. “You’d be a cheap date.” This easy inebriation, he says, would have been a disadvantage to the monkeys without the mutation, making them more easily get sick—or drunk—off fruit, enough so that they couldn’t defend their territory and seek out food. Primates with the new mutation could get more food, his group hypothesizes, and the gene was selected for in the human and chimpanzee lineage.

Carrigan says the discovery might explain why human brains evolved to link pleasure pathways with alcohol consumption—ethanol was associated with a key food source. “It’s not a whole lot different from the addictions some people have towards food,” he explains. “At the right dose, when you didn’t have alcohol and candy at every corner, it was hard to get too much of this sort of stuff, so when you found it, you wanted to be programmed to overconsume.”

The new data on when each version of ADH4 evolved also help put dates on when and where different branches of the primate family tree arose, Benefit says. Because the human primate ancestor already had a different ADH4 than other monkeys by 10 million years ago, she says, it casts doubt on the idea that Ouranopithecus, an Asian ape described by 8-million- to 9-million-year-old fossils, was a step in the evolution of humans and chimpanzees. By this point in history, the human branch of the evolutionary tree would have already separated from the Asian apes, which have the less effective ADH4, Benefit explains.

The findings also help paint a picture of how primates may have interacted with each other 10 million years ago. “We always wondered why some primates became terrestrial and not others,” Benefit says. “So to find out that they had an adaptation that allowed them to eat fermented, fallen fruit, it fits in so perfectly.”

*Correction, 8 December, 4:10 p.m.: The article originally omitted the fact that this research was performed at the Foundation for Applied Molecular Evolution.