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“Around this same time, the Earth cooled off, food sources changed, and this primate ancestor started to explore life on the ground,” Carrigan says. 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.”[/quote