That’s at least how a new theory published in the Proceedings of the National Academy of Sciences describes it. The consumption of alcohol, which otherwise wouldn’t have been palatable and might have been poisonous, was an evolutionary boon when our ancestors descended from the trees and started looking for food.

“Evolutionary biologists such as myself study so much peculiar and fascinating examples of organisms adapting to their environment, there is a cliche in our field: ‘Life will find a way,'” lead author Matthew Carrigan wrote in an e-mail to The Washington Post. “So when I began this research, I thought, ‘If ethanol is present in naturally fermenting fruit, why shouldn’t some frugivores adapt the molecular machinery to digest it?”

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At the center of this molecular machinery is an enzyme called ADH4. This is a very important enzyme: first, because it’s found in a primate’s throat, stomach, and tongue; and second, because it is the first enzyme that can metabolize alcohol, including ethanol and other alcohols found in plants. So Carrigan and other researchers went about the task of analyzing ADH4’s evolutionary history by resurrecting ancient enzymes, believing it would tell them when such consumption began.

Carrigan thought he would have found enzymes that could metabolize ethanol at least 40 million years ago, when some believe our ancestors started eating fruit in a big way. Wrong, he found out. “I had expected an adaptation to metabolize ethanol would have occurred much earlier in our evolutionary history than 10 million years ago,” he said.

There was a mutation “10 million years ago,” says the article, “that endowed our ancestors with a markedly enhanced ability to metabolize ethanol. This change occurred approximately when our ancestors adopted a terrestrial lifestyle and may have been advantageous to primates living where highly fermented fruit is more likely.”

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The semi-terrestrial life, which coincided with that emergence of that enzyme, wasn’t kind in the beginning. Sometimes food wasn’t easy to come by, and our ancestors had to make do with what they found on the ground. There, they found fruit — but not the Whole Foods kind. Carrigan suspects the animals developed an enzyme to consume fruit that otherwise would have been trouble: “Of course, fermented fruit is generally not as desirable as non-fermented fruit because of the toxic by-products of metabolizing ethanol, so I suspect fermented fruit was a second choice.”

The remarkable part: Our ancestors begrudgingly eating rotting food 10 million years spawned modern drinking and alcoholism. The findings piggyback on earlier research published in Integrative & Comparative Biology in 2004, which found that the “nutritional rewards” derived from eating fruit “underlie contemporary patterns of alcohol consumption and abuse.”