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While alcohol (called dietary ethanol by scientists) is a toxin, humans are able to tolerate a moderate amount of it without getting sick. We owe this ability to digestive enzymes, particularly alcohol dehydrogenase enzyme (ADH4), which are found in our guts, throats and tongues. All primates have ADH4 proteins, but not all of them can metabolize alcohol effectively; baboons and lemurs, for example, have a less effective version of ADH4.

Some scientists suspected that humans developed the ability to digest alcohol around the time they started making it themselves. According to archaeological evidence, the first modern humans began turning fermented fruit and other foods into their own boozy concoctions some 9,000 years ago. The residue of the earliest known alcoholic beverage (a mixed fermented drink of rice, honey and fruit) was found on early pottery used in Jiahu, a Neolithic village in China’s Yellow River Valley dating to 7,000-6,600 B.C. This evidence slightly predates that of similar beverages, including barley beer and grape wine, brewed in the Middle East.

In a new study, however, researchers suggest that our ability to digest alcohol may have far deeper roots. Using an experimental approach known as paleogenetics, in which gene sequences from contemporary species are analyzed to estimate how enzymes evolved over time, the scientists in the new study sequenced the genes for ADH4 proteins from 19 modern primates. They then worked backward to determine what the sequence of the protein would have been at different points in the past. According to their findings, published earlier this week in the Proceedings of the National Academy of Sciences, the oldest forms of ADH4, found in primates some 50 million years ago, broke down small amounts of ethanol very slowly and inefficiently. About 10 million years ago, however, a single genetic mutation occurred that enabled a common ancestor of humans, chimpanzees and gorillas to develop a version of the ADH4 proteins that was 40 times more efficient at metabolizing ethanol.

According to the scientists, this abrupt shift occurred at a time when the Earth’s climate was rapidly changing, causing the forest ecosystem of East Africa, home of these human ancestors, to be replaced by more widely dispersed forests and grasslands. While earlier human ancestors had lived their lives mostly in trees, now they made the transition to a more land-based lifestyle. As the new study’s lead author Matthew Carrigan, an evolutionary biologist at Santa Fe College in Gainesville, Florida, told Science Magazine: “Around this same time, the Earth cooled off, food sources changed, and this primate ancestor started to explore life on the ground.”

Crucially, the new ability to digest ethanol helped human ancestors eat rotting fruit that fell to the floor of the forest when other food was scarce. Such fallen fruit wouldn’t have been their first choice, scientists think, but it would have allowed them to survive. Other primates without this mutation were more likely to get sick or drunk off the fermented fruit, and be less effective at defending their territory or finding more food.

Carrigan claimed his team’s discovery might help explain why human brains evolved to link alcohol with pleasure—because it was identified with a necessary source of food—even though it is harmful and even poisonous in larger quantities. “It’s not a whole lot different from the addictions some people have towards food,’ he told Science. “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.”

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