We used to think that our predecessors began imbibing alcohol around 9,000 years ago with the advent of food storage and our manipulation of natural fermentation processes. Not so, according to an analysis of ancient alcohol-metabolizing enzymes. Ancestral apes first started consuming ethanol 10 million years ago -- around the same time they climbed down from the trees and adapted to a terrestrial life on the forest floor. The study was published in the Proceedings of the National Academy of Sciences this week.

Many diseases that we have today can be attributed to the incompatibility between our current environment and the environment that our genome was adapted for. Our recent increase in sugar consumption, for example, has led to obesity, diabetes, and hypertension. But an animal that’s fully adapted to a sugar-rich diet wouldn’t be expected to get these diseases, suggesting that we simply haven’t had enough time to adapt to this modern diet. Similarly, a recent introduction of ethanol -- a mere 9,000 years ago -- would indicate that alcoholism in modern humans reflects an incomplete genomic adaption due to insufficient time. On the other hand, according to an alternative model, primates were ingesting ethanol through fleshy fruits as early as 80 million years ago.

To investigate, a team led by Matthew Carrigan from Santa Fe College studied alcohol dehydrogenase class IV (ADH4) -- the first enzyme exposed to ethanol in the digestive tract that’s capable of metabolizing ethanol. The team resurrected ancestral ADH4 enzymes from 28 different mammals representing various points along the last 70 million years of primate evolution. Then they measured the ability of these ancient enzymes to metabolize ethanol.

The researchers identified a single genetic mutation that occurred about 10 million years ago that endowed our ancestors (and the ancestors of chimpanzees and gorillas) with a dramatically enhanced ability to metabolize ethanol. “Around this same time, the Earth cooled off, food sources changed, and this primate ancestor started to explore life on the ground,” Carrigan tells Science.

The ability to metabolize ethanol occurred long before human-directed fermentation, and it was likely advantageous to primates living where lots of highly fermented fruit is available, especially when food is scarce. Because fruit collected on the ground contains higher concentrations of fermenting yeast and ethanol than the same fruits hanging on trees, this transition may have been the first time our ancestors were exposed to substantial amounts of dietary 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 adds. “You’d be a cheap date.” Getting sick or drunk off fruit would make you unable to defend your territory or go looking for food, so selection favored those with the new mutation.

Additionally, these findings might explain why our brains evolved to link pleasure pathways with alcohol consumption, Science explains: Ethanol was associated with a key food source.

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