Our Early Ancestors Evolved the Ability to Process Alcohol 10 Million Years Ago

​Our ancestors were likely getting drunk as long as 10 million years ago—long before modern humans or even Neanderthals had evolved, a new study suggests.

The ADH4 gene, one that is important in the digestion of ethanol, evolved in the last common ancestors of humans, apes, and chimpanzees, according to an analysis of the history of that gene by Matthew Carrigan of Santa Fe College in Gainesville, Florida.

The evolution of the gene coincid​ed with the time that early hominids moved from living primarily in trees to living primarily on the ground, which makes a lot of sense, when you think about it.

These hominids were eating fruit that was often found on the ground, which, well, can sometimes be alcoholic because it's sitting there for a while.

"Fruit collected from the forest floor is expected to contain higher concentrations of fermenting yeast and ethanol than similar fruits hanging on trees," Carrigan wrote i​n a paper published in Proceedings of the National Academy of Sciences. "This transition may also be the first time our ancestors were exposed to (and adapted to) substantial amounts of dietary ethanol."

There are two ways of looking at the evolution of Carrigan's findings: Being able to process alcohol was either a random, useless mutation that developed in early hominids millions of years ago that didn't become useful until humans started getting drunk on purpose, or it played an important evolutionary role. He argues in the paper that it's the latter—that ethanol likely played a nutritionally important role in the lives of our ancestors.

That's where the fruit-on-the-ground scenario comes in.

"The transition to an increasingly terrestrial life would likely have exposed [our ancestors] to fruit with higher ethanol content. In this context, the increased activity of ethanol-metabolizing enzymes could provide a selective advantage, particularly during a time of large-scale ecological transitions and extinctions brought about by climate change," he wrote.

In other words, animals that could eat alcoholic fruits without getting sick or dying were better suited to survive. It's an idea that​'s been put forth before, several times. But the history of the ADH4 gene is some of the first hard proof we have.

So, what the heck does this 10-million-year-old adaptation mean to you and me? Quite a lot, actually. Cardigan muses that the ethanol found in ancient fermented fruit is "remarkably similar in concentration and form to the moderate ethanol consumption now recognized to be healthy for many humans."

He also suggests that various mutations and variations in ADH4 expression, many of which have not yet been characterized by science yet "may play an important role in the variability of human ethanol metabolism and risk for developing alcoholism or ethanol-related cancers of the upper gastrointestinal tract."

The finding is also something of a break from current thinking about alcoholism in general.

Though some scientists did believe that our ancestors had developed the ability to process alcohol, another thought was that the ability to process alcohol evolved after humans started fermenting alcohol, just 9,000​ years ago. If that were true, Carrigan notes that "alcoholism as a disease [would] reflect insufficient time since humans first encountered ethanol for their genome to have adapted completely to ethanol."

That school of thought appears to be disproven by his finding. Instead, today's alcoholics could be overly attracted to et​hanol because being able to metabolize it was once a very beneficial trait. With alcohol (and food) being plentiful these days, it's not evolutionarily advantageous anymore.

"In this version [of history], our contemporary attraction to ethanol is an 'evolutionary hangover' that ceased to be beneficial once that attraction became redirected to beverages with high concentrations of ethanol," he wrote.

"Evolutionary han​gover" it turns out, is a very appropriate phrase in this case.