Down there on all fours, ancestors had access to fruit that had fallen from the trees and was fermenting — so it had a buzzy kick. And that's when that ADH4 enzyme seemed to really come in handy.

"The emergence of ADH4 in our ancestors wasn't slow and gradual; it was a rather abrupt shift of a large magnitude," Carrigan tells The Salt.

To figure out when the enzyme might have become a regular in our gut, Carrigan used paleogenetics, an experimental approach in which gene sequences from contemporary species are used to estimate how proteins, and in this case enzymes, evolved over time.

This ability to eat fermented fruit — not just ripe fruit — and use the alcohol for energy, as well as the sugars, vitamins and proteins in that fruit might have helped us survive the changing climate, Carrigan says. But he says, it also elucidates another dimension of our long relationship with alcohol.

"There are hypotheses that the reason humans consume ethanol is because of our recent transition to farming, and how we learned how to ferment, grains or fruit, maybe because we wanted to escape consciousness," he says. "But my study shows that maybe it has its roots in our ancient history as fruitivores."

The findings have intriguing implications for research into the evolutionary origins of alcoholism, Carrigan says. We humans have only been fermenting alcohol for 9,000 years, but his research shows we've actually been drinking it for millions of years. So when and why did our relationship to booze become problematic? That's a mystery that remains to be solved.

Copyright 2014 NPR.

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