Matthew Carrigan of Santa Fe College in Gainesville, Florida, and his team found that a genetic mutation in our evolutionary past made ADH4 40 times better at breaking down ethanol.
The mutation was effectively ubiquitous in our ancestors by 10 million years ago, which might be significant. This is around the time that those ancestors started adapting to a terrestrial lifestyle and probably first encountered high ethanol content in fruits rotting on the forest floor. This point in prehistory also coincided with a period of climate change that saw forests in Africa shrink while grasslands expanded. In the new environments, fresh fruit would have been harder to come by.
Fallen, over-ripe fruit often lies uneaten for longer than the sought-after fresh and hanging ripe fruit, so it contains more ethanol. As the shift to a terrestrial life was underway, digesting ethanol quickly would have been life-saving for our ancestors, who were still spending half of their time climbing and swinging in trees some 10 to 20m above ground, says Carrigan.