TUESDAY, Dec. 2, 2014 (HealthDay News) -- Raise a glass of holiday cheer to this: New genetic research suggests that humans and their ape forebearers may have been relishing alcohol for 10 million years.
As anyone who's had too much knows, alcohol -- called ethanol by scientists -- can be a toxic chemical. And humans' ability to tolerate drinking relies on one enzyme, called ADH4, the researchers explained.
The new study suggests that, beginning about 10 million years ago, humans' primate ancestors developed a gene mutation to produce ADH4. In times when food was scarce, this enabled them to safely consume a noxious byproduct of rotting fruit -- alcohol.
"One hypothesis has been that our ancestors -- ancestors of humans that we also share with gorillas and chimps -- may have been exposed to ethanol millions of years ago as a result of a long history as a frugivore [fruit eater]," explained study lead author Matthew Carrigan. Carrigan is an assistant professor in the department of natural sciences at Santa Fe College in Gainesville, Fla. "Because fruit can naturally ferment, we may have adopted naturally to the consumption of these foods."
The researchers noted that the ADH4 class of proteins is found in the upper digestive tracts of many vertebrates, including humans, horses, mice and rats.
In their study, published online Dec. 1 in the Proceedings of the National Academy of Sciences, Carrigan's team worked with paleogeneticists, researchers who study ancient proteins found in now-extinct animals and species.
Together, they resurrected "enzymes from different points in our ancestors' history," Carrigan explained, looking at samples of ancient ADH4 drawn from key points over 70 million years of primate evolution.
"The evidence suggests that it was actually about 10 million years ago that our ancestors' enzymes changed as a result of exposure to ethanol, which means it's a very long-standing exposure," Carrigan said.
The finding might change the way scientists look at both alcohol and human evolution, he added.
"The same time our adaptation to ethanol took place, our ancestors also adapted to life on the ground," Carrigan explained. "Prior to this our ancestors were living almost exclusively in the trees, rarely coming to the ground, which means they weren't eating fruit off the ground. But once they did, they were being exposed to fruit which is older than fruit found in the tree -- and more likely to be fermented," he said.
"So my suspicion is that our ancestors who developed this mutation were using fermented fruit found on the ground as a 'fallback food' -- it probably wasn't their first choice. But if times got tough, if the climate changed perhaps, and forced our ancestors to adapt to life on the ground, this might be the driver behind their adaptation to ethanol," said Carrigan.
"I should say that at this point this is all speculation," he added. "But basically it suggests that we adapted to ethanol simply so we could tolerate it."
The findings might even offer new insights into alcoholism, Carrigan suggested.
"If it turns out to be the case that ethanol has been a beneficial and important part of our diet for some time, then the question is how does this understanding affect our thinking about alcoholism?" he said.
How a particular person's genes deal with alcohol might give clues to alcoholism prevention and treatment, Carrigan believes.
"It's important to appreciate that there is natural amount of diversity in [individuals'] genes," he explained. "And understanding these differences -- and how these differences might affect a person's risk for addiction -- could be really important when trying to understand how people interact with ethanol differently."
There's more on the genetics of alcoholism at the U.S. National Institute on Alcohol Abuse and Alcoholism.