Humans inherited their love of booze from primate ancestors who gorged on fruit containing alcohol.
The “drunken monkey” theory is based on modern day apes who seek out over-ripe fermented fruit for the alcohol.
Researchers believed that the smell of alcohol led our ancestors to ripe and nutritious fruit millions of years ago.
Although just a theory when it was first proposed by University of California Berkley biologist Professor Robert Dudley in 2014, new evidence amongst modern monkeys seems to support it.
The latest study by primatologist Professor Christina Campbell of California State University, Northridge (CSUN), looked at fruit collected from black-handed spider monkeys in Panama.
Along with her team, they found the alcohol content in the ripened fruit discarded was typically between 1-2% by volume, about half the strength of beer.
Researchers collected urine from these free-ranging monkeys and found it contained secondary metabolites of alcohol.
This result shows that the animals were actually using the alcohol for energy – it wasn’t just passing through their bodies.
Prof Campbell said: “For the first time, we have been able to show, without a shadow of a doubt, that wild primates, with no human interference, consume fruit containing ethanol.
“This is just one study, and more need to be done, but it looks like there may be some truth to that ‘drunken monkey’ hypothesis, that the proclivity of humans to consume alcohol stems from a deep-rooted affinity of fruit-eating primates for naturally-occurring ethanol within ripe fruit.”
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In his book outlining his ‘drunken monkey’ theory, Prof Dudley said that some fruits known to be eaten by primates have a naturally high alcohol content of up to 7%, half that of wine.
But at the time, he did not have data showing that monkeys or apes preferentially sought out and ate fermented fruits, or that they digested the alcohol in the fruit.
For the new study the CSUN researchers teamed up with Dudley to analyse the alcohol content in the fruits.
They are currently conducting a parallel study of the alcohol content in the fruit-based diet of chimpanzees in Uganda and the Ivory Coast.
Prof Dudley said: “The new study is a direct test of the drunken monkey hypothesis.
“What we don’t know is how much of it they’re eating and what the effects are behaviourally and physiologically. But it’s confirmatory.”
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However Prof Dudley said that he doubts that the monkeys get drunk in the same way humans do.
He said: “They’re probably not getting drunk, because their guts are filling before they reach inebriating levels.
“But it is providing some physiological benefit. Maybe, also, there’s an anti-microbial benefit within the food that they’re consuming, or the activity of the yeast and the microbes may be predigesting the fruit. You can’t rule that out.”
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