New research supports the 'drunken monkey' theory, which suggests our primate ancestors developed a taste for alcohol because it is naturally present in ripe, fermenting fruit. The theory, proposed by Professor Robert Dudley of UC Berkeley in 2014, argues that alcohol is not an unnatural substance for humans, as it is widely found in nature.
Around 100 million years ago, plants began producing sugary fruits to attract animals for seed dispersal. Yeast in the environment can feed on the sugar in very ripe fruit, producing alcohol. In hot, humid rainforests, this creates an 'ethanol plume' above fruit-laden trees, which may have served as an olfactory signal for primates searching for food.
Primatologist Aleksey Maro, a colleague of Professor Dudley, explained that ethanol plumes could help monkeys locate calorie-rich fruit in dense forests. The smell of ethanol, combined with other complex molecules from different fruit species, provides layered cues that guide animals to the source of fermentation.
Initial scepticism about the theory arose because scientists believed primates rarely ate overripe fruit and lacked a taste for alcohol. However, field observations have since documented chimpanzees and other primates consuming fermenting fruit. Dr Kimberley Hockings of the University of Exeter has observed wild chimps eating fermenting breadfruit and sharing it with each other.
A recent study by Maro's team, published in Science Advances, measured alcohol consumption in wild chimpanzees in Uganda and Ivory Coast. They found that chimps consume an average of 14 grams of ethanol per day, equivalent to two glasses of wine for a human. While the chimps did not appear drunk, the findings suggest alcohol is a routine part of their diet.
Humans share a common ancestor with chimpanzees from about six million years ago. Earlier this year, researchers discovered that an enzyme for breaking down alcohol became more efficient in primate ancestors around 10 million years ago, when our ancestors began spending more time on the forest floor where overripe fruit is more common. This evolutionary history may explain why most modern humans enjoy alcohol, despite its potential negative effects.
