You may have wondered why we're attracted to alcohol, and why some people self-destructively consume it to excess.

Of course, we drink alcohol because we like it, but are there deeper evolutionary explanations as to why our brain is so responsive to the stuff? And are there other animals that are exposed to alcohol in natural environments? It is the yeasts, after all, that produce alcohol while fermenting sugars within ripe fruits, nectars, and other sugar-rich substances. Fruitflies figured out a long time ago that the smell of alcohol indicates a great source of calories upwind, both for themselves and for their larvae when they lay eggs on decomposing fruit. But what about birds and mammals, many of which eat ripe fruit for a living?

The fruits of fermentation

The drunken monkey hypothesis proposes that our attraction to booze derives from a powerful sensory bias associating alcohol with nutritional reward. Primates evolved as fruit eaters in tropical rainforests, where yeasts abound and where fermentation is fast because of the warm and moist climate. Ripe fruit can be hard to find, but the smell of alcohol will lead you to the source. And once there, alcohol can stimulate feeding, just as it does in modern humans via the aperitif effect. It's best to eat up these resources quickly before others get there.

So the psychoactive effects of alcohol, as contained within sugar-rich fruit pulp, may have evolved to let hungry primates more efficiently find and consume scarce calories in the forest. This is part of our ancestral sensory and behavioral baggage that is retained into modern times. We even obtain health benefits from low-level alcohol consumption relative to either abstention or high levels of drinking. Similar effects pertain to fruitflies, suggesting genetically based adaptation to alcohol in the diet. The molecular pathways underlying inebriation are also similar between us and flies. Animals usually don't get drunk in the wild, however, because the concentrations of alcohol within fruit are fairly low. Even though our closet living relatives, the chimpanzees, eat ripe fruit as more than 90 percent of their diet, their stomachs fill up before they consume enough alcohol to get drunk.

Winos in the mist

Things can go badly wrong, however, when we have access to large amounts of cheap, high-concentration booze. There is a mismatch between what our great ape ancestors ate, and what we can create today via agriculture, controlled fermentation, and the relatively recent process of distillation to obtain strong spirits. By separating liquid alcohol from fermenting grains or fruits, we can obtain high levels of psychoactive reward while drinking that sometimes results in excessive consumption. As with obesity and diabetes rates being fueled by today's unlimited access to cheap calories in the supermarket, so too can excessive drinking can be viewed as a disease of nutritional excess associated with our modern technological environment. In this sense, alcoholism can informally be thought of as an evolutionary hangover.

So the next time you see either a barfly or a fruitfly, or are simply enjoying a glass of wine, think about evolutionary time, tropical forests, fermenting fruit, and our primate ancestors. And, as with everything else that we enjoy as modern humans, please drink in moderation.



Robert Dudley is a professor of biology at UC Berkeley and author of The Drunken Monkey, now available on Kindle.