If your mouth waters every time you think of that 1990 Bordeaux you've been waiting to open, consider that your desire to drink could be the result of an evolutionary hangover.
Humans may be hardwired with an instinctual attraction to alcohol, theorizes Robert Dudley, a biomechanics professor at the University of California, Berkeley. He and a small group of other scientists are exploring the possible evolutionary origins of drinking, hoping to shed light on the relationships between humans, alcohol and health. This Darwinian approach to medical science has fermented debate in the research community.
Ethanol is found widely in ripe wild fruit, Dudley explained. When wild yeast lands on the fruit and feeds on the sugars, fermentation occurs. The riper the fruit, the more alcohol it produces.
Many birds and mammals, including our primate ancestors, depend heavily on fruit, Dudley said, and they may have learned to find this food source quickly by following the scent of ethanol. Basically, the smell may act as a chow bell, signaling animals from afar that dinner's ready. (In turn, the plants benefit, as their seeds get to hitch a ride, spreading to new areas through the animals' waste.)
Primates appear to have a highly developed sensitivity to the smell of ethanol, Dudley said, which may give them an edge over other fruit-eating animals. And this sensitivity may have been passed on to humans. Today, we continue to be attracted to foods that benefited our ancestors.
Dudley first proposed the idea in 2000, in papers published in The Quarterly Review of Biology and the medical journal Addiction. He pulled together information from more than 100 other studies on topics such as the rate at which fruits ferment, the natural occurrence of yeast in the wild, gorilla behavior and genetic models of alcoholism in modern humans. The subject generated enough interest to be the focus of a symposium at the Society for Integrative and Comparative Biology's conference earlier this year.
"Dudley's hypothesis helps us understand why, from an evolutionary standpoint, humans are so attracted to ethanol," said Doug Levey, an ecologist at the University of Florida and a speaker at the symposium. "There is often a big nutritional reward associated with being attracted to ethanol," Levey said, adding that this may help account for why "there are health benefits associated with low-to-moderate levels of ethanol consumption."
Dudley believes that because ethanol occurs naturally and existed before humans, "there must be a historical perspective on a modern disease such as alcoholism." If that relationship could be determined, it could help lead to a treatment. (With our culture of abundance, he suggested, we may overconsume foods that our body cues us to seek as nourishment.)
As an indicator that wild animals are attracted to ethanol, Levey said, there have been "rare but consistent accounts of birds and mammals consuming fermented fruits and becoming drunk."
Pigeons, fish and fruit-eating butterflies, among others, have been seen acting inebriated, said Dudley, who has spent years observing birds, butterflies and flying reptiles from Panama to Southeast Asia. "While no one has officially reported an animal drunk in the wild, there is lots of anecdotal evidence, such as a drunken warthog running on a rampage through a village." He even found a group of cedar waxwings that had died from alcohol poisoning after eating too much overripe fruit.
There are still gaps in the hypothesis, said Levey, such as how one makes the leap from low-level consumption of ethanol in wild fruits to the drinking habits of modern society to full-fledged alcoholism.
One of the missing links in the evolution argument is that there are no verifiable reports of drunken primates, said Katharine Milton, a primatologist at UC, Berkeley. Milton, who also spoke at the conference, has spent 30 years doing field research in the jungles of Central and South America, Africa and southeast Asia and never noticed an intoxicated primate. Nor have the more than 20 colleagues she asked.
Dudley suggests that the amount of ethanol in ripe fruit may generally be too low to get drunk, or perhaps some animals are better able to digest alcohol than humans. Next summer, one of Dudley's graduate students will test this by measuring the ethanol levels in fruits fed to chimpanzees in Uganda and monitoring their blood alcohol levels to see how well they metabolize the substance.
Milton was set to publish a contradiction of Dudley's ideas this month, in a paper (titled "Ferment in the Family Tree") in the journal Integrative and Comparative Biology. She believes that inebriation is a human luxury in which other primates can't indulge. "You can't afford to have even a mild sense of euphoria when you are a primate," Milton said, "because you will get eaten or fall out of a tree and onto your head."