Forbidden fruit: understanding alcoholism in the context of evolution

AUSTIN, Texas—Researchers studying alcoholism and addiction would do well to take primate evolution and patterns of fruit consumption into account, according to a University of Texas at Austin biology professor. Dr. Robert Dudley believes scientists studying addiction should not assume that alcoholism is a historically recent aberration.

Alcohol has been available for tens of thousands of years to primates and other animals wherever there are fruit trees, according to Dudley, an associate professor in the integrative biology section of the College of Natural Sciences. "Exposure to small amounts of alcohol from time to time in the form of partially fermented fruit may have played an important role in the evolution of today's humans," Dudley said. He believes that a better understanding of the reasons that humans are motivated to drink in the first place -- especially reasons based on biology -- could help improve treatment of alcoholism.

"Treatment could be enhanced through an understanding of the basic biological mechanisms that underlie the motivation to consume alcohol," Dudley said. "This hypothesis doesn't try to justify or promote consumption of ethanol (alcohol), it merely tries to explain why that might be an evolved outcome."

An article by Dudley on this topic has just been published in the Quarterly Review of Biology. Dudley, a native of Edinburgh, Scotland, has taught at UT Austin since 1992. He also is a research affiliate with the Smithsonian Tropical Research Institute in Panama. An expert in the evolution of animal flight, Dudley became interested in the interaction between primates and fruit bearing plants during his five years of research in tropical forests.

Dudley's focus is on the impact on human evolution of very small amounts of ethanol that occur only at certain seasons -- not the large amounts available today. "Standard fermentation basically gives you a natural occurrence of ethanol of less than one percent in most fruits. Present day availability of ethanol occurs at concentrations and in volumes that -- in evolutionary terms -- were never experienced," Dudley said.

His article notes that birds, bats, butterflies and a host of fruit-munching mammals, including monkeys and human beings, may have gained strategic survival advantages from consuming alcohol in the wild. Fruit-bearing plants gained advantages from this as well.

In tropical forests, wild fruit trees can be few and far between. The appearance of ripe fruit is highly seasonal. To disperse their seeds at the right time, it's important for fruit bearing plants to inform the consuming public just when dessert is on the table.

Odor plumes (including ethanol) that waft from ripe fruit into the jungle air constitute an ideal mechanism for advertising fruit availability over long distances between these trees, Dudley said. The high sugar content, the pungent smell and extra calories created by fermentation and the presence of ethanol as the fruit begins to decay make it attractive to many consumers. This means fruit seeds and pits are dispersed over a wider area, improving survival chances for the plants.

Research also suggests that the ability to forage efficiently for fruit has been a survival advantage to various critters for the past 40 million years. Many of today's primates feed primarily on fruit and so, undoubtedly, did many of humanity's hairy ancestors. That means evolution favored fruit eaters -- including those best able to sniff out over-ripe, alcohol-laced fruit.

It also may have favored adaptations in the way humans metabolize small amounts of alcohol to make the best use of its physiological benefits. Dudley said this possibly could explain why scientists have been able to demonstrate that consumption of moderate amounts of alcohol lowers cardiovascular disease and appears to be associated with a longer life span. Dudley said research has shown that "one glass of wine or beer a day is correlated with enhanced longevity in many people."

Dudley's conclusion? Alcohol, appearing intermittently with the seasons, has been a part of the animal diet for eons and has played an important part in human evolution. The ability to sniff your way to alcohol, sugar and calorie laden fruit is an advantage when fruit is wild and scarce. It is not an advantage when a society built on mass production makes wine coolers available at every shopping center.

"This hypothesis is similar to research linking high rates of obesity and diabetes to the historically unusual volume of fats and carbohydrates available in industrialized societies," Dudley said. "Genetically-based behaviors adaptive in the ancestral environment become disadvantageous in a modern human environment that provides unlimited access to nutritional substrates, including ethanol."

Dudley said that the role of evolution largely has been ignored in studies of human alcoholism. In looking at the biology that underlies addiction, he said, researchers need to consider that the interaction between hominids and alcohol may have a much broader influence on the behavioral responses of today's problem drinkers than currently is realized.

For more information, contact Dr. Robert Dudley at (512) 471-3499 or the Office of Public Affairs at (512) 471 3151.