It is tempting to start this review with a nod to Monty Python’s Philosopher’s Drinking Song. But there is a dark side to our use and especially abuse of alcohol, lethal traffic accidents being just one of them. Why are we so enamoured with our booze? With The Drunken Monkey, Professor of Integrative Biology Robert Dudley puts forward the idea that it is linked to the dietary preferences of our primate ancestors who used alcohol as a cue to identify ripe fruit. Is this another evolutionary just-so story?

Dudley’s drunken monkey hypothesis revolves around the symbiotic relationship between flowering plants and their animal seed dispersers. One strategy plants employ to ensure their seeds colonise new areas, away from their parent, is to wrap them in a sweet, fleshy shell: fruit. Animals eat these, the seeds pass through their digestive tract and come out the other end in (hopefully) a different place.

As we all know, sweet fruit tastes the best, and plants use sugar to tempt animals into eating their fruit. But we’re not the only ones who love sugar. I have touched on part of this story in my review of The Rise of Yeast: How the Sugar Fungus Shaped Civilisation, as this fungus also feeds on sugar and in the process produces alcohol, likely as a strategy to kill bacterial competitors). Alcohol, specifically ethanol, is a volatile compound that can be smelled at a distance and can act as another cue to detect ripe fruit. (How do you think those fruit flies always find your bowl of overripe bananas?)

So, Dudley says, our primate ancestors, through the consumption of ripe fruit, had a fairly constant, low-level exposure to alcohol, though the many anecdotal reports of drunken animals are often exaggerated. The maximum theoretical alcohol production, before the substance starts inhibiting yeast’s biochemistry, is below 15%. It took the invention of distillation to produce harder liquor.

And this is the crux of the argument: what used to serve us in our evolutionary past no longer does so in our modern, urban environment. This is the tenet behind the discipline of evolutionary medicine, which seeks to explain human health problems by looking at our evolutionary past. As also popularised in books such as Mismatch: The Lifestyle Diseases Timebomb and Mismatch: How Our Stone Age Brain Deceives Us Every Day And What We Can Do About It, the argument goes that our bodies and minds have not yet had the time to adapt. We crave sugar, salt, and fat, but this instinct backfires in a world where these substances are cheap, easy to obtain, and overabundant. The resulting epidemic of obesity and diabetes is another prominent consequence (see e.g. The Hungry Brain: Outsmarting the Instincts That Make Us Overeat).

“We crave sugar, salt, and fat, but this instinct backfires in a world where these substances are cheap, easy to obtain, and overabundant.”

It’s a neat idea, but is it true? Encouragingly, Dudley is not shy about the gaps in our knowledge and I found The Drunken Monkey to be very level-headed in that regard. There is a dearth of field data on alcohol levels in ripening tropical fruit and in wild animals after eating fruit. Similarly, how important the odour of alcohol is in attracting wild animals in comparison with fruit colour, and how well both function in the complex three-dimensional environments of tropical forest canopies has never been studied in the wild. Many entertaining though challenging field experiments await those brave enough to tackle these questions.

Furthermore, what we do know is quite limited and perhaps not all that relevant. Experiments with fruit flies have provided evidence for a health benefit of systemic, low-level exposure to alcohol. Similarly, epidemiological studies suggest the same might be true for humans, but direct experimental data in vertebrates is lacking. What little data has been obtained in laboratory settings involves animals drinking alcoholic liquids directly, which does not capture the natural context of eating fermented food.

Similarly, Dudley is critical of our clinical diagnosis of alcoholism. It has been notoriously flexible through time, and even today varies by practitioner, cultural group, and country. And importantly, he says, it does not depend on quantitative measures of alcohol intake, but on qualitative measures of impacts of excessive drinking. The search for a genetic explanation of why some people can hold their liquor where others become drunkards is also fraught with difficulty. Twin studies give heritability estimates between 20% to 60%, indicating a large environmental component. Searches for “the” gene for alcoholism (candidate gene approaches) have been replaced by the sobering insight that alcoholism is a complex trait depending on many genes of small effect, requiring so-called genome-wide association studies. (For much more on twin studies, the genetic underpinnings of our behaviour, and complex traits, see my review of Blueprint: How DNA Makes Us Who We Are.)

“[…] these evolutionary mismatch stories assume we haven’t had the time to evolve to better match our new environment. This boils down to the question of how fast evolution happens, which is hotly debated.”

So far, so convincing; Dudley is appropriately cautious. What I am mildly sceptical about with these evolutionary mismatch stories is the assumption that we haven’t had the time to evolve to better match our new environment. As Dudley mentions, findings from both archaeology and the study of fossil teeth suggest intentional fermentation started happening some time between 10,000 to 5,400 BCE (see e.g. Uncorking the Past: The Quest for Wine, Beer, and Other Alcoholic Beverages and Ancient Wine: The Search for the Origins of Viniculture, as well as my reviews of The Tales Teeth Tell: Development, Evolution, Behavior and Evolution’s Bite: A Story of Teeth, Diet, and Human Origins). Is that really not enough time?

This all boils down to the question of how fast evolution happens, which is hotly debated. Traditionally considered a slow process, many argue there are various processes to speed it up, from Stephen Jay Gould’s ideas (see Punctuated Equilibrium), evolution probing multidimensional spaces of possible protein sequences (see Arrival of the Fittest: Solving Evolution’s Greatest Puzzle), the famous domestication experiments with silver foxes (see How to Tame a Fox (and Build a Dog): Visionary Scientists and a Siberian Tale of Jump-Started Evolution, and my review here), or the role of epigenetics (see my review of Extended Heredity: A New Understanding of Inheritance and Evolution). See also my review of Rates of Evolution: A Quantitative Synthesis. Then again, Nathan Lents pointed out, we are a walking comedy of errors: evolution is often limited by the material at hand and the legacy of the past (see my review of Human Errors: A Panorama of Our Glitches, from Pointless Bones to Broken Genes). The problem is that this potentially allows you to invoke slow or fast evolution depending on what your explanation requires, something not addressed here.

That notwithstanding, The Drunken Monkey is a well-paced and level-headed book. It provides all the background you need to understand Dudley’s hypothesis, makes no secret of limitations and gaps in our knowledge, and carefully lays out a research programme. A fine example of striking the right balance between cautious scholarship and imaginative, convincing explanations.

Disclosure: The publisher provided a review copy of this book. The opinion expressed here is my own, however.

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