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Alcoholism

Evolution

Overview

Alcoholism is an incredibly complex and difficult problem to understand and solve in our society. The evolutionary perspective is often overlooked but is a key perspective in understanding this disease. The evolution of alcoholism is thought to originate at the consumption of fermented fruits. Those that are able to find and successfully consume ripe fruit had an advantage because of the additional source of nutrients. This led to an association of ethanol (found in fermenting fruits) with energy. Ethanol is produced inside of ripening fruits which contain significant amounts of nutrients and high caloric value. Natural selection favoring primates attracted to alcohol, even if the benefits were not direct, is one hypothesis for why some people are more susceptible to alcoholism than others.  This is an example of Darwinian medicine, and is part of the explanation for why some people may be more susceptible, the whole story about who is more susceptible to alcoholism also includes, genetics, environment, family background, and other stressors, all of which are important and tend to be studied more than the evolutionary aspect. Alcoholism is a disease of nutritional excess, similar to obesity. Early human consumption of ethanol was a byproduct as well as a source of nutrients, but in an industrial society where there is an excess amount of alcohol, this consumption can become a problem.

Fermented Fruit Consumption

Early humans regularly ingested ethanol which was made from yeast - based fermentation of naturally occurring fruit sugars. The sugars found in fruit are an incentive for dispersers to consume and then eventually disperse seeds; the fruit pulp also serves as the base for ethanol production. The development of ethanol in fruits occurs during the ripening process which leaves fruits more available for consumption by dispersers. Unripe fruits contain seeds that have not matured, and if those seeds were to be eaten and dispersed it would be maladaptive. Unripe fruits are also less available to microbes to consume. The ripening of fruit can be seen as a race between dispersers and microbes. Ethanol inhibits the growth of microbes but it also typically makes fruit inedible to vertebrates as well. So, when an organism is able to consume alcohol, those fruits are available to them and not others. There is also an additional advantage to ethanol consumption which is the high caloric value of ethanol. The caloric value of ethanol is 7.1 kcal/g which is nearly twice that of carbohydrates which is 4.1 kcal/g.

Ancestral Ethanol Consumption

Humans originate from a primarily frugivorous (fruit eater) lineage of primates. A large part of primate evolution occurred in warm equatorial climates where fruit fermentation occurred quickly and regularly. The ancestors of human and nonhuman primates were routinely exposed to low levels of ethanol through their fruit eating. This led to corresponding adaptation and preference for ethanol that has been preserved in modern humans.

Hormetic Effect

The Hormetic effect or Hormesis is another aspect of the ancestral relationship humans have with alcohol. The Homertic effect is the idea that low concentrations to stressors, in this case ethanol, can be beneficial, but higher concentrations are stressful and cause harm. The evolutionary explanation for hormesis is based on the assumption that natural selection maximises relative fitness. This is an explanation for why organisms developed the metabolic machinery to consume ethanol in order to maximise its benefits. The Homertic effect in relation to alcohol consumption has not been studied thoroughly in humans but has in the fruit fly genus, Drosophila. The longevity of Drosophila is enhanced at very low concentrations of ethanol but is decreased at higher concentrations. Additionally, the ability to produce an abundant amount of offspring increases in the low concentration presence of ethanol. Other organisms whose diet consists of fermenting fruit share these same characteristics and this may also include humans, seeing as we do have the ability and metabolic equipment to have hormetic advantages from ethanol at low concentrations.

Humans Frugivory

Humans have a far reaching frugivorous dietary heritage. Frugivorous adaptations among primates is thought to have started at least 40 million years ago, though likely earlier. Our closest relatives, the chimpanzees, have a predominantly frugivorous diet which supports the idea of our common ancestor’s frugivorous dietary heritage. Additionally, gibbons and orangutans are almost exclusively frugivory, while gorillas which are partially frugivory. Because of this shared evolutionary history, nonhuman primates have been used as models to understand alcoholism. Researchers have used macaques to test whether natural selection supports genes for traits that lead to excessive alcohol consumption because these same traits may enhance fitness in other contexts. Because of our close lineages, this may be true as well for humans.

Modern Alcoholism

In our prehistoric ancestry, there were advantages to our consumption of ethanol in fermenting fruits. But as the world changed and our living conditions turned to resemble our current modern industrial society, our access to ethanol changed as well. Similar to sugars and fats, ethanol was only found in very low concentrations and because of its tie to fruit sugars, our consumption of it was necessary. So, just like we crave sugar and fat because prehistorically they are only minimally obtainable and necessary for bodily functions, ethanol can also be craved and be over consumed. In our society sugar, fats and ethanol are readily available and in combination with our craving for it, both obesity and alcoholism can be considered diseases of nutritional excess.