Frugivory By-Product Hypothesis
As ethanol provides cues to fruit ripeness, the human penchant for consuming alcohol may be a by-product of adaptive fondness for ripe fruit.
Alcohol pervades most, if not all, human cultures, and its consumption is regarded as a desired accompaniment to a wide variety of activities. From serving as a social lubricant to playing a role in addiction, many researchers have taken an interest in studying how alcohol is consumed and the consequences of that consumption, resulting in an impressive literature on alcohol. More elusive, however, are explanations for why humans as a species consume alcohol at all despite its significant toxicity (Brooks 1997). The current entry describes the hypothesis put forth by Dudley (2000) that alcohol (ethanol) tolerance and consumption may have emerged as a by-product of our ancestors’ adaptive penchant for ripe fruit.
The Evolution of Frugivory Diets
According to fossil records, hominoids have historically survived on a diet consisting mainly of fruits and plants (Andrews 1981). As descendants of the hominoid line, all modern apes, including humans, are similarly adapted for frugivorous diets. Primates such as the lowland gorillas, orangutans, and chimpanzees have all been observed to consume a wide range of fruits in copious amounts (Rodman 1977; Tutin et al. 1991).
Fruits offer the highest dietary benefits when they are ripe. When fruits ripen, a variety of changes occur. Starch is converted to sugar, resulting in a hike in sugar concentration levels and an increase in sweetness. Frugivores are adapted to find the sweet taste of sugar palatable, which then motivates them to seek and consume fruits particularly when those fruits are in a ripened state where caloric and nutritional content is highest. Other biochemical processes that occur during ripening, such as changes in acidity, color, aroma, and texture (Dudley 2004), also entice frugivores’ consumption of ripe fruits. In return, frugivores aid fruits and plants in seed dispersion through a mutually beneficial coevolutionary process. Indeed, fruits that are unripe and carry seeds that have yet to mature have chemical and physical safeguards to prevent untimely consumption by frugivorous seed dispersers (Benner et al. 2002).
Ripened fruits, however, are not always accessible or available, especially in tropical forests where harsh or sporadic weather patterns can hinder fruit development (Leigh 1999). Coupled with intense competition between species of frugivores for scarce ripe fruit, frugivores evolved to be extremely competent foragers. It has been argued that the advanced cognitive and visual capabilities of frugivorous primates are attributable to the selection pressures brought on by the desire for ripe fruit (Milton 2006).
Alcohol Consumption as a Byproduct of Frugivory Diets
Noting the adaptive challenge of acquiring and consuming ripe fruit, Dudley (2000) surmised that frugivores likely evolved to exploit cues that could facilitate the acquisition and intake of optimally ripe fruit. Fermentation occurs during fruit ripening, which is a complex biochemical process whereby yeast feeds on sugar and produces ethanol. Through fermentation, the odorous plumes of volatile ethanol that emanate from ripe fruit may serve as far-ranging olfactory signals for frugivores. Moreover, ethanol has a distinct taste, which can serve as an additional indicator of ripeness alongside the fruit’s sweetness. Thus, utilizing the smell and taste of ethanol enables frugivores to predict a fruit’s caloric value (Singh 1985), and frugivores that could detect the odor of fermenting fruit from afar also had a better chance of acquiring the nutrition and calories needed for survival.
Furthermore, as fruits that are optimally ripe are likely to be seized and consumed quickly, many frugivores have to settle for leftover overripe fruits that contain substantial amounts of ethanol (Dudley 2004). When metabolized, the caloric content of ethanol is estimated to be 7.1 kcal/g, which is actually higher than that of carbohydrates. As some ethanol consumption may have been unavoidable and ethanol can also provide a source of nutritional supplement, frugivores possibly evolved to be somewhat tolerant of ethanol’s toxicity. As descendants of early hominoids who may have evolved to be amenable to ethanol as a by-product of pursuing frugivorous diets, humans may likewise be adapted to not only tolerate the toxicity of ethanol but also have a taste for it.
The frugivory by-product hypothesis suggests that alcohol consumption in humans and our subsequent fondness for the substance may originate from our frugivorous ancestral heritage. Through selection pressures borne from the need to acquire nutritious food sources, frugivores may have evolved to respond favorably to ethanol in order to find and consume ripe fruit. Patterns of alcohol use by humans in contemporary environments may therefore reflect a co-option of ancestral nutritional strategies, specifically the adaptive sensory biases that lead frugivores to associate ethanol with nutritional reward.
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