Ethanol, fruit ripening, and the historical origins of human alcoholism in primate frugivory

Ethanol is a naturally occurring substance resulting from the fermentation by yeast of fruit sugars. The association between yeasts and angiosperms dates to the Cretaceous, and dietary exposure of diverse frugivorous taxa to ethanol is similarly ancient. Ethanol plumes can potentially be used to localize ripe fruit, and consumption of low-concentration ethanol within fruit may act as a feeding stimulant. Ripe and over-ripe fruits of the Neotropical palm Astrocaryum standleyanum contained ethanol within the pulp at concentrations averaging 0.9% and 4.5%, respectively. Fruit ripening was associated with significant changes in color, puncture resistance, sugar, and ethanol content. Natural consumption rates of ethanol via frugivory and associated blood levels are not known for any animal taxon. However, behavioral responses to ethanol may have been the target of natural selection for all frugivorous species, including many primates and the hominoid lineages ancestral to modern humans. Pre-existing sensory biases associating this ancient psychoactive compound with nutritional reward might accordingly underlie contemporary patterns of alcohol consumption and abuse.