Human attraction to alcohol may derive from an evolutionary association between ethanol and fruits consumed by animals in nature. Fermentative yeasts are widespread in the terrestrial biosphere, and simple carbohydrates underpinning ethanol production are commonplace within fruits. We determined ethanol concentrations within fruits representing a substantial portion of the diet of our closest living relatives, the chimpanzees. Ripe fruit pulp from 20 angiosperm species in Côte d’Ivoire and Uganda contained an average value of 0.31 (± 0.21 SD) and 0.32% (± 0.20) ethanol (weight/weight), respectively, as scaled by annual chimpanzee feeding time per species at each site. Chimpanzees typically eat ~4.5 kilograms of fruit per day, corresponding to an estimated ethanol ingestion of 14 grams (±9), or the equivalent of 1.4 (±0.9) standard drinks by international standards. These findings are consistent with the hypothesis that ethanol is widespread within tropical fruits and that modern predisposition to alcohol consumption derives from ancestral exposure to this psychoactive substance among frugivorous primates.