Obviously, alcohol's been a part of human society for a very long time. We started making it at roughly the same time we figured out agriculture, 9,000 years ago. A new study on our now-extinct ancestors revealed that they developed the ability to metabolize alcohol when they started going down to forest floors, about 10 million years ago.
It's not exactly a surprise that fruit that's fallen from the tree is more fermented, and therefore higher in alcohol content, then fruit on trees. Other animals make use of this today. So of course surviving on the ground rather than in the trees would require the ability to metabolize ethanol.
This study, published in the latest issue of PNAS, was an attempt by the researchers to aid in the understanding of alcoholism and determine whether it could be the result of "incompatibility between our current environment and the environment for which our genome is adapted." In other words, is there simply more alcohol in the modern world than we've evolved to handle?
The study points to two historical models in opposition: One saying that ethanol is relatively new to our diet and one arguing that we adapted to it long ago:
In one historical model, ethanol was not a significant part of the hominin "Paleolithic diet" and was also absent from the diets of earlier ancestors. Rather, the model holds that ethanol entered our diets in significant amounts only after humans began to store surplus food (possibly because of the advent of agriculture) and subsequently developed the ability to intentionally direct the fermentation of food (∼9,000 y ago), perhaps as a means of preservation. In this model, alcoholism as a disease reflects insufficient time since humans first encountered ethanol for their genome to have adapted completely to ethanol.
... In an alternative model, primates may have ingested ethanol via frugivory as early as 80 million y ago (Ma), a time corresponding to the origin and diversification of primates and when angiosperm plants first produced fleshy fruits that can become infected by yeast capable of the accumulating ethanol via fermentation. In one version of this model, small amounts of ethanol present in slightly fermenting fruit attached to trees attracted arboreal primates foraging in the trees. In this version, our contemporary attraction to ethanol is an "evolutionary hangover" that ceased to be beneficial once that attraction became redirected to beverages with high concentrations of ethanol, made possible only after humans developed the tools allowing them to intentionally direct fermentation (and enhanced with the advent of technology to distill ethanol to higher concentrations). Another version of the "ethanol early" model for ethanol exposure recognizes that ethanol itself, as well as the food naturally containing it, can be a significant source of nutrition. This model posits that any organism with metabolic adaptations that permit the exploitation of ethanolic food would have access to a specialized niche or important fallback foods unavailable to organisms without this metabolic capacity.
The researchers focused on the enzyme ADH4, which plays an important role in preventing alcohol from entering the bloodstream. Looking at ADH4 in modern humans, our ancestors, and our genetic relatives, the researchers found that ADH4 became ethanol-active about 10 million years ago, emerging with our last common ancestor with the chimpanzee and gorilla. The ADH4 that emerged in humans, gorillas and chimpanzees was found to be 40 times better at clearing ethanol from our systems than earlier ancestors. Our last common ancestor with orangutans, for example, was not able to metabolize ethanol, but was able to metabolize the kinds of alcohols found in plant leaves. The researchers hypothesized that, while it was possible that the super-ethanol active ADH4 served no purpose until we started fermenting things, it was more likely that it was a useful adaptation to living on the ground. The evidence of that shift coincides with the emergence of the ethanol-active ADH4 and:
Saps, nectar, and fruit (in situ) also ferment naturally, thereby exposing both terrestrial and arboreal animals to ethanol. Overripe fruit that has fallen to the ground is generally older (and more damaged) than ripe fruit picked directly from trees and, as such, has had more time to ferment, potentially leading to higher concentrations of ethanol. Therefore, the transition to an increasingly terrestrial life would likely have exposed the HCG ancestor to fruit with higher ethanol content. In this context, the increased activity of ethanol-metabolizing enzymes (e.g., ADH4) could provide a selective advantage, particularly during a time of large-scale ecological transitions and extinctions brought about by climate change.
... Having adapted to a more terrestrial lifestyle than its predecessors, it is possible that the ancestor of [humans, chimpanzees, and gorillas] had less access to the fallback [tree-based food of] arboreal gibbons or orangutans, likely leading to greater selective pressures to exploit alternative fallback food strategies. Our study indicates that the [humans, chimpanzees, and gorillas] ancestor possessed the capacity for a novel strategy: an ethanol-active digestive ADH4 enzyme that would permit exploitation of food containing increased concentrations of ethanol. Furthermore, because frugivorous vertebrates generally avoid "rotting" (and presumably more ethanolic) fruit when other options are available, there may have had been little competition for this resource.
Our ancestors may have survived coming out of the trees because they developed the ability to consume fermented fruit 10 million years ago.