The 'drunken monkey' hypothesis proposes that alcohol, and primarily the ethanol molecule, is routinely consumed by all animals that eat fruits and nectar. As first worked out by Louis Pasteur in the 19th century, fermentation is a natural process deriving from the metabolic action of yeasts on sugar molecules. The molecules produce alcohol to kill off their bacterial competitors, and the booze accumulates at low concentrations within fruits and nectar. It also wafts into the environment, producing a downwind vapor trail that reliably indicates the presence of fruits and sugars. Any animal that can sense and follow this odor upwind will come to the source of ethanol and, of course, the sugars within the fruit. In tropical forests, ripe fruit occurs patchily, so any ability to find it over long distances is beneficial.
Around 10 million years ago, as our ape ancestors progressively became more upright and began to walk about bipedally, an interesting change occurred in their physiological ability to process alcohol. Based on DNA sequence data and modern-day reconstruction of ancestral enzymes, we now know that the ability of these early apes to metabolize alcohol increased about 20-fold due to a single-point mutation in their genes, consistent with greater dietary exposure to this molecule. These animals were walking about the forest floor and within savannahs, and might have simply been obtaining greater access to fallen fruits that had been fermenting longer, and thus that contained more alcohol. Whatever the initial advantages might have been for this particular mutation, we have retained it into modern times. What once helped to find food more efficiently in the wild has become a major part of human culture, with alcohol both loved and abused throughout the world.
I find it to be very interesting that our taste for alcoholic beverages really seems to be a naturally occurring thing. It looks as we evolved, we were able to find more places that alcoholic beverages were available. This is a lot closer to our behavior than you might think. The easier the access to the bar, the more often you might visit it. Alcohol exposure is an ancient and persistent part of animal diets suggests that today's drinking behavior is motivated, in part, by deeply rooted reward pathways within our brain. And these responses we share with creatures as diverse as fruit bats, flies and even chimpanzees.