One of the things that makes humans an unusual animal is that we like a drink – sometimes more than is good for us.

Most animals lack the taste for booze and the ability to effectively metabolize the alcohol in it.

For non-humans, very small amounts of alcohol can result in disorientation, clumsiness and illness, which are the perils we associate with drinking to excess. These are significant disadvantages in a natural world full of predators and competitors.

This has led many biologists to ponder what many of us wonder after overindulging at a party: Why do we do this to ourselves? Why do humans tolerate – and even desire – alcohol?

Chimpanzees and gorillas share our tolerance for alcohol. (copyright Delphine Bruyere, cc-by-sa-3.0)

According to biologist Matthew Carrigan, from Santa Fe College in Gainesville, Fla., the answer may lie in an ancient adaptation that our ancestors acquired when we moved out of the trees.

In an interview with CBC Radio’s Quirks & Quarks, Dr. Carrigan explained that the key to human tolerance is an enzyme that helps break down alcohol, called ADH-4. He says humans have a particularly effective form of this enzyme.

"The difference is dramatic. There’s a 40-fold increase in the efficiency of our enzymes compared to almost all other primates.”

As a result, he says, we can tolerate large amounts of alcohol and even break it down chemically into energy-providing fuel.

Dr Carrigan investigated the evolutionary history of this enzyme by comparing the form we carry with the one found in apes, monkeys and other primates.

What he found was that about 10 million years ago a mutation appeared in the common ancestor we share with gorillas and chimpanzees, which allowed that primitive ape to tolerate alcohol.

But 10 million years ago there were no wineries or holiday parties, so Dr. Carrigan needed an explanation for how this mutation provided an adaptive advantage that would make it spread.

“We started wondering what other scientists observed occurring at about the same time, 10 million years ago,” says Dr Carrigan.

What he found was that this was about the time when our ancestors moved out of the trees, and also a time of climate change, shrinking forests and scarce food resources.

One kind of food possibly available to our newly ground-dwelling ancestors would have been fallen fruit, which would have fermented on the ground. Most animals would take a pass on this potentially dangerous alcoholic fruit, but perhaps not our ape ancestor.

The amount of alcohol in wine, beer and spirits far exceeds what would be found in rotting fruit.

It wouldn’t have been really attractive food, thinks Dr. Carrigan, “but given the choice between starving and eating rotten fruit, it appears eating rotten fruit is advantageous, as long as you have the enzyme to process the alcohol and not get intoxicated.”

That could explain why we can tolerate some alcohol - but the amount of alcohol in wine, beer and spirits far exceeds what would be found in rotting fruit.

And humans have clearly developed a desire, sometimes a quite unhealthy one, for alcohol in relatively large amounts. Dr. Carrigan says this may be because alcohol became the signature of a newly desirable food.

“It’s possible that our neurological reward system became hard-wired to associate reward with the nutritional benefit from this food item.”

Clearly then, it just takes one rotten apple to make the whole barrel worth drinking.