When it comes to alcohol, human metabolism left us in a tough position. Our bodies are equipped with enzymes that can digest alcohol, which allows us to drink it, but they don’t work all that well, so we end up with traffic accidents and various other regrettable incidents as well as hangovers, and if we drink enough, liver damage.
If we lacked the ability to metabolize alcohol at all we’d never have any of these problems.
Curious as to how we humans developed our paradoxical relationship to alcohol, Matthew Carrigan of Santa Fe College in Florida began to investigate the biochemical history of alcohol metabolism.
“Almost everyone does it,” he said about drinking – meaning that alcohol drinking exists in all cultures except those that have laws forbidding it (and such laws probably wouldn’t exist if some people didn’t want to do it.) “Is this something that’s unique to human consciousness or something that goes much further back?” he asked.
What he found was that it probably all started with a dirty trick played on us by plants. As a strategy to avoid being eaten, many plants evolved the ability to produce a number of different noxious varieties of alcohol. As a countermeasure, animals have developed enzymes that break down alcohols and even in some cases harvest them for energy.
Carrigan looked one of the most prominent enzymes that does this job – called alcohol dehydrogenase – and found that in most animals, it isn’t very good at breaking down our favorite form of alcohol, known as ethanol. And then something happened.
By studying the different versions of this enzyme carried by various mammals, he concluded that it underwent a mutation in our lineage about 10 million years ago, when our ancestors were also the ancestors of chimpanzees and gorillas. That mutation changed the structure of our alcohol dehydrogenase in a way that enabled it to break down small amounts of ethanol.
The results are published in this week’s issue of the journal Proceedings of the National Academy of Sciences.
As soon as he discovered this, Carrigan started calling primatologists, asking whether they knew of any chimps or gorillas who imbibed. Many had not, he said, but a few said that they’d caught our fellow primates nosing around fermented fruit.
Perhaps, he reasoned, this mutation spread right around the time our ancestors started spending less time in trees and more on the ground. There, they might have started encountering fruit that was older, and therefore getting a bit alcoholic. Those who could eat those fruits might have had a big survival advantage over those who were sickened by them.
Breaking down ethanol is a complicated business because it produces a toxic byproduct known as acetaldehyde. And the body has to the break that down before it accumulates and kills us. So creatures that break down alcohol too fast may get sick before they get drunk, and those who break it down too slowly may get drunker than others and find it hard to sober up. That might be particularly disadvantageous 10 million years ago when our ancestors still did some climbing in trees to escape predators.
Our problem, he said, may come down to concentration. Fermented fruit is likely to have a much lower concentration of alcohol even than beer, let alone most other popular drinks. A sip or two of wine with dinner may give us a dose similar to what our distant ancestors consumed. We’re not adapted to drink whiskey, he said. At least not yet. The study leaves open a number of questions, including why so many people find the neurotoxic effects of alcohol so much fun. Carragan says that’s the subject of future investigations.
