Darwin’s bottle: the evolution of wine and drinking
James L. Cambias
Most people don’t want to drink toxic waste, but that is exactly what every wine fancier does after pulling the cork on an aged Bordeaux or a crisp Chardonnay. Alcohol is a waste product given off by yeast cells, and it is poisonous to many other organisms. So why do we humans find it good to drink?
At the January 2004 conference of the Society for Integrative and Comparative Biology in New Orleans, a group of biologists led by Dr. Robert Dudley of UC Berkeley, gathered to look for the evolutionary roots of alcohol formation and consumption.
The seminar was partly a debate. Back in 2001, Dudley published a paper in the journal Addiction on the evolutionary roots of alcoholism. In that paper, he suggested that alcohol addiction in humans could have been inherited from our primate ancestors, who sought out fermented fruits as a way to tell when they were ripe. At the New Orleans meeting, some other scientists presented results that could either confirm that hypothesis or shoot it down.
Dr. Douglas Levey of the University of Florida has studied the ecology of alcohol production. It’s part of a three-way struggle among fruit-bearing plants, fruit-eating animals, and the insects and microorganisms which cause decay.
Each of these three has its own agenda. For the plants, the whole purpose of making fruit is to “bribe” animals to disperse the plant’s seeds widely, increasing the chance that they will wind up in a good place to grow. The plants fill their fruit with sugars to attract seed-dispersing animals.
But this strategy only works if the fruit doesn’t spoil. Animals don’t like to eat rotting fruit, and the microorganisms or insects that do can’t disperse seeds. A rotten fruit’s seeds drop right next to the parent plant, which means that the offspring wind up competing with the larger parent for light and nutrients–not a productive evolutionary strategy.
So fruiting plants evolve defenses against microorganisms. They armor their fruit with thick rinds, or divide the interior into segments to prevent decay from spreading easily. They also lace the fruit with chemicals that can kill or repel microorganisms–some examples familiar to winegrowers are tannin and citric acid. We consider them pleasant or interesting flavors, but to many kinds of fungi and bacteria, they’re chemical weapons.
The micro-organisms fight back in a variety of ways. They evolve a tolerance for the plant defenses, and improved ways of getting at the sugars contained in fruit. Yeasts, for instance, have a two-stage strategy for attacking large fruit. The yeast spores are everywhere in the air, so they are present inside most fruits as they form. The yeast cells remain dormant as long as the fruit is unripe, but as soon as the ripening enzymes start turning starches into sugar in the fruit, the yeast goes to work.
When the fruit is mostly intact and its interior is sealed off from the outside air, yeasts use anaerobic reactions to metabolize the sugars into alcohol, releasing energy the yeasts use to live. As oxygen penetrates the decaying fruit, the yeasts can make a second pass at the alcohol using aerobic reactions, getting another harvest of energy. As Levey puts it, “I think it’s probably best to view ethanol as an intermediate compound rather than as a waste substance.”
The alcohol also poisons other microorganisms which might compete against the yeast. The Saccharomyces genus of yeast is among the champion alcohol producers, and one of them, S. cerevisiae, is better known as brewer’s yeast.
In addition to battling plants, the micro-organisms are also fighting the fruit-eating animals. If you’ve evolved to live in fruit, the digestive system of an animal is likely to be a deadly place. So micro-organisms evolve methods of repelling animals, like releasing toxic waste products into the fruit to make it unappetizing. Alcohol can certainly have that effect at high concentrations: Above about 1% concentration, most animals are repelled by alcohol.
Animals that eat fruit thus have to contend with a whole barrage of chemical weapons produced by this ongoing evolutionary war between plants and micro-organisms. They must be able to tolerate the plant defenses and the various wastes produced by the microorganisms. They also have to find the fruit in the first place.
Nathan Dominy, of the University of Chicago, is interested in how animals find ripe fruit and why it’s important to them. Unripe fruit is harder to digest, and may contain higher concentrations of defensive chemicals. There’s a reason why eating green apples gives you a stomach ache. So any fruit-eating animal wants to eat ripe ones. The animals also want to avoid eating rotten fruit, which may be laced with potentially harmful waste products given off by micro-organisms.
To determine ripeness, animals use the same methods one sees at the produce counter–smelling the fruit, feeling it to see if it’s soft, and looking at its color. Primates are particularly good at this, as they have good vision to judge color, hands to feel the softness of a fruit, and a sharp sense of smell. Interestingly, primates are sensitized to the smell of alcohol. They can detect it at concentrations other animals cannot. A human can smell alcohol better than a rat can.
Ripening fruit naturally contains ethanol produced by yeast cells. In general, the riper it is, the more ethanol a fruit contains. So to a primate, the scent of alcohol indicates something good to eat. But too much can be trouble.
This is reflected in the way humans process alcohol. Our bodies produce the enzymes Alcohol Dehydrogenase (ADH) and Aldehyde Dehydrogenase (ALDH) to break down alcohol, aldehydes and ketones into simpler molecules the body can use. The stomach can produce limited amounts of these enzymes, allowing it to directly break down alcohol at concentrations of less than 1%. Interestingly, this is about the concentration level found in very ripe fruit.
Above 1%, the liver has to take over. The liver is the body’s defense against many toxic chemicals, and can produce larger amounts of ADH and ALDH to break down alcohol. But to reach the liver, the alcohol has be absorbed into the bloodstream, which means it can affect all other parts of the body as well–including the brain.
It’s noteworthy that many fruit-eating birds have large livers capable of producing quantities of ADH. An experiment on starlings in 1996 showed just how remarkable their ability to process alcohol really is. Researchers gave European starlings a dose of 10% alcohol and found it was completely metabolized within two hours. Examining their livers, the scientists found levels of ADH 14 times higher than in humans.
Despite this ability to handle alcohol, birds don’t appear to be attracted to it. Nor are other large animals. Levey’s tests of birds in Central America, and work on fruit bats by a team from Ben-Gurion University in Israel, showed that given a choice between ripe fruit with low alcohol and fruit with high alcohol, the creatures tended to go for the low-alcohol fruit. Fruit flies, however, are a different story. They actively home in on the smell of alcohol–something to remember the next time you’re having a glass outdoors.
The New Orleans seminar concluded with Dr. Arthur Klatsky, of the Kaiser Permanente HMO system, speaking about the health effects of alcohol. According to Klatsky’s research using the vast database of patients available to Kaiser Permanente, there is a small but real reduction in mortality among patients who are light consumers of alcohol–less than three drinks per day. In that range, everything else being equal, the light drinkers tend to live longer than nondrinkers.
Klatsky speculates that this is due to the effect of blood alcohol on arterial plaque. Above three drinks a day, however, the effects are also quite clear: the more you drink, the shorter your lifespan.
So what does all this mean about the evolution of drinking? Apparently, humans can consume alcohol because it naturally occurs in fruit, and our ancestors ate fruit. At the levels those ancestors encountered it–1% or less–we can tolerate it and may even gain some benefit from it. At higher levels it’s bad for us. Are we “naturally” inclined to like alcohol? Scientists still don’t agree. The animal evidence says no: Birds and bats avoid alcohol. And yet we drink it. As Dudley points out, “Many modern humans clearly consume ethanol on a regular basis; this could be termed a preference. Others avoid it altogether. Some overindulge.” Something to think about over a drink.
(James Cambias is a writer and game designer based in Deerfield, Mass. Contact him through email@example.com.)
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