If alcohol permeates our galaxy and universe, it should come as no surprise that sugar fermentation (or glycolysis) is thought to be the earliest form of energy production used by life on Earth. Some four billion years ago, primitive single-celled microbes are hypothesized to have dined on simple sugars in the primordial soup and excreted ethanol and carbon dioxide. A kind of carbonated alcoholic beverage would thus have been available right from the beginning.
Today, two species of single-celled yeasts (Saccharomyces cerevisiae and S. bayanus), encompassing a large group of wild and domesticated strains, carry on in this grand tradition and serve as the workhorses that produce the alcohol in fermented beverages around the world. Although hardly primitive—they have most of the same specialized organelles as multicellular plants and animals, including a central nucleus which contains the chromosomal DNA—these yeast thrive in oxygen-free environments, such as we imagine existed on Earth when life began.
If we accept this scenario, then the alcohol generated by these first organisms must have been wafting across the planet for millennia. It and other short-chained carbon compounds eventually came to signal the presence of a convenient, high-energy sugar resource. The pungent, enticing aroma of alcohol announced to later sugar-loving animals of the world, ranging from fruit flies to elephants, where the banquet was to be found. When fruit-bearing trees appeared around 100 million years ago (mya), during the Cretaceous period, they offered an abundance of both sugar and alcohol. The sweet liquid that oozes out of ruptured, ripened fruit provides the ideal combination of water and nutrients that allows yeast to multiply and convert the sugar into alcohol.
Animals became superbly adept at exploiting the sugary cocktail of fruit trees, which in turn benefited from the animals’ dispersal of their seed. The close symbiosis between a tree and the animals that pollinate its flowers, eat its fruit, and carry out a host of other mutually advantageous functions is astonishing. Take the fig tree, with some eight hundred species spread throughout the world. These trees do not bloom in the conventional sense: instead, they have male and female inflorescences that are tightly encased in a succulent-tasting sac called the syconium, and they cannot be pollinated directly because they flower at different times on the same tree. A species of wasp unique to each fig species must carry out this task. The female adult wasp bores through the tip of the syconium, ruining her wings and eventually dying. She lives long enough, however, to deposit her eggs and transfer the pollen from another tree to the female flowers. When the wasp eggs hatch, the wingless male, trapped within the syconium and eventually dying there, impregnates the female and chomps an opening with his powerful jaws for her to escape with another load of eggs. Sustained by sucking alcoholic nectar through her long, strawlike proboscis from the deep corollas of the fig flowers, she goes on to pollinate another fig tree.
While the fig wasps carry on with their secret sex life, air flowing into the hole created by the escaping female causes the syconium to ripen into the fig “fruit.” The yeast goes to work and generates the alcoholic fragrances that alert animals to the potential feast. As many as one hundred thousand figs on a large tree can be devoured by birds, bats, monkeys, pigs, and even dragonflies and geckos in a feeding frenzy.
The fig tree illustrates the intricacy and specificity of the web of life. Other plant sugars, such as evergreen saps and flower nectars, have their own stories to tell. A much sought-after and luscious honey in Turkey, for example, is made from pine honeydew. This is a sugar-rich secretion produced by a scale insect, Marchalina hellenica, which lives in cracks in the bark of the red pine tree (Pinus brutia) and feeds exclusively on the resinous sap. Bees collect the honeydew, and with a specific enzyme, invertase, break down its sugar (sucrose) into simpler glucose and fructose. The final product, honey, is the most concentrated simple sugar source in nature, with the specific plant species from which it derives, red pine in this instance, contributing special flavors and aromas.
Entomologists have exploited insects’ taste for fermented beverages by smearing the substances on the bases of trees in order to capture them. Charles Darwin employed a similar tactic: when he set out a bowl of beer at night, a tribe of African baboons were lured in and were easily gathered up as specimens the next morning in their inebriated state. Intemperate slugs—mollusks lacking shells—are less fortunate, as they self-indulgently drown in beer traps. In one carefully constructed set of experiments, it was shown that common fruit flies (Drosophila melanogaster) lay their eggs where there are intense odors of ethanol and acetaldehyde, another by-product of alcohol metabolism. The fermenting fruit guarantees that their larvae will be well-fed with sugar and high-protein yeast, as well as alcohol, for which they have highly efficient energy pathways.
Nature’s hidden rationale and complex ecological interplay centered on sugar and alcohol resources can have a seriocomic side. Elephants, which consume fifty thousand calories a day, sometimes overindulge in their consumption of fermenting fruit. They can perhaps be excused, as they work hard for their pleasure: they have to remember where to find the trees and travel many miles to reach them at the time of ripening. Unfortunately, they also have a weakness for the human-produced equivalent. In 1985, about 150 elephants forced their way into a moonshine operation in West Bengal, ate all the sweet mash, and then rampaged across the country, trampling five people to death and knocking down seven concrete buildings. This episode highlights problem drinking in higher mammals, including humans.
Birds are also known to gorge themselves on fermenting fruit. Cedar waxwings feasting on hawthorn fruit have suffered ethanol poisoning and even death. Robins fall off their perches. Maturing fruits concentrate sugar, flavor and aroma compounds, and colorants that announce to birds and mammals that they are ripe for eating. As the fruit passes its prime, however, it becomes the target of a host of microorganisms, including yeasts and bacteria, that can threaten the plants. Ominously, the plants defend themselves by generating poisonous compounds. The manufactured plant toxins, including alkaloids and terpenoids, inhibit the growth of virulent microorganisms, as well as fending off noxious insects. A placid-looking apple orchard or idyllic vineyard may not look like a battleground, but a host of creatures are vying for supremacy and defining the balance of power through chemical compounds.
Sometimes, one creature unwittingly steps on a chemical landmine intended for another. If alcohol is dangerous when consumed in excess, plant toxins can be lethal. In one well-documented incident near Walnut Creek, California, thousands of robins and cedar waxwings apparently found the scarlet, mildly sweet berries of holly (Ilex spp.) and firethorn bush (Pyracantha) too good to pass up. Over a three-week period, the birds overdosed on the berries and their toxins and began crashing into cars and windows. Autopsies revealed that their gullets were bursting with the fruit. (By contrast, the normal, demure courtship behavior of cedar waxwings involves passing a single berry back and forth between the male and female until the gift is finally accepted and the pair copulate.)
As with the West Bengal elephants, the California birds’ drunken behavior was due to the excessive consumption of a mind-altering compound. Intriguingly, the compounds in holly berries—caffeine and theobromine—are the same ones humans enjoy today in coffee, tea, and chocolate. Native Americans in the woodlands of the north and the jungles of Amazonia also showed their appreciation for these substances: Spanish colonists observed that they brewed up a bitter but aromatic “black drink” by steeping toasted holly leaves in hot water.
A VERY PECULIAR YEAST
Just as a plant will defend its territory with a chemical arsenal, the invisible world of microorganisms engages in a similar struggle for supremacy and survival. A finely tuned enzymatic system and the production of alcohol are the weapons of choice for S. cerevisiae, the principal yeast used by humans in making alcoholic beverages. About the same time that fruit trees were proliferating around the globe, S. cerevisiae appears to have acquired an extra copy of its entire genome. F
urther rearrangements enabled it to proliferate in the absence of oxygen, and the alcohol it produced destroyed much of its competition. Other microorganisms, including many spoilage- and disease-causing yeasts and bacteria, simply cannot tolerate alcohol in concentrations above 5 percent, but S. cerevisiae survives in fermenting substances with more than twice this concentration of alcohol.
The yeast pays a cost for its success. In producing more alcohol, it forgoes making more of the compound adenosine triphosphate (ATP), which provides living organisms with the energy for essential biological processes. Pure aerobic metabolism yields thirty-six molecules of ATP from glucose. S. cerevisiae makes only two molecules of ATP in air, channeling the rest of the glucose into the production of alcohol to be deployed against its competitors.
S. cerevisiae’s apparent loss later becomes its gain. Because of the doubling of its genome, each yeast cell develops two versions of the gene that controls the production of alcohol dehydrogenase (ADH). This enzyme converts acetaldehyde, an end product of glycolysis, into alcohol. One version of the enzyme (ADH1) reliably processes sugar into alcohol in an oxygen-free environment, whereas the other (ADH2) is activated only after most of the sugar has been consumed and oxygen levels start to rise again. For S. cerevisiae, this happens after many competing microbes have been destroyed. Then ADH2 springs into action, converting alcohol back into acetaldehyde and ultimately generating more ATP. Of course, other microorganisms, such as acetic acid–producing bacteria, which can tolerate high alcohol levels, wait in the wings. They are ready to turn any remaining alcohol into vinegar unless another hungry organism acts faster or is able, like a human, to improvise a way to preserve the alcohol.
It is still a mystery why varieties of S. cerevisiae live on the skins of certain fruits, especially grapes, or in honey, where they are able to tolerate high sugar levels. This yeast is not airborne but can take up residence in special microclimates, like the breweries around Brussels, with their lambic beers, or the rice-wine factories of Shaoxing in China (see chapter 2): both beverages are fermented without intentionally adding yeast. The yeast apparently lives in the rafters of the old buildings, from where it falls into the brew; when the rafters have been covered during renovations, brewers have been unable to start their fermentations. The yeast most likely was carried there by insects, especially bees and wasps, who inadvertently picked it up when they fed on the sweet juice oozing out of damaged fruit, and were drawn to the buildings by the aromas of the sweet worts and juices or musts.
ENTER HOMO IMBIBENS: MAN, THE DRINKER
Our world is awash in ethanol. In 2003, some 150 billion liters of beer, 27 billion liters of wine, and 2 billion liters of distilled spirits (mainly vodka) were produced worldwide. This amounts to about 8 billion liters of pure alcohol, representing about 20 percent of the world’s total ethanol production of 40 billion liters. Now that alternative energy sources are a priority, fuel ethanol, made mainly from sugar cane and corn, accounts for the lion’s share (70 percent in 2003, and more today). The industrial sector of chemicals and pharmaceuticals produces the remaining 10 percent. For the foreseeable future, the fuel sector will probably continue to expand, while the production of alcoholic beverages will show only modest gains to keep pace with the world’s population. The world’s total annual production of pure alcohol for beverages now exceeds 15 billion liters and is projected to reach 20 billion liters by 2012.
Fifteen billion liters of pure alcohol in naturally fermented and distilled beverages would provide every man, woman, and child on Earth with more than two liters a year. This estimate is likely too low, as illegal production is widespread and traditional home-brewed beverages, consumed globally in great quantities, are not included. Considering that most fermented beverages have an alcohol content of 5 to 10 percent and children generally do not imbibe, there is obviously plenty of alcohol to go around.
How has it come about that humans everywhere drink so much alcohol? Practically speaking, alcoholic beverages supply some of the water that we need to survive. Our bodies are two-thirds water, and the average adult needs to drink about two liters daily to stay hydrated and functioning. Untreated water supplies, however, can be infected with harmful microorganisms and parasites. Alcohol kills many of these pathogens, and humans must have recognized at an early date that those who drank alcohol were generally healthier than others.
Alcoholic beverages have other advantages. Alcohol spurs the appetite, and in liquid form, it also satiates feelings of hunger. The process of fermentation enhances the protein, vitamin, and nutritional content of the natural product, adds flavor and aroma, and contributes to preservation. Fermented foods and beverages cook faster because complex molecules have been broken down, saving time and fuel. Finally, as we have learned from numerous medical studies, moderate consumption of alcohol lowers cardiovascular and cancer risks. People consequently live longer and reproduce more. This was crucially important in antiquity, when life spans were generally short.
Drinking an alcoholic beverage, however, has meant much more to humankind than gains in physical health and longevity. To understand its broader biological and cultural dimensions, we must travel back to the period when Homo imbibens first walked the planet. By necessity, our tour guides are archaeologists, DNA researchers, and other detectives of the past, who have patiently excavated and studied the fragmentary remains of our ancestors and the genetic evidence encoded in our bodies today.
By examining the skeletal and dental evidence from early hominid fossils, dating from between about 4.5 to 2 mya, inferences can be drawn about how they lived and what foods they ate as they traversed the African jungle and savannah. Many of the fossils come from the Great Rift Valley of East Africa, including Australopithecus afarensis, best represented by the skeleton known as Lucy. Her forty-seven bones show that she could walk on two legs as well as climb trees. These traits would have served her and the rest of her “first family” well, enabling them to stretch tall and clamber through branches to reach sweet fruit.
The smaller molars and canines of early hominids (and the great apes), going as far back as Proconsul and other fossils around 24 mya, are also well adapted to consuming soft, fleshy foods like fruit. These dentitions are broadly comparable to those of modern apes, including gibbons, orangutans, and lowland gorillas, who get most of their calories from fruit. Chimpanzees, whose genome is the closest to our own, have a diet consisting of more than 90 percent plants, of which more than 75 percent is fruit. In other words, early hominids and their descendants have favored fresh fruit for millions of years.
If fruit was the food of choice at the beginning of the hominid odyssey, alcoholic beverages were probably not far behind. Especially in warm tropical climates, as the fruit matured, it would have fermented on the tree, bush, and vine. Fruits with broken skins, oozing liquid, would have been attacked by yeast and the sugars converted into alcohol. Such a fruit slurry can reach an alcohol content of 5 percent or more.
Visually oriented creatures that we are, we can imagine that the bright colors of the fermenting fruit, often red or yellow, would have attracted hominid interest. As our early ancestors approached the ripe fruit, other senses would have come into play. The intense aroma of alcohol from the fermenting fruit would have alerted them to the source of nourishment, and tasting it would have brought new and enticing sensations.
We cannot be sure how close to reality such a reconstruction is, since the ancient fossils tell us nothing about the easily degradable sensory-organ tissues. The taste and smell sensitivity of modern humans does not rate particularly high in the animal kingdom, despite the occasional super-taster among us. Early hominids might have had much more acute senses than ours, like the macaque, an Old World monkey, which has exquisite sensitivity to alcohol and other smells.
THE DRUNKEN MONKEY HYPOTHESIS
The biologist Robert Dudley has proposed that alcoholism among humans is rooted in the evolutionary history of primates. This thought-provoking hypoth
esis, dubbed the drunken monkey hypothesis, draws on the often fragmented and debatable pieces of the archaeological record and what is known about modern primate diets. If we grant that early hominids were primarily fruit eaters, at least up until about 1–2 mya, when they began consuming more tubers and animal fat and protein, then perhaps our early ancestors gained an advantage from imbibing moderate amounts of alcohol, whose benefits have been shown by recent medical research, and adapted biologically to it. On average, both abstainers and bingers have shorter, harsher life spans. The human liver is specially equipped to metabolize alcohol, with about 10 percent of its enzyme machinery, including alcohol dehydrogenase, devoted to generating energy from alcohol. Our organs of smell can pick up wafting alcoholic aromas, and our other senses detect the myriad compounds that permeate ripe fruit.
Among modern humans and other primates, the thirst for alcohol sometimes far exceeds any obvious nutritional or medical benefit (see plate 1). On the remote tropical island of Barro Colorado in Panama, Dudley reports, howler monkeys could not get enough of the ripe fruit of a palm (Astrocaryum standleyanum). You might think that monkeys would know better than to binge, in the same way that they avoid unsafe, even poisonous, plants in the natural world, but these monkeys gorged themselves on the bright orange fruit, ingesting the equivalent of about ten standard drinks, or two bottles of 12 percent wine, in twenty minutes. Obviously, there are diminishing returns to life and health if a monkey gets too drunk, misses a leap from one branch to another, and falls or is impaled by a sharp palm spine.
Malaysian tree shrews, who belong to a family dating back more than 55 mya that is believed to be ancestral to all living primates, have a similar penchant for fermented palm nectar. As documented by Frank Wiens and colleagues, they provide elegant testimony in support of Dudley’s hypothesis. These small creatures, resembling flying squirrels, often lap up alcohol in excess of the cross-species benchmark for intoxication (1.4 grams pure alcohol per kilogram weight) over the course of a night. That equates to about nine glasses of wine for the average-sized human. Yet the shrews show no signs of intoxication as they make their way deftly through the sharp spines of the palm trees to one oozing flower bud after another. The inflorescences of the bertam palm (Eugeissona tristis) are like miniature fermenting vessels where nectar accumulates year-round. In the tropical climate, the resident yeast rapidly converts it to a frothy, strongly scented palm wine with an alcohol content as high as 3.8 percent. The symbiotic relationship between palm and shrew is remarkable: while the animal guzzles, it pollinates the plant. Humans may have lost some of the genetic machinery to metabolize alcohol as efficiently as the tree shrew, but they have emulated its behavior by fermenting the sugar-rich saps and nectars of numerous palm tree species in Africa (chapter 8) and elsewhere.
Uncorking the Past Page 2
