An excerpt from

An Orchard Invisible

A Natural History of Seeds

Jonathan Silvertown

John Barleycorn: Beer

There was three Kings into the east,
Three Kings both great and high,
And they hae sworn a solemn oath
John Barleycorn should die.
They took a plough and plough’d him down,
Put clods upon his head,
And they hae sworn a solemn oath
John Barleycorn was dead.
But the cheerfu’ Spring came kindly on,
And show’rs began to fall;
John Barleycorn got up again,
And sore surpris’d them all.
—Robert Burns, “John Barleycorn”

What the three kings did to John Barleycorn after he was full grown and beginning to yellow with age deserves an Adult rating for wanton violence. It involved sickles, thrashing, hanging, a darksome pit full of water, scorching flame, and crushing between two stones, and it ended in an orgy of drunkenness. The bowdlerized version of the recipe for making beer from barley is less entertaining but more enlightening. Barley seeds are first sprouted so that the enzymes which become active upon germination convert the starch stored in them to the sugar maltose; roasting the sprouting seeds converts some of the sugar to malt, and then the mash is fermented to convert the sugar to alcohol.

The importance of beer to human health and happiness should not be underestimated. In the days before clean water supplies, “small beer” (beer with low alcohol content) was a safer drink than polluted well water. The Roman historian Pliny the Elder commented in his monumental, twenty-four-volume work Natural History that “the nations of the West also have their own intoxicant, made from grain soaked in water. There are a number of ways of making it in the various provinces of Gaul and Spain.… Alas!, what wonderful ingenuity vice posses! A method has actually been discovered for making even water intoxicated.”

Pliny’s remarks betray the disdain for beer of a patrician wine drinker commenting on the uncivilized customs of barbarians who lived at the far-flung reaches of the Roman Empire. Pliny was less disparaging of barley as food and wrote that it was ancient and venerated by the Athenians. Even in Rome, gladiators were once known as hordearii, or “barley eaters.”

Archaeology proves Pliny right about the ancient role of barley in the human diet. Barley was important in the transition from hunter-gathering to settled agriculture and was one of the first three grain crops to be domesticated at the dawn of Old World agriculture in the Fertile Crescent. This region, curved like a sickle blade, sweeps in a broad arc from Israel and Palestine on the eastern shores of the Mediterranean, northward into Syria and Turkey and then southeast into the valleys of the Tigris and the Euphrates in Iraq. The seeds of barley, preserved as charred remains, tell the story of how the Neolithic revolution, which gave birth to settled agriculture, probably started. Excavations at the Neolithic settlement of Netiv Hagdud, near the town of Jericho, have uncovered one the earliest episodes in this story. Netiv Hagdud was occupied for only three hundred years, ending about 8500 bc, so its early remains were not obliterated by later occupation. The people who lived there hunted a great variety of wild animals and also harvested grains using flint sickle blades that have been found at the site. The variety of wild plant foods collected was large, including figs, pistachios, acorns, and almonds, but the abundance of remains at the site suggest that barley was the staple food plant.

Were the barley grains found at Netiv Hagdud from domesticated crops, or were they collected wild? The answer can be found by microscopic examination of the plant remains themselves. The seeds of grasses stay attached within the flower as they develop, forming a unit called a “spikelet.” The seed heads, or “spikes,” of wild grasses such as wild barley (Hordeum vulgare subsp. spontaneum) shatter and disperse their spikelets as they ripen, leaving a scar where the spikelet detached cleanly from the head. By contrast, the spikes of domesticated cereals retain their spikelets and do not shatter, but are removed after harvest by threshing, which leaves diagnostic signs of breakage from the spike. When examined, the vast majority of barley remains found at Netiv Hagdud showed signs of shattering, rather than threshing, indicating that the grains were harvested from wild populations.

Two genes control whether barley shatters or not, and a small percentage of plants in wild populations are nonshattering genetic variants. Plants like these were crucial to the domestication of barley, which the archaeological record suggests may have begun around the time that Netiv Hagdud was abandoned. The transition from shattering wild barley to a nonshattering domesticated crop took no more than three centuries. What must have happened is a typical example of the process known as artificial selection, in which evolutionary change in an animal or plant population is driven by human preference, intentional or not, for particular characteristics. The different breeds of dog, all descended from a common ancestor, are a familiar example.

In the case of the domestication of barley, the preferences of early farmers were possibly unwitting because harvesting wild cereals with sickles would anyway have preferentially collected the seeds of nonshattering plants. Imagine gathering the stems of wild grasses that are on the point of ripeness. The blow of the sickle would dislodge seeds from the top of the spike, where they ripen first, and these would be lost. The harvest you brought home would consist of a disproportionate amount of unripe seeds and ripe seeds from nonshattering plants. Now, use some of those seeds to replant next year and repeat. Each year the proportion of nonshattering plants will increase, to the point at which the crop is fully domesticated and cannot disperse its seeds unaided.

But this was just the first stage of domestication. Through artificial selection, domestication wrought other important changes upon wild barley in the Fertile Crescent. Barley spikelets occur in triplets, arranged alternately up the spike, but in the wild species only the middle grain in each triplet develops. In the mature spike this results in two opposite rows of spikelets on the spike, and the first domesticated barley also had this two-row structure. A single gene controls the fertility of the lateral spikelets in each triplet. Where there are genes, there are mutations, and at some point soon after two-rowed barley was domesticated, barley with a genetic mutation causing all three spikelets to be fertile came under artificial selection. This mutant has six rows of spikelets on the spike, making it three times more likely that it will be resown than two-rowed barley, without any need for conscious selection on the part of the sower. Why the same numerical advantage does not cause the six-rowed mutant to replace the two-rowed one in wild populations of barley is a puzzle. The mutant must possess some disadvantage under natural conditions which prevents natural selection favoring six-rowed barley.

The Neolithic domestication of barley involved at least one probable instance of conscious selection on the part of early farmers. The seeds of wild barley and most of its cultivated forms remain wrapped in a part of the flower after threshing. This hulled grain is used in brewing and for animal feed, but “naked” grain without the hull was preferred by traditional farming communities for use in food. The naked/hulled characteristic of barley seed is controlled by a single gene, so some small proportion of plants in wild and early domesticated barley crops must have produced naked grain, but naked, six-rowed barley soon became a distinct crop variety through artificial selection. Since the presence/absence of the hull around the seed does not become apparent till after threshing, it does not seem likely that the harvest process itself would favor naked seeds. Farmers must have chosen naked grains to resow.

Two other cereals, emmer and einkorn wheat, were domesticated in the Fertile Crescent at the same time as barley and showed parallel evolutionary changes from shattering to nonshattering spikes as a consequence of artificial selection. Lentils and peas are two other important seed crops that were early domesticates in the Fertile Crescent. In these plants too, artificial selection blocked the normal seed dispersal mechanism of their wild progenitors, producing plants whose seed pods remained closed at harvest time. The large grains and fat peas and lentils we enjoy today are the result of artificial selection for larger seed size, another evolutionary legacy of a trend started in the Neolithic.

There is not just one instance of domestication in the production of beer, but two. In addition to barley, another organism essential to beer production is, of course, brewer’s yeast (Saccharomyces cerevisiae). This species is also used in wine and bread production, but how it was domesticated is only a recent discovery. Brewer’s yeast, just like domesticated barley and grapes, must have wild relatives, but S. cerevisiae is not that common in the wild. The closest relatives of brewer’s yeast found in nature occur in fermenting fruit, in the sugary sap of trees, and in clinical samples from patients with a compromised immune system.

Because it is rare in nature, some have suggested that “wild” populations of S. cerevisiae may be escapees from domestication. At first this may sound far-fetched, but feral cats and pigeons infest cities, so why not feral yeast? Imagine the implications: our immune system generally protects us from yeast infections, but patients with compromised immune systems are susceptible to infections of all kinds. When you take a friend in hospital a bunch of grapes, the general idea is that the patient eats the fruit, not that yeast in the bloom on the grapes eats the patient!

To investigate the origins of wild yeast, Justin Fay and Joseph Benavides of Washington University School of Medicine in St. Louis, Missouri, constructed an evolutionary tree for eighty-one yeast strains collected from domesticated and wild sources all over the world. An evolutionary tree is like a family tree, showing the relationships among people through their common ancestors. It answers the question “How far back do you have to go before a particular pair of individuals share a common ancestor?” For close relatives like brothers and sisters, the answer is just one generation. For first cousins it is two generations, for second cousins it is three generations and so on. One of the differences between an evolutionary tree and a family tree is that evolutionary trees stretch back over thousands, or many millions, of years and show how change occurred over time.

The evolutionary tree for Saccharomyces cerevisiae revealed several interesting things. First, it settled the question as to whether wild strains were descended from domesticated ones. It turned out that they were not. For example, of eleven yeast strains isolated from infections in immuno-compromised patients, ten were like wild yeasts and only one appeared to be related to a yeast strain from a vineyard. This news will no doubt come as a relief to anyone in the habit of taking grapes to the bedside of hospital patients. Yeasts isolated from wild sources such as tree sap were also found not to be descended from domesticated strains. In fact, quite the reverse.

The yeast strains with the deepest, and hence oldest, origin in the evolutionary tree were from African palm wine that is made from the sap of the oil palm. Because wild yeast occurs naturally on tree sap, this may be how it first became associated with humans—through the fermentation of palm wine in Africa. Of course, the human species also originated in Africa, which raises the tantalizing possibility that Saccharomyces cerevisiae was used for wine fermentation in Africa long before barley and other cereals became domesticated and beer or bread entered our diet. Cereals were domesticated in the Fertile Crescent around ten thousand years ago, well after Homo sapiens had spread out of Africa.

Fay and Benavides’ evolutionary tree also showed that the yeast used today to produce rice wine (sake) in Japan and the yeast used to produce grape wine belong to separate branches of the tree and represent two separate domestications, one in the East and one in the West. This is not unexpected, given that rice was domesticated in the Far East independently of cereal domestication in the Fertile Crescent. In theory, it should be possible to date the earliest common ancestor of the Eastern and Western branches of the S. cerevisiae tree. But, in practice, this requires some large assumptions to be made about, among other things, the length of the average generation time in yeast. Generation time is measured by the number of years between an individual producing its first offspring and those offspring themselves starting to reproduce. In other words, it is the time from when you become a parent to when you become a grandparent. This varies among human populations, but averages twenty to twenty-five years. A yeast generation is about three hours.

Assuming a generation time of three hours, Fay and Benavides estimated the split between the Eastern and Western yeast to have occurred more than twelve thousand years ago, but they admitted that if they had underestimated the generation time of yeast, the split could have occurred more than a hundred thousand years ago. The earliest evidence of fermented drinks so far uncovered is from China, where residues from the insides of Neolithic pottery jars indicate that they once contained an alcoholic beverage probably made by fermenting rice, honey, and fruit. The jars were nine thousand years old, so the estimate of yeast domestication twelve thousand years ago is in the right ballpark.

However the dates for yeast domestication pan out, and estimates will certainly become more accurate with further evolutionary and archaeological research, it is clear that humans have brewed alcoholic beverages since at least the Neolithic, and probably for a lot longer than that. Why, though, does Saccharomyces cerevisiae oblige us by producing alcohol? Does this substance, which is toxic to most microbes, serve some purpose for yeast? Possibly, but there is another, simpler explanation too: maybe it just can’t help but produce it.

As you will know if you have brewed your own beer or fermented your own wine, yeast will turn sugar into alcohol (ethanol) only if it is deprived of oxygen. Let air into the fermentation vessel and all the yeast will do is turn sugar into water, carbon dioxide, and more yeast cells. However, in anaerobic conditions yeast is unable to oxidize sugar fully, and the reaction stops at an incomplete stage with the production of ethanol. There is so much chemical energy still locked up in ethanol that it can be used to run an internal combustion engine.

When deprived of oxygen, and hence an efficient energy source, yeast cells divide much more slowly, so there is little doubt that the anaerobic conditions in which ethanol is produced are suboptimal. But that is not the end of the story, because evolution has a habit of turning adversity to advantage. The advantage to ethanol production is that it poisons other microbes, hence its well-known preservative properties. When vice admiral Lord Horatio Nelson lay dying from a sniper’s bullet on the day of his famous victory at the battle of Trafalgar, he asked that his body not be buried at sea, as was naval custom. Legend has it that to preserve Nelson’s body for the long passage home it was stored in a cask of Royal Navy rum. Unfortunately, when Nelson’s ship reached Portsmouth the rum had vanished. Sailors had tapped the cask with a small hole and drunk the contents.

Yeast has a sailor’s tolerance of ethanol, and by producing this compound it can poison the well from which its teetotal microbial competitors might otherwise drink. Better even than that, yeast has evolved a gene that enables it to use accumulated ethanol as an energy source, so it can drink from the poisoned well. These biochemical talents equip S. cerevisiae supremely well to capture, protect, and then consume the sugars found in fruit.

Saccharomyces cerevisiae is not the only species of yeast able to produce alcohol, but it is uniquely equipped to consume it as an energy source. Alcohol-producing yeasts share a gene for an enzyme called alcohol dehydrogenase (ADH) that is used in ethanol production, but S. cerevisiae also has a second, modified copy of the gene that enables it to reverse the process with an enzyme called ADH2. It is ADH2 that makes S. cerevisiae such a successful alcoholic.

When did S. cerevisiae acquire ADH2? Was this an adaptation to the long association of this yeast species with wine fermentation, perhaps? Was ADH2 the product of yeast domestication? Studies of molecular evolution suggest not, and offer instead an even more intriguing possibility. The origin of the ADH2 gene in the evolutionary history of S. cerevisiae has been dated to about eighty million years ago, in the Cretaceous. This is when the flowering plants really got going and fleshy fruit evolved. Of course, fruit are the natural habitat of yeast.

The human species is a very recent player in evolutionary history and we would do well to remember that the bread and the beer that are furnished us by nature have their own evolutionary tales to tell that are mostly much more ancient than our own. The twists and turns of evolution are full of surprises, but a recurring theme can be recognized beneath the anarchy of adaptation and counteradaptation that evolves among species when they interact with one another. We could call it John Barleycorn’s theme, because, no matter what the adversity, somewhere a seed of evolutionary success always sprouts from the clay burial ground of defeat.

Copyright notice: Excerpt from pages 145–54 of An Orchard Invisible: A Natural History of Seeds by Jonathan Silvertown, published by the University of Chicago Press. ©2009 by The University of Chicago. All rights reserved. This text may be used and shared in accordance with the fair-use provisions of U.S. copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires the consent of the University of Chicago Press. (Footnotes and other references included in the book may have been removed from this online version of the text.)

Jonathan Silvertown
An Orchard Invisible: A Natural History of Seeds
©2009, 224 pages, 21 halftones
Cloth $25.00 ISBN: 9780226757735

For information on purchasing the book—from bookstores or here online—please go to the webpage for An Orchard Invisible.

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