At the DNA level, all the major cereals—wheat, rice, maize, millet, barley, and so on—are surprisingly alike. But despite their genetic similarity, maize looks and acts different from the rest. It is like the one redheaded early riser in a family of dark-haired night owls. Left untended, other cereals are capable of propagating themselves. Because maize kernels are wrapped inside a tough husk, human beings must sow the species—it essentially cannot reproduce on its own. The uncultivated ancestors of other cereals resemble their domesticated descendants. People can and do eat their grain; in the Middle East, for example, the wild barley harvest from a small piece of land can feed a family. By contrast, no wild maize ancestor has ever been found, despite decades of search. Maize’s closest relative is a mountain grass called teosinte that looks nothing like it (teosinte splits into many thin stems, whereas maize has a single thick stalk). And teosinte, unlike wild wheat and rice, is not a practical food source; its “ears” are scarcely an inch long and consist of seven to twelve hard, woody seeds. An entire ear of teosinte has less nutritional value than a single kernel of modern maize.
The grain in wild grasses develops near the top of the stem. As it matures, the stem slowly breaks up—shatters, in the jargon—letting the seed dribble to the ground. In wild wheat and barley, a common single-gene mutation blocks shattering. For the plant the change is highly disadvantageous, but it facilitates harvest by humans—the grain waits on the stem to be collected. The discovery and planting of nonshattering grain is thought to have precipitated the Neolithic revolution in the Middle East. Like other grasses, teosinte shatters, but there is no known nonshattering variant. (At least sixteen genes control teosinte and maize shattering, a situation so complex that geneticists have effectively thrown up their hands after trying to explain how a nonshattering type might have appeared spontaneously.) No known wild ancestor, no obvious natural way to evolve a nonshattering variant, no way to propagate itself—little wonder that the Mexican National Museum of Culture claimed in a 1982 exhibition that maize “was not domesticated, but created”—almost from scratch.
In the 1960s Richard S. MacNeish, of Phillips Academy, in Andover, Massachusetts, led an archaeological team that meticulously combed Puebla’s Tehuacán Valley for signs of early agriculture. Like the Peruvian littoral, the Tehuacán Valley lies in a double rain shadow, sandwiched between two mountain ranges. The aridity similarly helps preserve archaeological evidence. MacNeish’s team sifted through fifty caves before they found anything. In site No. 50, a rockshelter near the village of Coxcatlán, the team found maize cobs the size of a cigarette butt.
Ultimately, MacNeish’s team found 23,607 whole or partial maize cobs in five caves in the Tehuacán Valley. This ancient refuse became ammunition in a long-running academic battle between Harvard botanist Paul C. Mangelsdorf and George Beadle, a geneticist who worked at Stanford, Caltech, and the University of Chicago. In the late 1930s both men proposed theories about the origin of maize. Mangelsdorf said that it descended from the mix of a now-vanished wild ancestor of maize and wild grasses from the genus Tripsacum. Teosinte, he said, played no role in its development. Beadle had a simpler theory: maize was directly descended from teosinte. Mangelsdorf treated this idea with disbelieving scorn. By now the reader will not be surprised to learn that an apparently arcane debate about the distant past could become vehemently personal. Relations between the two men became cold, then bitter, then explosive. Botanists chose sides and wrote caustic letters about each other.
Mangelsdorf worked with MacNeish and classified the 23,607 ancient maize cobs. The smallest and oldest, he proclaimed, were maize’s true wild ancestor, which Indians had then crossed with Tripsacum to make modern maize. So powerful did the evidence of Mangelsdorf’s tiny cobs seem that in the 1960s it buried the teosinte hypothesis, even though the latter’s champion, Beadle, had for other research won a Nobel Prize. Beadle’s ideas were taken up in revamped form by University of Wisconsin botanist Hugh Iltis in 1970. Maize originated, Iltis postulated, in a strange, wholesale mutation of teosinte, to which Indians added and subtracted features through intensive breeding. Mangelsdorf’s side found itself on the defensive; Iltis had gleefully pointed out that the “wild maize” cobs from the Tehuacán Valley were identical to those of an unusual, fully domesticated variety of popcorn from Argentina. By then the dispute over the origin of maize had filled almost as much paper—and became as acrimonious—as the battle over Clovis.
In 1997 Mary W. Eubanks, a Duke University biologist, resuscitated the hybridization theory in a new variant. Maize, she suggested, might have been created by repeatedly crossing Zea diploperennis, a rare maize relative, and another cousin species, Eastern gamagrass. When species from different genera hybridize, the result can be what the biologist Barbara McClintock called “genomic shock,” a wholesale reordering of DNA in which “new species can arise quite suddenly.” In Eubanks’s theory, Indians came upon a chance combination of Zea diploperennis and gamagrass and realized that by mixing these two species they could shape an entirely new biological entity. As proof, she announced the creation of a Zea diploperennis–gamagrass hybrid in the laboratory that displayed the attributes of ancient maize.
The teosinte faction remained skeptical. A consortium of twelve maize scientists harshly attacked Eubanks’s work in 2001 for, in their view, failing to demonstrate that her hybrid was actually a Zea diploperennis–gamagrass mix and not an accidental blend of Zea diploperennis and modern maize. (Such errors are a constant threat; in a busy lab, it is all too easy for biologists to use the wrong pollen, mislabel a tray, or mistake one analysis for another.) Meanwhile other geneticists pinpointed teosinte mutations that could have led to modern maize, including sugary 1, a variant gene that alters maize starch in a way that gives tortillas the light, flaky texture celebrated at Itanoní.
Because maize is many steps removed from teosinte, these scientists argued that the modern species had to have been consciously developed by a small group of breeders who hunted through teosinte stands for plants with desired traits. Geneticists from Rutgers University, in New Brunswick, New Jersey, estimated in 1998 that determined, aggressive, knowledgeable plant breeders—which Indians certainly were—might have been able to breed maize in as little as a decade by seeking the right teosinte mutations.
From the historian’s point of view, the difference between the two models is unimportant. In both, Indians took the first steps toward modern maize in southern Mexico, probably in the highlands, more than six thousand years ago. Both argue that modern maize was the outcome of a bold act of conscious biological manipulation—“arguably man’s first, and perhaps his greatest, feat of genetic engineering,” Nina V. Federoff, a geneticist at Pennsylvania State University, wrote in 2003. Federoff’s description, which appeared in Science, intrigued me. It makes twenty-first-century scientists sound like pikers, I said when I contacted her. “That’s right,” she said. “To get corn out of teosinte is so—you couldn’t get a grant to do that now, because it would sound so crazy.” She added, “Somebody who did that today would get a Nobel Prize! If their lab didn’t get shut down by Greenpeace, I mean.”
To people accustomed to thinking of maize in terms of dark or light yellow kernels of corn on the cob, the variety in Mexican maize is startling. Red, blue, yellow, orange, black, pink, purple, creamy white, multicolored—the jumble of colors in Mesoamerican maize reflects the region’s jumble of cultures and ecological zones. One place may have maize with cobs the size of a baby’s hand and little red kernels no bigger than grains of rice that turn into tiny puffs when popped; in another valley will be maize with two-foot-long cobs with great puffy kernels that Mexicans float in soup like croutons. “Every variety has its own special use,” Ramírez Leyva explained to me. “This one is for holidays, this one makes tortillas, this one for niquatole [a kind of maize gelatin], this one for tejate,” a cold drink in which maize flour, mamey pits, fermented white cacao beans, and other ingredients are marinated in water ov
ernight and then sweetened and whipped to a froth. As a rule domesticated plants are less genetically diverse than wild species, because breeders try to breed out characteristics they don’t want. Maize is one of the few farm species that is more diverse than most wild plants.
More than fifty genetically distinguishable maize “landraces” have been identified in Mexico, of which at least thirty are native to Oaxaca, according to Flavio Aragón Cuevas, a maize researcher at the Oaxaca office of the National Institute for Forestry, Agriculture, and Fisheries Research. A landrace is a family of local varieties, each of which may have scores of “cultivars,” or cultivated varieties. As many as five thousand cultivars may exist in Mesoamerica.
Maize is open pollinated—it scatters pollen far and wide. (Wheat and rice discreetly pollinate themselves.) Because wind frequently blows pollen from one small maize field onto another, varieties are constantly mixing. “Maize is terribly promiscuous,” Hugo Perales, an agronomist at the think tank Ecosur, in Chiapas, told me. Uncontrolled, open pollination would, over time, turn the species into a single, relatively homogeneous entity. But it does not, because farmers carefully sort through the seed they will sow in the next season and generally do not choose obvious hybrids. Thus there is both a steady flow of genes among maize landraces and a force counteracting that flow. “The varieties are not like islands, carefully apart,” Perales explained. “They are more like gentle hills in a landscape—you see them, they are clearly present, but you cannot specify precisely where they start.”
San Juan Chamula, a mountain town in central Chiapas, near the border with Guatemala, is an example. Located outside the colonial city of San Cristóbal de Las Casas, it has a sixteenth-century church with a brilliant blue interior that is a popular tourist destination. But beyond the souvenir kiosks in the cathedral square, most of the 44,000 inhabitants of Chamula scratch a living from the dry mountain slopes outside town. Almost all are Tzotzil, a southwestern branch of the Maya; in 1995, the most recent date for which census data are available, about 28,000 did not speak Spanish. According to a survey by Perales, 85 percent of the farmers plant the same maize landraces as their fathers, varieties that have been passed on and maintained for generations. The crop in the field today is the sum of thousands of individual choices made by community members in the past.
Landrace maize from Oaxaca
Indian farmers grow maize in what is called a milpa. The term means “maize field,” but refers to something considerably more complex. A milpa is a field, usually but not always recently cleared, in which farmers plant a dozen crops at once, including maize, avocados, multiple varieties of squash and bean, melon, tomatoes, chilis, sweet potato, jicama (a tuber), amaranth (a grain-like plant), and mucuna (a tropical legume). In nature, wild beans and squash often grow in the same field as teosinte, the beans using the tall teosinte as a ladder to climb toward the sun; below ground, the beans’ nitrogen-fixing roots provide nutrients needed by teosinte. The milpa is an elaboration of this natural situation, unlike ordinary farms, which involve single-crop expanses of a sort rarely observed in unplowed landscapes.
Milpa crops are nutritionally and environmentally complementary. Maize lacks digestible niacin, the amino acids lysine and tryptophan, necessary to make proteins and diets with too much maize can lead to protein deficiency and pellagra, a disease caused by lack of niacin. Beans have both lysine and tryptophan, but not the amino acids cysteine and methionine, which are provided by maize. As a result, beans and maize make a nutritionally complete meal. Squashes, for their part, provide an array of vitamins; avocados, fats. The milpa, in the estimation of H. Garrison Wilkes, a maize researcher at the University of Massachusetts in Boston, “is one of the most successful human inventions ever created.”
Wilkes was referring to the ecological worries that beset modern agribusiness. Because agricultural fields are less diverse than natural ecosystems, they cannot perform all their functions. As a result, farm soils can rapidly become exhausted. In Europe and Asia, farmers try to avoid stressing the soil by rotating crops; they may plant wheat one year, legumes the next, and let the field lie fallow in the year following. But in many places this only works for a while, or it is economically unfeasible not to use the land for a year. Then farmers use artificial fertilizer, which at best is expensive, and at worst may inflict long-term damage on the soil. No one knows how long the system can continue. The milpa, by contrast, has a long record of success. “There are places in Mesoamerica that have been continuously cultivated for four thousand years and are still productive,” Wilkes told me. “The milpa is the only system that permits that kind of long-term use.” Likely the milpa cannot be replicated on an industrial scale. But by studying its essential features, researchers may be able to smooth the rough ecological edges of conventional agriculture. “Mesoamerica still has much to teach us,” Wilkes said.
To Wilkes’s way of thinking, ancient Indian farming methods may be the cure for some of modern agriculture’s ailments. Beginning in the 1950s, scientists developed hybrid strains of wheat, rice, maize, and other crops that were vastly more productive than traditional varieties. The combination of the new crops and the greatly increased use of artificial fertilizer and irrigation led to the well-known Green Revolution. In many ways, the Green Revolution was a tremendous boon; harvests in many poor countries soared so fast that despite burgeoning populations the incidence of hunger fell dramatically. Unfortunately, though, the new hybrids are almost always more vulnerable to disease and insects than older varieties. In addition to being too costly for many small farmers, the fertilizer and irrigation can, if used improperly, damage the soil. Worst, perhaps, in the long run, the exuberant spread of the Green Revolution has pushed many traditional cultivars toward extinction, which in turn reduces the genetic diversity of crops. Wilkes believes that some or all of these difficulties may be resolved by reproducing features of the milpa in a contemporary setting. If this occurs, it will be the second time that the dissemination of Mesoamerican agricultural techniques will have had an enormous cultural impact—the first time being, of course, when they originated.
From today’s vantage it is difficult to imagine the impact maize must have had in southern Mexico at the beginning, but perhaps a comparison will help. “Almost pure stands” of einkorn wheat covered “dozens of square kilometers” in Turkey, Iraq, Syria, and other parts of the Middle East, according to Jack R. Harlan and Daniel Zohary, two agronomists who pored over the area in the 1960s to determine the distribution of wild cereals. “Over many thousands of hectares” in those countries, they wrote in Science, “it would be possible to harvest wild wheat today from natural stands almost as dense as a cultivated wheat field.” In the Middle East, therefore, the impact of agriculture was thus less a matter of raising the productivity of wheat, barley, and other cereals than of extending the range in which they could be grown, by developing varieties that could flourish in climates and soils that daunt the wild plant. By contrast, the Americas had no wild maize, and thus no wild maize harvest. Stands of teosinte have been seen in the wild, but because the “ears” are tiny and constantly shattering they are difficult to harvest. Thus before agriculture the people of Mesoamerica had never experienced what it was like to stand in a field of grain. Grain fields—landscapes of food!—were part of the mental furniture of people in Mesopotamia. They were an astounding novelty in Mesoamerica. Indians not only created a new species, they created a new environment to put it in. Unsurprisingly, the reverberations sounded for centuries.
Maize in the milpa, the Yale archaeologist Michael D. Coe has written, “is the key…to the understanding of Mesoamerican civilization. Where it flourished, so did high culture.” The statement may be more precise than it seems. In the 1970s the geographer Anne Kirkby discovered that Indian farmers in Oaxaca considered it not worth their while to clear and plant a milpa unless it could produce more than about two hundred pounds of grain per acre. Using this figure, Kirkby went back to the ancient cobs excav
ated from Tehuacán Valley and tried to estimate how much grain per acre they would have yielded. The cob sizes steadily increased as they approached the present. In Kirkby’s calculation, the harvest broke the magic two-hundred-pound line sometime between 2000 and 1500 B.C. At about that time, the first evidence of large-scale land clearing for milpas appears in the archaeological record. And with it appeared the Olmec, Mesoamerica’s first great civilization.
Based on the Gulf Coast side of Mexico’s waist, on the other side of a range of low mountains from Oaxaca, the Olmec clearly understood the profound changes wreaked by maize—indeed, they fêted them in their art. Like the stained-glass windows in European cathedrals, the massive Olmec sculptures and bas-reliefs were meant both to dazzle and instruct. A major lesson is the central place of maize, usually represented by a vertical ear with two leaves falling to the side, a talismanic symbol reminiscent of a fleur-de-lys. In sculpture after sculpture, ears of maize spring like thoughts from the skulls of supernatural beings. Olmec portraits of living rulers were often engraved on stelae (long, flat stones mounted vertically in the ground and carved on the face with images and writing). In these stela portraits, the king’s clothes, chosen to represent his critical spiritual role in the society’s prosperity, generally included a headdress with an ear of maize emblazoned on the front like a star. So resonant was the symbol, according to Virginia M. Fields, curator of pre-Columbian art at the Los Angeles County Museum of Art, that in later Maya hieroglyphics “it became the semantic equivalent of the highest royal title, ahaw.” In the Maya creation story, the famous Popul Vuh, humans were literally created from maize.
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