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By Root 1773 0

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