Corn: It's Not for Cocktails

Ancient history. Traces of the oldest known corn were found under this boulder in Mexico. Early farmers domesticated unappetizing teosinte (left in inset), transforming it into edible form (right).

(rock) Anthony Ranere; (teosinte/maize) John Doebley

Pity the first corn eaters. The ancestor of the plant that gives us its succulent yellow kernels is an unappetizing grain known as teosinte, whose ears harbor only five to 12 rock-hard grains. Scientists have now found the earliest known traces of corn--or maize--at a site in central Mexico dated to nearly 9000 years ago. And although this ancient plant was probably tough on the teeth, the find suggests that early farmers did indeed eat it--rather than turn it into alcoholic beverages, as some researchers have suggested.

Scientists have spent decades trying to figure out where maize (Zea mays) was first domesticated and why. Many researchers suspected a link between teosinte and today's corn, but the evidence was not conclusive. In 2002, a team led by geneticist John Doebley of the University of Wisconsin, Madison, found the smoking gun. They used genetic testing to show that maize is a very close relative of a variety of teosinte that grows today in Mexico's Balsas River area. What's more, Doebley's team found that maize had been domesticated only once, about 9000 years ago, and then spread throughout the Americas.

That and other discoveries led Dolores Piperno, an archaeobotanist at the Smithsonian Institution in Washington, D.C., and her co-workers to begin searching for archaeological evidence of the earliest maize in the Balsas River region. Until then, the earliest known maize cobs, found near Oaxaca, Mexico, had been dated to only 6200 years ago. Piperno and her colleagues began using new techniques to identify early domesticates using microscopic plant fossils, called phytoliths, as well as starch grains, both of which are often preserved on the stone tools used by early farmers in the humid tropics. In 2005, a team led by Piperno and Anthony Ranere, an anthropologist at Temple University in Philadelphia, Pennsylvania, hit pay dirt: Under a giant boulder in the Balsas River region, called the Xihuatoxtla Shelter, the researchers discovered a trove of prehistoric grinding stones to which phytoliths and starch grains from maize were still adhering. Radiocarbon dating of charcoal found with the earliest of the stones pegged the corn as 8700 years old, bearing out the genetic dating by Doebley's group.

The findings, which the team reports in two online papers today in the Proceedings of the National Academy of Sciences, also appear to discount a leading hypothesis for why early farmers would bother domesticating the unappealing teosinte plant in the first place. Several researchers, including Michael Blake, an archaeologist at the University of British Columbia in Vancouver, Canada, have suggested that the plant was valued not for its tough kernels but for its sugar-laden stalk, which might have been used to make early alcoholic drinks. Only later, after it had come under cultivation, did teosinte undergo the genetic changes that turned it into maize and a staple food crop, according to the hypothesis. But none of the phytoliths and starch grains at Xihuatoxtla come from the stalk; even at this very early period of domestication, the team found, the people who used the shelter were grinding the kernels.

Blake concedes that the new findings do not support his idea and that the "primary interest" of the Xihuatoxtla people appears to have been the maize kernels rather than the stalks. Nevertheless, he argues, it is still possible that the farmers squeezed the juice out of the stalks while they were still in the fields and fermented it there rather than taking it back to the shelter. But most importantly, Blake says, the new discovery raises hopes that researchers will eventually find larger, visible fragments of cobs, kernels, and stalks at early farming sites, which would provide critical insights into how they were processed and domesticated.

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