Scientists have been trying for decades to nail down when, where, and how corn was domesticated. Genetic evidence indicates that the New World maize arose from the wild grass called teosinte that's found in the moist Balsas River Valley of Mexico. Now a study pushes back by several hundred years the date by which ancient americans had turned the grass into corn.
The only archaeological evidence for early corn comes from the San Marcos cave in Tehuacán and the Guilá Naquitz cave in Oaxaca, both in the Mexican highlands. Cobs from the San Marcos cave appear to be about 5500 years old, according to recent radiocarbon dating. Any direct dating of the cobs from Guilá Naquitz was impossible when the caves were first excavated in the 1960s, because radiocarbon dating techniques at the time would have destroyed the cobs.
Now archaeologist Kent Flannery of the University of Michigan, Ann Arbor, has dated the corn cobs from Guilá Naquitz using a technique called accelerator mass spectrometry, which gives a direct and accurate date and only requires a sesame-seed-sized piece of the cob. He found that the cobs date to 6250 years ago--700 years earlier than the previous oldest specimen.
A companion report provides additional information about the form and structure of the Guilá Naquitz cobs. The cobs, says ethnobotanist Bruce Benz of Texas Wesleyan University in Fort Worth, show classic signs of domestication: Seeds are held tightly to a rigid cob, and would therefore have depended on humans to break off and plant the seeds. Teosinte seeds, on the other hand, are held on a brittle structure and fall off easily. The Guilá Naquitz cobs appear to be from an early stage of domestication, Benz says, because they don't have the multiple rows of seeds that characterize the later corn from Tehuacán.
Still, no one has discovered the earliest stage of domestication, which would require evidence of teosinte used as food. When he originally excavated Guilá Naquitz, Flannery found no visible plant remnants of teosinte. And in the current work, Flannery's collaborator Dolores Piperno of the Smithsonian Tropical Research Institute in Panama looked for microfossil evidence of teosinte at Guilá Naquitz. Finding none, she concludes that corn was domesticated elsewhere in Mexico and then brought to the Oaxacan cave.
The two studies, which appear in the 13 February issue of the Proceedings of the National Academy of Sciences, point to the need for more archaeology, says Bruce Smith of the Smithsonian's National Museum of Natural History. "All of our evidence for early domestication of corn, beans, and squash come from a total of five caves in Mexico," he explains. "We need a lot more data."