R. JOHN
DAVENPORT
Academic Press  | | Early
agriculture. The Guil‡ Naquitz cave in Oaxaca is home to the oldest known domesticated
corn. Credit: Kent Flannery |
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 Tehuac‡n and the Guil‡ 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 Guil‡ 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 Guil‡ 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 Guil‡ 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 Guil‡ 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 Tehuac‡n.
Still, no one has
discovered the earliest stage of domestication, which would require evidence of
teosinte used as food. When he originally excavated Guil‡ 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 Guil‡ 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."
|
