An analysis of squash seeds and other table scraps dug up in a Mexican cave suggests that people in the Americas gave up hunting and gathering for farming at least 8000 years ago--centuries earlier than previously thought. The findings, reported in tomorrow's issue of Science,* paint a brand-new picture of the transition to agriculture in the New World.
Kent Flannery of the University of Michigan found the squash fragments in 1966 in a dry cave in Oaxaca, Mexico; he quickly identified them as domesticated varieties of the squash Cucurbita pepo, the same species to which modern pumpkins and summer squash belong. Flannery gauged the age of the oldest squash parts by dating charcoal from the same layers as the squash, concluding that the fragments were nearly 10,000 years old. However, using a modern dating technique called accelerator mass spectrometry (AMS), researchers discovered in the late 1980s that the oldest domesticated corncobs--from a nearby dig--were much younger than previously estimated. That sparked an impassioned debate about the accuracy of the dates for all the crop fragments from the Mexican caves.
One of the well-preserved squash seeds (Cucurbita pepo) (13.8 millimeters long) excavated from Guilá Naquitz cave in Oaxaca, Mexico, that has been dated by the accelerator mass spectrometer carbon-14 technique. The seeds provide evidence that this plant species was first domesticated in Mexico 10,000 years ago.
This revisionism led Bruce Smith, director of the archaeobiology program at the National Museum of Natural History in Washington, D.C., to pay a visit to the Instituto Nacional de Antropología e Historia in Mexico City last year, where he examined the 13 seeds and other Cucurbita material Flannery had extracted. Smith's AMS dating of the seeds later bore out Flannery's original estimates: Six seeds ranged from 8400 to 10,000 years old. The reanalysis has effectively squelched the squash squabble. "What's exciting about Smith's data," says Flannery, is that the first American farmers "aren't getting younger--in fact, they may be a little older than we thought."
Because the corn and bean fragments still seem to be much younger than the squash seeds, Smith's findings suggest that farming took hold more gradually in the Americas than in other parts of the world. In the Near East and China, for instance, vigorous agricultural economies had sprung up by 9000 and 7500 years ago, respectively, within a thousand years of the domestication of the first crops. The gap between the squash dates and those for other crops implies that Mesoamericans spent thousands of years planting gourds such as squash--even selecting strains for certain characteristics--without "making any other substantial transition to growing their own food," says Gayle Fritz, an archaeobotanist at Washington University in St. Louis. "They were still practicing their old hunting, gathering, and fishing ways."