Squash Seeds Yield New View of Early American Farming

Just as a wisp of hair or a speck of blood can sway the verdict in a criminal trial, the tiniest morsels of evidence can swiftly undercut an ascending scientific theory. According to a view that has attracted many supporters over the last few years, human beings in the Americas gave up hunting and gathering for farming 5000 to 3500 years ago—much more recently than previously thought, and several millennia behind Near Eastern and Asian civilizations. But a reanalysis of a handful of oversized squash seeds and other table scraps dug up in a Mexican cave has upset this theory, leading to a brand-new picture of the transition to agriculture in the New World.

Curiously enough, the new evidence comes from a researcher who, until recently, insisted that early Americans were latecomers to agriculture. Archaeologist Bruce Smith, director of the archaeobiology program at the National Museum of Natural History in Washington, D.C., based his case on an improved carbon-14 dating technique called accelerator mass spectrometry (AMS); it had revealed the oldest fragments of domesticated squash, corn, and beans from caves in Mexico to be no more than 5000 years old—several thousand years younger than had been thought. Then last year, Smith traveled to the Instituto Nacional de Antropología e Historia in Mexico City to gather an additional set of squash fragments for the same kind of dating. And, as he reports on page 932, these fragments imply that agriculture got an early start after all: They turn out to be 8000 to 10,000 years old.

Because the corn and bean fragments still seem to be much younger, Smith's new 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.”

The debate about the emergence of New World agriculture turns on a few bits of squash stashed in a drawer at the Mexico City institute. Found in the 1960s by the University of Michigan's Kent Flannery at a dry cave in Oaxaca, Mexico, they are the only evidence of agriculture's earliest days in the Americas—along with a few other crop fragments discovered by Flannery's mentor, Richard “Scotty” MacNeish, in four rock shelters and caves in Tamalipas and the Tehuacán Valley. Most archaeologists who go to Mexico “want to dig up Mayan tombs, fancy temples, and caches of jade,” explains Flannery. “They're not interested in these preceramic hunter-and-gatherer cultures.”

When Flannery found his squash fragments in 1966, he quickly identified them as domesticated varieties of the squash Cucurbita pepo, the same species to which modern pumpkins and summer squash belong. The seeds and stems were larger, the rinds thicker, and the outer skin more colorful than the corresponding parts of the wild Cucurbita gourds still growing in some regions of the Americas. But he couldn't date them directly. The radiocarbon-dating technique then available required pulverizing large chunks of material, which would have destroyed the fragments.

So Flannery resorted to an indirect technique. In each era of human occupation of the cave, called Guilá Naquitz, residents added a new layer of detritus to the cave floor. Flannery gauged the age of the oldest squash parts by dating charcoal from the same layers as the squash. He concluded that the fragments were nearly 10,000 years old—implying that the transition from foraging to farming began more or less simultaneously in the Fertile Crescent and the Americas. MacNeish used the same strategy to estimate slightly younger ages for the crop fragments from his caves.

There matters stood until the 1980s, when many archaeologists began using AMS. In both conventional and AMS radiocarbon dating of organic samples, the ratio of the slowly decaying isotope carbon-14 to normal carbon yields an estimate of the time elapsed since the organism's metabolism ceased. But whereas conventional radiocarbon dating measures carbon-14 by counting decay events, AMS counts the carbon-14 atoms directly, meaning that much less sample material is needed.

In 1989, Austin Long, a geochemist at the University of Arizona, Tucson, and Bruce Benz, a paleobotanist now at the University of Guadalajara in Mexico, used AMS to date the oldest domesticated corncobs directly from MacNeish's digs in Tehuacán. Astonishingly, they found the cobs to be only 4700 to 1600 years old, some 800 to 2300 years younger than MacNeish and his collaborators had estimated.

That finding sparked an impassioned debate among archaeologists about the accuracy of the dates for all the crop fragments from the Mexican caves. In a 1994 Current Anthropology article, entitled “Are the First American Farmers Getting Younger?,” Fritz wrote that “One begins to wonder how many, if any, of the [domesticated species] from the Tehuacán excavations actually precede 3000 B.C.” Fritz argued that burrowing rodents or people digging fire pits might have disturbed the cave floor's layers, casting doubt on all of MacNeish's dates from charcoal in the layers.

This revisionism also led Smith to wonder about the reliability of Flannery's dates for the Guilá Naquitz squash material. If the other crop fragments were as young as they now seemed, Flannery's dates would mean that several thousand years had passed between the domestication of the first crop, squash, and the widespread domestication of other crops, such as beans and corn. To Smith, that seemed implausibly long, in light of the abrupt appearance of farming-based civilizations in the Near East and Asia. “All these different lines of evidence seemed to suggest that it was worth taking a step out onto thin ice and predicting that [when dated using AMS] the Guilá Naquitz squash would turn out to be younger than people had initially proposed,” says Smith.

In retrospect, that step wasn't warranted. In Mexico City last year, Smith retrieved the 13 seeds and other Cucurbita material Flannery had extracted from zones B through E of the floor of Guilá Naquitz, layers spanning the period from 10,800 to 8700 years before the present. Smith's AMS dating of the seeds bore out Flannery's original estimates: One zone C seed proved to be 9900 years old, while five zone B seeds ranged from 8400 to 10,000 years old.

This 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.” Question marks still hang over the plant artifacts from MacNeish's Tehuacán sites; although most researchers agree that the new AMS dates there are trustworthy, MacNeish isn't ready to concede that his dating of the cave-floor layers was flawed.

Still, by all accounts, AMS dating is forcing ethnobotanists and archaeobiologists to rethink their definition of “farmer.” “There's been a dichotomy of views: People can either be hunter-gatherers or agriculturalists,” explains Smith. “In reality, there's a transition stage between these two, but we don't know much about it. And in Mesoamerica, it's clear that this ‘transitional phase' was 6500 years long”—longer, Smith notes, than the hunting-gathering and farming phases put together.

The only way to fill in this picture, say Smith and other researchers, is to search out new troves of agricultural remains. Until then, says MacNeish, there will be “a lot of room for speculation, and a lot of very opinionated people.”