Maize domestication grows older in Mexico.

Inhabitants of southern Mexico began to cultivate maize, the major grain crop of prehistoric societies in the Americas, by at least 6,300 years ago, a new study finds. This is around 800 years earlier than previous estimates.

Radiocarbon dates for minute samples taken from two maize cobs converge on the older age, according to a report in the Feb. 13 PROCEEDINGS OF THE, NATIONAL ACADEMY OF SCIENCES. Archaeologists excavated both cobs in 1966 at Guild Naquitz Cave in Mexico's southern highlands. The specimens are now housed in a Mexican museum.

Until now, the earliest evidence of maize growing in the New World came from a radiocarbon analysis of 5,500-year-old cobs from San Marcos Cave, located in southern Mexico's Tehuacan Valley.

The new findings can't resolve scientific debates over the precise location and timing of initial maize domestication, say coauthors archaeologist Dolores R. Piperno of the Smithsonian Tropical Research Institute in Balboa, Panama, and anthropologist Kent V. Flannery of the University of Michigan in Ann Arbor.

Previous genetic studies indicate, to some scientists, that a subspecies of the wild grass teosinte was the likely ancestor of maize. Maize domestication may have first occurred where this form of teosinte still grows, in the Central Balsas River Valley, which is located about 250 miles east of Guila Naquitz.

Other investigators have suggested that prehistoric people living much closer to Guild Naquitz concocted the earliest maize as a hybrid of teosinte and another wild grass species.

Whatever the case, maize cultivation didn't originate at Guila Naquitz, Piperno and Flannery assert. The researchers found no fossilized grains characteristic of either teosinte or maize in the site's layers of earth ranging from about 10,000 to 7,000 years old.

Maize cobs found at both Guild Naquitz and San Marcos contain securely attached grains and other features typical of modern maize, adds Bruce F. Benz of Texas Wesleyan University in Fort Worth. His analysis appears in the same issue Of PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES.

Remains at Guila Naquitz from around 10,000 years ago have also provided the earliest evidence of squash cultivation in the Americas (SN: 5/24/97, p. 322).

Scientific clues to the origins of Mexico's three major crops--squash, maize, and beans--come from only five caves excavated 40 to 50 years ago, remarks archaeologist Bruce D. Smith of the Smithsonian Institution in Washington, D.C., in a comment published with the new reports. The limited evidence nevertheless suggests that the domestication of both squash and maize first occurred in southern and southwestern Mexico, Smith says. Beans were probably first domesticated around 4,000 years ago in western Mexico, in his view.