On a coastal flood plain etched by rivers flowing through swamps and alongside fields of maize and beans, the people archaeologists call the Olmecs lived in a society of emergent complexity. It was more than 3,000 years ago, along the Gulf of Mexico around Veracruz.

The Olmecs, mobilized by ambitious rulers and fortified by a pantheon of gods, moved a veritable mountain of earth to create a plateau above the plain, and there planted a city, the ruins of which are known today as San Lorenzo. They left behind palace remnants, distinctive pottery and art with anthropomorphic jaguar motifs. Most impressive were Olmec sculptures: colossal stone heads with thick lips and staring eyes that are assumed to be monuments to revered rulers.

The Olmecs are widely regarded as creators of the first civilization in Mesoamerica, the area encompassing much of Mexico and Central America, and a cultural wellspring of later societies, notably the Maya. Some scholars think the Olmec civilization was the first anywhere in America, though doubt has been cast on that theory by recent discoveries in Peru.

Archaeologists have split sharply over how much influence the Olmecs had on contemporary and subsequent Mesoamerican cultures. Were Olmecs the "mother" culture? Or were they one among "sister" cultures whose interactions through the region produced shared attributes of religion, art, political structure and hierarchical society?

Last month the simmering pot of mother-sister controversy was stirred anew by Jeffrey P. Blomster, an Olmec archaeologist at George Washington University. In a report in the journal Science, he and other researchers described evidence of the widespread export of Olmec ceramics that they said supported "Olmec priority in the creation and spread of the first unified style and iconographic system in Mesoamerica."

Blomster's team analyzed the chemistry of 725 pieces of pottery decorated with symbols and designs in the Olmec style and collected throughout the region. The researchers compared the composition of the ceramics with local clays. They determined that most of these were not imitations of the Olmec style made by local potters. In a significant number of pots, the clay matched the chemistry of material found around San Lorenzo.

"The evidence is overwhelming that San Lorenzo, the first Olmec capital, was doing the exporting," Blomster said. "The Olmecs were disseminating their culture, and it was something of great interest to others."

The research, he added, showed that San Lorenzo did not appear to be importing artifacts emblematic of other cultures or that regional contemporaries were exchanging such material with one another. The city on the artificial plateau seemed to be the hub of regional culture, and central, he said, to understanding the origin and development of complex society in Mesoamerica.