A culture that flourished in Costa Rica more than 2,000 years before any previously known habitation in the region has been discovered under layers of Pompeii-like volcanic ash.

Radiocarbon dating shows that the first settlements, including ruins of round houses with elaborately decorated ceramics and cemeteries, were established around 2000 B.C. They were found beneath the first of 10 ash layers, indicating that the volcano repeatedly buried villages rebuilt at the site.

According to Payson Sheets, a University of Colorado archeologist who reported his findings to the American Anthropological Association meeting here last week, the people lived simply -- farming corn and shunning trade with other Central American civilizations.

Sheets speculates that this insular attitude, which he believes must have been deliberate, helped the people avoid social and political entanglements with the nearby expansionist Maya and Aztec empires.

Skeletons from the cemeteries indicate high infant mortality rates but little sign of disease or malnutrition among adults, whose life spans appeared similar to those of modern peoples.

The settlements clustered around a lake, today called Laguna de Arenal, at the base of the 5,000-foot Arenal volcano. Because the culture was simple and in balance with the rigorous tropical climate, Sheets speculates, the people were able to recover rapidly from the volcano's repeated assaults.