South American surprise: ancient farmers settled in Uruguay's wetlands.

One of the earliest complex societies in South America flourished in an unexpected corner of the continent. Around 4,200 years ago, an extensive, carefully designed farming settlement was built in the wetlands of what's now southeastern Uruguay; a team of researchers finds.

The ancient agricultural outpost, Los Ajos, was steadily occupied until about 500 years ago, say archaeologist Jose Iriarte of the Smithsonian Tropical Research Institute in Balboa, Panama, and his colleagues.

Discoveries at Los Ajos challenge the long-standing scientific assumption that early South American civilizations arose solely along the coast of Peru and in nearby highland valleys. Iriarte and his coworkers describe their findings in the Dec. 2 Nature.

Work at Los Ajos "exposes the potential for prehistoric culture in grasslands and wetlands, which were historically viewed as marginal areas," remarks archaeologist Peter W. Stahl of Binghamton (N.Y.) University in an editorial published with the new report.

Fifteen earthen mounds dot the landscape at Los Ajos. Pilot excavations in and around these mounds, conducted a decade ago by other researchers, didn't reveal a farming settlement but indicated that people had lived there approximately 4,000 years ago.

Excavations and radiocarbon measurements by Iriarte's team in 2003 established that the site's earliest residents lived in a complex village where they cultivated various crops.

Los Ajos households were arrayed around six flat-topped mounds arranged in a horseshoe formation. At one end of that formation, two dome-shaped mounds framed an oval plaza. Further work will explore structures embedded in the mounds.

Iriarte's findings indicate that ancient residents maintained separate public areas for food preparation and stone-tool making. Plant remains and starch grains uncovered at the site point to cultivation of maize, squash, and beans.

According to the researchers, similar findings are emerging from three sites near Los Ajos, each of which also contains earthen mounds.

South American river basins and rain forests underwent an agriculture-friendly transition around 4,000 years ago, Iriarte's team proposes. Earlier studies indicated that a sharp decline in rainfall at that time created grassy expanses near inland rivers, thus opening prime territory for farming.

Until now, evidence for farming settlements in these regions dated to no more than 1,000 years ago. For instance, Brazil's Amazonian rain forest contains the remains of villages and cultivation areas from A.D. 1200 to A.D. 1600.