A complex agricultural society in Uruguay’s La Plata basin, 4,800-4,200 years ago

A complex farming society developed in Uruguay around 4,800 to 4,200 years ago, much earlier that previously thought, Iriarte and his colleagues report in this week’s Nature (December 2). Researchers had assumed that the large rivers system called the La Plata Basin was inhabited by simple groups of hunters and gatherers for much of the pre-Hispanic era.

Iriarte and coauthors excavated an extensive mound complex, called Los Ajos, in the wetlands of southeastern Uruguay. They found evidence of a circular community of households arranged around a central public plaza. Paleobotanical analyses of preserved starch grains and phytoliths –tiny plant fossils- show that Los Ajos’ farmers adopted the earliest cultivars known in southern South America, including maize, squash, beans and tubers.

Over time, around 3,000 years ago, the mound complex architectural plan of Los Ajos exhibited sophisticated levels of engineering, planning, and cooperation revealing an earlier, new, and independent architectural tradition previously unknown from this region of southern South America. The formal and compact layout of the central part of the site (Inner Precinct) consists of seven imposing platform mounds surrounding a central plaza area.

Iriarte extracted a sediment core from nearby wetlands to reconstruct what the environment was like when this farming society arose. Combined analyses of preserved pollen and phytoliths indicated that, as in other regions of the world, the mid-Holocene was characterized by significant climatic and ecological changes associated with important cultural transformations. During this period, around 4,500 years ago, the climate was much drier than it is today and “Wetlands became biotic magnets for human habitation providing an abundant, reliable, and a resource-rich supply of foods and water. Furthermore, wetland margins offered an ideal place for the experimentation, adoption, and intensification of agriculture encouraging the Los Ajos’ community to engage into horticulture”, explains Iriarte, currently a post-doctoral fellow at the Smithsonian Tropical Research Institute (STRI) in Panama.

At Los Ajos, cultural artifacts are spread out over 12 ha. suggesting the presence of a large resident population. Moreover, as Iriarte indicates “Los Ajos is far from a lonely isolated community in southeastern Uruguay. In the ten square kilometers surrounding Los Ajos alone there are ten other large and spatially complex mound sites. These were thriving societies that probably were integrated into regional networks of towns and villages”. Iriarte believes that “this region was a locus of early population concentration in lowland South America.”