Zigzagging across the savanna of northeastern Bolivia are mysterious dirt embankments half a meter high, a meter wide, and up to 3.5 kilometers long. They were obviously built by humans and probably predate the Spanish conquest, but their function has mystified archaeologists. "These don't make sense for transportation, because they double the distance," says archaeologist Clark Erickson of the University of Pennsylvania in Philadelphia.
After studying a network of the structures that covers about 500 square kilometers, Erickson identified them as weirs, or fish traps, for capturing the fish that spill out onto the savanna during the rainy season. A telltale detail of their design tipped him off: There are funnel-shaped channels where fish could be trapped--a feature shared by most weirs--in the embankments, he writes in the 9 November issue of Nature.
Over several thousand years, the local Baures people also constructed canals, artificial ponds, and causeways that linked settlements and may have controlled water flow. Through these structures, the Baures intensively managed the landscape, enabling them to sustain a fairly large population in an environment where protein is scarce, Erickson explains.
The interpretation reinforces the view that although the Baures didn't build big like the Incas and Maya, they had a more sophisticated, coordinated society than previously thought, says anthropologist William Balee of Tulane University in New Orleans. Perhaps not what we'd call a state, he says, but a civilization complex enough to plan, build, and maintain these waterways.