Water engineers in ancient South America turned seasonally flooded Amazonian savannas into hotbeds of year-round maize farming.
Casarabe people built an innovative, previously unrecognized network of drainage canals and water-storing ponds that enabled two maize harvests annually, say geoarchaeologist Umberto Lombardo of the Autonomous University of Barcelona and colleagues. Large-scale maize cultivation during rainy and dry parts of the year fed the rise of Casarabe urban sprawl across Amazonian forests and savannas in what’s now northern Bolivia, the scientists report January 29 in Nature.
Previous excavations dated Casarabe society, which covered an area of 4,500 square kilometers, to between the years 500 and 1400. Casarabe people had access to a variety of foods and crops, including maize, starchy tubers, squash, peanuts and yams. But investigators have found no evidence of Casarabe agricultural fields, raising questions about how farmers grew enough food to sustain a substantial population.
Rather than exploiting a range of available crops, Casarabe people transformed savannas into maize-production centers, the researchers say. “As the population grew and environmental pressures increased, perhaps they looked for more reliable and stable sources of proteins,” Lombardo suggests. “Maize could have offered that to some extent.”
Using satellite images and ground surveys of Casarabe territory, Lombardo’s team identified clusters of human-made ponds in two savanna regions. Canals dug into the ground, mapped using a drone-mounted remote sensing technique called light detection and ranging, or lidar, connected to many ponds. Leading away from pond clusters, canals formed drainage networks consisting of increasingly deep channels.
Soil samples from the edges of drainage canals and ponds contained microscopic mineral formations, called phytoliths, characteristic of maize. Cultivation probably occurred along canal borders and around the margins of ponds, the scientists suspect.
Radiocarbon dates for seeds and leaves indicate that farmers used one pond from around 1250 to 1550. But the age of the drainage system and other ponds remains unknown.