Agriculture's roots go tropical.

As early as 7,000 years ago, prehistoric societies in the tropical forests of Central and South America changed over from foraging to food production by cultivating manioc and other plants with edible, starchy roots, a new study finds.

Although cultivation appeared later there than in the Middle East, the data support a controversial theory that tropical-forest dwellers cultivated roots and tubers long before such practices emerged elsewhere among Native Americans, says a team led by archaeologist Dolores R. Piperno of the Smithsonian Tropical Research Institute in Balboa, Panama.

Piperno's group recovered starch grains from milling stones found at a Panamanian site dated at between 7,000 and 5,000 years old.

Microscopic analysis of the grains identified examples of manioc, arrowroot, and yams, the researchers report in the Oct. 19 NATURE. Earlier microscope observations by Piperno had uncovered characteristic grain shapes for these and many other modern species of wild and domesticated plants.

The ancient milling stones also contained starch grains from maize, indicating that the site's prehistoric residents grew seed crops as well as root crops, the scientists say.

Piperno suspects that the cultivation of manioc, a staple food in the tropics, first occurred in South America and then spread northward. Other researchers have uncovered manioc grains at two sites in Belize that date to 4,700 years ago.