In the Arikara tribe, which lived along the Missouri River in the Dakotas between the 14th and 19th centuries, the women did all the farming. Historical accounts relate that they produced so much corn by the 1850s that they had fat surpluses for trade.

Now Arikara bones have furnished direct testimony about their lives. Daniel Westcott of the University of Missouri, Columbia, and Deborah Cunningham of the Smithsonian Institution in Washington, D.C., examined between 95 and 160 pairs of male and female arm and leg bones from a period spanning nearly 4 centuries. They measured indications of mechanical load, including the area of the weight-bearing cortex and how bone cross sections departed from circularity.

The study, to appear in the July Journal of Archaeological Science, found that as agriculture intensified, women's leg bones changed. By the late 1700s, their left legs showed signs of having borne greater loads, which the authors suggest stemmed from "pushing off" on the left leg while working the fields. Anthropologist Christopher Ruff of Johns Hopkins University in Baltimore, Maryland, says this makes sense. "Lower limbs tend to be 'right dominant' in things like kicking a ball, but the left is used to stabilize the body, which is actually more stressful biomechanically."

While the women were in the fields, the men were developing a different asymmetry, the authors report: Their right arms became larger, probably as they relied increasingly on rifles rather than bows and arrows, which put stress on both arms.

CREDIT: EDWARD S. CURTIS/COURTESY DARTMOUTH COLLEGE LIBRARY