A scan of an upper arm bone from a prehistoric woman agriculturalist. This bone is from a North African population, which was not part of the study, but is an example of the research method. (University of Cambridge)

Modern humans have historically skinny bones. Our skeletons reflect a departure from the labors of hunting and gathering. (Speeding to McDonald's in search of novelty chicken nugget sauce doesn't count.) Anthropologists looking backward in time have found that, in contrast with our well-rested skeletons, prehistoric leg and arm bones were thicker. Human bones, once built up with exertion, began to shrink after the shift to agriculture.

Yet the bulk of research pairing past behavior to bones has focused on male limbs. “There has been little work done yet, and what does exist has focused on men largely because the relationship between behavior and bone is a bit less complex in men than in women,” said Alison Macintosh, who studies skeletal biomechanics at the University of Cambridge's anthropology department. Women's bones work double duty: They not only need to be strong, she pointed out, but the bones also store minerals that are used during pregnancy and lactation.

Macintosh is the author, along with other researchers at Cambridge and the University of Vienna, of a new report that shines a spotlight on women workers from 5,500 years ago through today. The study, published in Science Advances on Wednesday, paired the scans of ancient female bones with those of modern humans. Until the medieval period, women were performing manual labor that produced thick arm bones.

“We've largely been underestimating the scale of this work,” Macintosh said.

The anthropologists made laser scans of old bones, from the upper arms and shins of prehistoric women. The women all lived in Central Europe and were part of farming societies, but they represented different times: the Neolithic, Bronze, Iron or Middle Ages. The researchers then asked living women to undergo CT scans, including semiprofessional athletes  — soccer players, rowers and runners — as well as non-athletes.

That “the study makes a nice comparison with living women makes it quite appealing,” said Habiba Chirchir, a biological anthropologist who studies skeletal anatomy at Smithsonian Natural History Museum, who was not involved with this research.

Cross sections of the tibia, the lower leg bone, revealed that modern runners and soccer players had the most strain in their legs. The average modern woman's shin bones were like that of the prehistoric women.

Neolithic, Bronze and Iron Age women, though, had pronounced effects of strain in their upper arm bones, the humerus. Even members of female crew teams couldn't quite compare.

“The extent to which the daily activities of prehistoric women put strain on their arm bones was likely quite similar, or even more so, than the amount of strain that these rowers are putting on their arm bones,” Macintosh said.

Comparing prehistoric women with rowers indicated that these long-ago laborers “were experiencing remarkable levels of activity,” Chirchir said. She cautioned, however, that central European subsistence practices were specific to that region, so these findings “cannot be generalized for all prehistoric populations throughout the world.”

Macintosh said it was difficult to pinpoint, exactly, what caused the strong arms. Instead she listed a wide range of agricultural tasks: “Before the plow was invented, they would have been using digging sticks, flint sickles inserted into wooden handles, etc., to till the soil and harvest the grain, and stone querns to grind the grain into flour by hand. Women also were likely involved in looking after domestic livestock, milking them, processing milk, meat, hides and wool into textiles, and making pottery and manufacturing other items.”

By the time the medieval period rolled around, Macintosh said, technology may have made grinding grain less arduous. A device called the rotary quern, a type of hand mill, became common in Central Europe in the late Iron Age. It was faster and easier to use than other stone grinding tools. The new culinary invention “may be one of the main reasons why women's upper limb bone strength starts to decline between the Iron Age and the medieval period in this region of Europe,” Macintosh said.

Behavior is not the only factor that strengthens bones. Genes and nutrition play a role, too. But the study author said that “the influence of mechanical loading on the skeleton is likely explaining the largest portion of the differences.”

Macintosh said the team is working to glean more information about a person's life from bones, notably about their muscle structures and fat. The researchers also plan to continue their investigations of arm bone strength — in prehistoric men, in a somewhat rare inversion of the usual scientific order of things.

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