A new study of the bones of hundreds of humans who lived during the past 33,000 years in Europe finds the rise of agriculture and a corresponding fall in mobility drove humans’ shift to lighter bones.

“There was a lot of evidence that earlier humans had stronger bones and that weight-bearing exercise in modern humans prevents bone loss, but we didn’t know whether the shift to weaker bones over the past 30,000 years or so was driven by the rise in agriculture, diet, urbanization, domestication of the horse or other lifestyle changes,” said Prof Christopher Ruff of the Johns Hopkins University School of Medicine, lead author of a paper published online in the Proceedings of the National Academy of Sciences.

“By analyzing many arm and leg bone samples from throughout that time span, we found that European humans’ bones grew weaker gradually as they developed and adopted agriculture and settled down to a more sedentary lifestyle, and that moving into cities and other factors had little impact.”

The scientists focused on Europe because it has many well-studied archeological sites and because the population has relatively little genetic variation, despite some population movements.

For their study, they took molds of bones from museums’ collections and used a portable X-ray machine to scan them, focusing on two major bones from the legs and one from the arms.

“By comparing the lower limbs with the upper limbs, which are little affected by how much walking or running a person does, we could determine whether the changes we saw were due to mobility or to something else, like nutrition,” Prof Ruff said.

When the scientists analyzed the geometry of bones over time, they found a decline in leg bone strength between the Mesolithic era, which began about 10,000 years ago, and the age of the Roman Empire, which began about 2,500 years ago. Arm bone strength, however, remained fairly steady.

“The decline continued for thousands of years, suggesting that people had a very long transition from the start of agriculture to a completely settled lifestyle. But by the medieval period, bones were about the same strength as they are today,” Prof Ruff said.

“Paleolithic-style bones are still likely achievable, at least for younger humans, if they recreate to some extent the lifestyle of their ancestors, notably doing a lot more walking than their peers.”

“The difference in bone strength between a professional tennis player’s arms is about the same as that between us and Paleolithic humans.”

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Christopher B. Ruff et al. Gradual decline in mobility with the adoption of food production in Europe. PNAS, published online May 18, 2015; doi: 10.1073/pnas.1502932112