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 a shift to lighter bones, rather than urbanization, nutrition or other factors. The discovery sheds light, researchers say, on a monumental change that has left modern humans susceptible to osteoporosis, a condition marked by brittle and thinning bones.
     At the root of the finding is the knowledge that putting bones under the 'stress' of walking, lifting and running leads them to pack on more calcium and grow stronger. "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," says Christopher Ruff, Ph.D. , a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine. "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," Ruff added.
     The researchers took molds of bones from museums' collections and used a portable X-ray machine to scan them, focusing on the tibia, femur, and humerus from 1,842 people from sites throughout Europe as old as 33,000 years and as recent as the 20th century. "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," Ruff says.
     The researchers 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," Ruff says. "The difference in bone strength between a professional tennis player's arms is about the same as that between us and Paleolithic humans," he says.

Edited from Popular Archaeology (18 May 2015)