A recent study found ancient hunter-gatherer ancestors had much heavier bones than modern humans, and these changes correspond with the rise of agriculture and drop in mobility.

The findings provide insight into changes that left modern humans more susceptible to osteoporosis, and bones that are put under the stress of exercise tend to be stronger and richer in calcium, Johns Hopkins Medicine reported.

"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 Christopher Ruff, 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."

To make their findings, the researchers took molds of bone representing humans living in Europe over the past 33,000 years. They used a portable X-ray machine to can the samples, and focused on major bones from the arms and legs.

"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 said.

The researchers noticed a decline in leg bone strength occured between the Mesolithic era (about 10,000 years ago) and the age of the Roman Empire (about 2,500 years ago); arm bone strength proved to remain relatively standarf.

"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 said. "But by the medieval period, bones were about the same strength as they are today."

The researchers believe humans could regain this Paleolithic bone strength in they lived the same lifestyle as their ancestors.

 "The difference in bone strength between a professional tennis player's arms is about the same as that between us and Paleolithic humans," Ruff concluded.

The findings were published in a recent edition of the journal Proceedings of the National Academy of Sciences.