Our modern, more sedentary lifestyles have not only made humans heavier, but also noticeably lighter than our hunter-gatherer ancestors - at least in the bones.

That's according to a new study published in the journal Proceedings of the National Academy of Sciences, which adds to the growing evidence that agriculture is to blame for humans having lighter, more brittle bones. Thanks to the switch to farming, modern humans are more susceptible to osteoporosis, characterized by weak and brittle bones.

Scientists in the past had argued that urbanization, nutrition or other factors contributed to our lighter bones, but after examining the bones of hundreds of humans who lived during the past 33,000 years in Europe, a team from Johns Hopkins finds the rise of agriculture and a corresponding fall in mobility drove the change.

At the root of the finding, the researchers say, 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," researcher Christopher Ruff, a professor of functional anatomy and evolution at the Johns Hopkins University School of Medicine, said in a statement.

"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," he added. (Scroll to read on...)

The team focused on Europe because it has many well-studied archeological sites, and because the population has relatively little genetic variation, despite some population movements - meaning any changes observed could be attributed more to lifestyle than to genetics.

For the study, the researchers took molds of bones from various museum 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," Ruff explained.

When they analyzed the geometry of bones over time, 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. Interestingly, arm bone strength, remained about the same.

"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."

But don't worry; this research isn't to say that all humans are doomed to be brittle beings with osteoporosis. The researchers say that it's still possible at least for younger humans to achieve Paleolithic-style bones. Though, they would have to recreate the lifestyle of their ancestors, for example, by doing a lot more walking or doing weight-bearing exercises.

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