Washington, May 19 (IANS): Rather than urbanisation or diet, rise of agriculture and fall in mobility has led to lighter, brittle bones among humans today, interesting research has found.

The study of the bones of hundreds of humans who lived during the past 33,000 years in Europe sheds light 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.

Earlier humans had stronger bones and that weight-bearing exercise in modern humans prevents bone loss.

"By analysing 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," said Christopher Ruff from the Johns Hopkins University's school of medicine.

Modern lifestyles have famously made humans heavier, but, in one particular way, noticeably lighter weight than our hunter-gatherer ancestors: in the bones.

"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.

Better bones are still 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 authors said.

The paper appeared in the early edition of the journal Proceedings of the National Academy of Sciences.