When humans invented the wheel and domesticated horses thousands of years ago, enabling a transition from hunting and gathering to farming, they also started developing weaker bones that make modern people more susceptible to fractures and osteoporosis, according to a new study.

Scientists analyzed the remains of Europeans who lived at various times over the past 33,000 years and found the more stationary lifestyle afforded by farming - not the rise of cities or changes in diet - appears to have led to thinner, more brittle bones in modern humans, compared to our “caveman” ancestors.

"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 if the shift to weaker bones was driven by the rise in agriculture, or by other causes like diet or urbanization," said Dr. Christopher Ruff, director of the Center for Functional Anatomy and Evolution at Johns Hopkins Medical Center in Baltimore who led the study published May 18 in the Proceedings of the National Academy of Sciences.

Past research has found that load-bearing play and exercise in childhood, paired with adequate calcium and vitamin D, is the best way to build strong bones for a lifetime.

Ancient humans, however, had denser bones at any age compared to modern counterparts. To understand when and why that difference arose, Ruff and colleagues examined long bones in the legs and arms from 1,842 individuals who lived during the Paleolithic to Neolithic eras of hunting and gathering in Europe, often known as the Stone Age, as well as during the Roman empire, Medieval times, the Industrial Revolution and the 20th century.

They X-rayed the bones, and made silicone putty molds of the bone surface. Then they did a computer analysis of the data to help pinpoint the moment in human history that coincided with a structural shift to thinner bones.

Bone strength started declining in the femur and tibia bones of the leg about 7,000 years ago in the Neolithic period, as rudimentary farming practices began to take hold, and continued until about 2,000 years ago in the Roman period, the analysis found.

"The decline continued for thousands of years, suggesting that people had a very long transition from the start of agriculture to a completely sedentary lifestyle," Dr. Ruff said. "By the medieval period, bones were about the same strength that they are today." Changes in the humerus bone in the upper arm were smaller and less consistent.

Looking at both the upper and lower body, and showing a greater reduction in bone strength in the lower limb, "rules out a change like less protein or less calcium in the diet," said Dr. Steven Churchill, an evolutionary anthropologist at Duke University in Durham, North Carolina, who provided some of the Stone Age data but was not involved in the study.

In the legs, researchers also found that strength related to front-to-back motions associated with running or walking long distances declined more dramatically than strength linked to side-to-side motions tied to bending, lifting and pivoting.

The findings only apply to Europeans, the study team notes, and they cannot say whether the same pattern of changes occurred in other parts of the world.

Given how much more sedentary lifestyles have become since Roman times, it's surprising that the bones of 20th century humans didn't become even weaker as technology enabled even less exertion in daily life, Dr. Simon Mays, a human skeletal biologist at English Heritage in Portsmouth and visiting lecturer at the University of Southampton in the U.K., said by email.

Mechanization has risen particularly in the past 50 years, said Mays, who wasn't involved in the study. "I wonder whether improved nutrition in the 20th century has led to thicker bones, canceling out any bone-thinning effects of increased sedentism."