Human bones have lost density and become fragile since the dawn of agriculture, according to a new study that says increasingly sedentary lifestyles are to blame for the negative evolution.

Modern orangutans, whose bones are just as strong as those of hunter-gatherers from 7,000 years ago, help illustrate this point, according to the Cambridge University research team.

Foraging humans had a bone mass approximately 20 percent higher than those evolved since the farming age, and the researchers say it would take just three months of weightlessness to lose that amount of bone mass.

Researchers say they ruled out dietary changes and variations in body structure as possible causes for the change in bone mass over time and they believe there is no anatomical reason why a person born today could not achieve this bone mass.

Yet not even the most physically active people today manage to do so because their lifestyles don't measure up to those of hunter-gatherers in terms of the load and stress on bones, say the researchers.

"Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations," says co-author Dr. Colin Shaw from the University of Cambridge's Phenotypic Adaptability, Variation and Evolution (PAVE) Research Group. "Sitting in a car or in front of a desk is not what we have evolved to do.

The findings support the theory that exercising could be more important than diet in preventing bone fractures and the onset of osteoporosis.

"You can absolutely morph even your bones so that they deal with stress and strain more effectively," says Dr. Shaw, who says there is much to be learned from the skeletons of prehistoric humans. "Hip fractures, for example, don't have to happen simply because you get older if you build your bone strength up earlier in life, so that as you age it never drops below that level where fractures can easily occur."

The type of bone tissue most affected by the evolutionary change is spongy bone, known as trabecular in the scientific community.

"Trabecular bone has much greater plasticity than other bone, changing shape and direction depending on the loads imposed on it," says Shaw. "It can change structure from being pin or rod-like to much thicker, almost plate-like. In the hunter-gatherer bones, everything was thickened."

The study was published in Proceedings of the National Academy of Sciences of the United States of America.