The bones of modern humans are more fragile than those of non-human primates and our hunter-gatherer ancestors, and we have the agricultural revolution to blame.

Researchers from Penn State and the University of Cambridge compared the top of the femur, where the long legbone joins the hip, in samples from early farmers, foraging populations and non-human primates. CT scans showed the honeycomb-like bone inside the femur head of hunter-gatherers was much denser than the more sedentary agriculturalists'.  

Writing in the Proceedings of the National Academy of Sciences, they say different activity patterns display significantly different bone structure. Co-author Tim Ryan said the findings suggest physical activity, especially when young, is critical to building strong bones throughout life.

The research could shed light on osteoporosis, a bone-weakening condition.

In another study in the same issue, researchers report our lightweight skeletons evolved relatively recently, about 12,000 years ago.  

Brian Richmond of George Washington University and his international team used high-resolution CT scans on different limb joints of modern humans and chimpanzees, as well as several hominid ancestors. They also found bone density decreased over the millennia, and it was more pronounced in the lower joints than in the upper ones -- like the shoulder, elbow and hand.

Richmond said, "this density drastically drops off ... when we started to use agricultural tools to grow food and settle in one place."