Two studies have determined that low-density bones evolved relatively recently in modern humans, with bone density decreasing in the past 12,000 years or so. This marked the change in modern humans, shifting from foraging to agriculture and becoming more sedentary.
Timothy Ryan at Pennsylvania State University in University Park and Colin Shaw at the University of Cambridge, UK, looked at the bone structure in the hip joint. The study looked at four human populations and primates from several hundred to several thousand years ago. They identified that thicker and denser bones existed in forager populations and thinner and lighter bones in agricultural groups, and similar to those of wild non-human primates.
Brian Richmond of the American Museum of Natural History in New York, Habiba Chirchir of the Smithsonian Institute in Washington DC and their colleagues analysed modern and early humans, chimpanzees and several fossil ancestor species. They looked at upper and lower limb bones in these animals. They found that bone density, especially in lower limb joints, decreased in humans only relatively recently.
This suggests that physical activity is important for bone strength.