Low-density bones evolved only relatively recently in modern humans as they shifted from foraging to agriculture and became more sedentary, according to two studies.

Habiba Chirchir of the Smithsonian Institution in Washington DC, Brian Richmond of the American Museum of Natural History in New York and their colleagues analysed upper and lower limb bones from modern and early humans, several fossil ancestor species and chimpanzees. They found that bone density, especially in lower limb joints, decreased in humans only in the past 12,000 years or so.

In a separate study, Timothy Ryan at Pennsylvania State University in University Park and Colin Shaw at the University of Cambridge, UK, examined bone structure in the hip joint in primates and in four human populations from several hundred to several thousand years ago. They discovered that bones from forager populations were much thicker and denser than those from agricultural groups, and were similar to those of wild non-human primates. The results support the idea that physical activity is important for bone strength.

Proc. Natl Acad. Sci. USA http://doi.org/xwr (2014); http://doi.org/xwq (2014)