The team scanned circular cross-sections of seven bones in the upper and lower limb joints in chimpanzees, Bornean orangutans and baboons. They also scanned the same bones in modern and early modern humans as well as Neanderthals, Paranthropus robustus, Australopithecus africanus and other Australopithecines. They then measured the amount of white bone in the scans against the total area to find the trabecular bone density. Crunching the numbers confirmed their visual suspicions. Modern humans had 50 to 75 percent less dense trabecular bone than chimpanzees, and some hominins had bones that were twice as dense compared to those in modern humans. Both studies have implications for modern human health and the importance of physical activity to bone strength. "The lightly-built skeleton of modern humans has a direct and important impact on bone strength and stiffness," says Tim Ryan. That's because lightness can translate to weakness—more broken bones and a higher incidence of osteoporosis and age-related bone loss. The researchers warn that with the deskbound lives that many people lead today, our bones may have become even more brittle than ever before. "We are not challenging our bones with enough loading," says Colin Shaw, "predisposing us to have weaker bones so that, as we age, situations arise where bones are breaking when, previously, they would not have."