If you compare a chimpanzee’s bones with those of a modern human, one difference will immediately jump out at you. Chimp bones are densely packed with microscopic structures known as spongy bone. Human bones aren’t. That relative lack of spongy bone makes our skeletons lighter and increases our risk of fractures and osteoporosis. But weaker skeletons and more broken bones don’t seem like great evolutionary strategies. So why the change? Two papers published online today in the Proceedings of the National Academy of Sciences propose an explanation. In the first, the authors compare skeletons from modern chimpanzees, the early human ancestor Australopithecus africanus, Neandertals, early Homo sapiens, and today’s modern humans. They found that chimpanzees, Australopithecus, Neandertals, and even the early modern humans had much higher densities of spongy bone than today’s humans (see the illustration above, which tracks the change through CT scans of hand bones from each species; left to right: chimps, Australopithecus, Neandertal, modern human). That suggests that the driving force behind the change might be modern human’s sedentary lifestyle, free of the bone-strengthening exercise of chasing down prey and spending hours foraging for food. The second paper further supports that hypothesis by comparing the density of spongy bone in the hip joints of nonhuman primates, ancient hunter-gatherers, and ancient farmers. The hunter-gatherers’ hip joints were about as strong as those of the apes, whereas the ancient farmers’ hips showed a significant loss of spongy bone. The researchers conclude that a lack of rigorous exercise, rather than any evolutionary pressure toward lighter skeletons, is the reason for modern human’s weak bones. So if you want a stronger skeleton, start exercising like a hunter-gatherer.