Compared with chimpanzees and our extinct ancestors, modern humans have fragile, lightly-built skeletons. Recent research shows that these lightweight bones evolved late in our evolutionary history, and were likely the result of reduced mobility thanks to our shift from foraging to farming lifestyles. The pair of findings (here and here) were published in the Proceedings of the National Academy of Sciences this week.
An international team led by Habiba Chirchir of the Smithsonian Institution examined the density of the "spongy" (or trabecular) bone that’s found throughout the skeleton of modern humans and several other primates spanning several millennia. Both the upper and lower limbs of recent humans were much less dense compared with chimps, now extinct human species, and even modern humans that lived before the Holocene, our presence epoch. And it wasn’t a gradual shift either: bone density remained high throughout human evolution until a dramatic decrease in modern humans around 12,000 years ago.
Furthermore, there was more of a decrease in the lower limbs (hip, knee, ankle) than the upper limbs (shoulder, elbow, hand), suggesting that it was a result of changes in mobility. This fits with our relatively recent shift from a nomadic hunter-gatherer way of life to a more settled-down agricultural one. "Modern humans have less bone density than seen in related species, and it doesn't matter if we look at bones from people who lived in an industrial society or agriculturalist populations that had a more active life," Chirchir says in a news release. "They both have much less bone density."
To assess how behavioral patterns affect skeletal structure, Cambridge’s Colin Shaw and colleagues compared the hip joint among 59 individuals from ancient human populations -- ranging from mobile foragers to sedentary agriculturalists -- as well as a 31 primates that are around today. They x-rayed the inside of the femoral head, the ball at the top of the femur which fits into the pelvis. It's one of our most load-bearing bone connections. 
Highly mobile foragers, they found, had significantly thicker and stronger bones in their hip joints: bone mass was around 20 percent higher. Also, the structure of foragers’ hip joints was comparable to that of similarly sized non-human primates. In these 2D microCT images through the femoral head, you can see how human agriculturalists (right) have more lightly-built skeletons compared to foragers (left).
While the 7,000-year-old foragers had vastly stronger bones than the 700-year-old farmers, neither of them come close to that of even earlier species from around 150,000 years ago. "Something is going on in the distant past to create bone strength that outguns anything in the last 10,000 years," Shaw says in a university statement. He adds: "Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations... Sitting in a car or in front of a desk is not what we have evolved to do."
Images: AMNH/J. Steffey and Brian Richmond (top), Timothy M. Ryan (middle)
