pending more time sitting on our butts isn't just a problem for obesity and heart disease. The shift to a more sedentary lifestyle has probably been bad for our bones, too. A pair of papers published today in the Proceedings of the National Academy of Sciences suggest that humans evolved lighter joint bones relatively recently in our evolutionary history as a response to changes in physical activity.
One study pinpoints the origin of these weaker bones at the beginning of the Holocene epoch roughly 12,000 years ago, when humans began adopting agriculture. "Modern human skeletons have shifted quite recently towards lighter-more fragile, if you like-bodies. It started when we adopted agriculture. Our diets changed. Our levels of activity changed," says study co-author Habiba Chirchir, an anthropologist in the Smithsonian's Human Origins Program.
The second study attributes joint bone weakness to different levels of physical activity in ancient human societies, also related to hunting versus farming. Both works 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, an anthropologist at Penn State University and a co-author on the second study. That's because lightness can translate to weakness-more broken bones and a higher incidence of osteoporosis and age-related bone loss.
Scientists already knew that the modern human skeleton is longer, thinner and generally weaker than that of its hominin predecessors, but no one was sure what has been driving this "gracility." Previous studies suggested that walking upright put more pressure on joints to go long and lean, while others argued that a decrease in physical activity or changes in diet has been behind these skeletal changes.
Recently, scientists have zeroed in on trabecular bone, the sponge-like material that's found at the ends of the bones that form joints. "Think of the end of a chicken bone: If you cut through it, then you see this meshwork of bone that's interwoven," says Chirchir. Modern humans have lower trabecular bone density within specific bones than their ancestors.
Spongy bone responds to mechanical stress, so Chirchir and her colleagues decided to take CT scans of the hand bones of primates, including humans, to see if the bone differed based on how the animals moved. "We though that if an orangutan climbs, it should have a different structure of trabecular bone than knuckle-walkers like chimpanzees," says Chirchir.
The team noticed that scans of human hands looked drastically different than those of their primate relatives. In the CT scans, air bubbles appear dark against a white backdrop of bone. "The human hand had very little white compared to the other primate hand bones," says Chirchir, indicating that it might be incredibly airy and light. "So that was the striking thing." She and her colleagues wondered how the density of trabecular bone in the rest of the human skeleton compared to that in other primates and early human ancestors.
