The switch from a physically demanding life of hunting and gathering to one of agriculture may explain why most modern humans have bones that are thinner, lighter, and more fragile compared to other primates.
Researchers came to this conclusion after comparing the hip joints in samples from foraging populations, early agriculturalists, and comparably sized nonhuman primates.
“We set out to test three potential explanations for modern human gracility and any one of them would have been interesting,” says Timothy M. Ryan, associate professor of anthropology and information science and technology, Penn State. “What we found was the most interesting.”
The most plausible explanation is that a lack of constant physical activity causes the bone in the head of the femur—the long bone in the thigh—to become thinner and lighter than that found in more mobile populations or modern primates such as chimpanzees, gorillas, and orangutans.
The other two possible explanations—that humans and nonhuman primates have different bone structure because of genetics, with humans evolving to a lighter, more gracile structure, or that the large joint surfaces required for upright, two-legged movement decrease the strain on bone and therefore the development of strong bones—do not appear to be true.
The human hip bones the researchers analyzed came from two agricultural groups and two foraging groups that once lived in what is now Illinois. The nonhuman bones came mostly from wild specimens in collections.
The researchers used noninvasive microcomputed tomography to scan the hip joint ends of the femurs. In all, the study included 59 adult humans and 229 nonhuman primates.
Ryan and Colin N. Shaw of the University of Cambridge compared the trabecular bone—the honeycomb-like bone that fills joint ends—among the three groups.
Farmers vs. foragers
“The results of the present study indicate that human populations with divergent activity patterns display significantly different trabecular bone structural characteristics in the proximal femur,” the researchers report online today in the Proceedings of the National Academy of Sciences.
The researchers found that the agriculturalists had significantly lower bone mass than the foragers. However, the bone characteristics of the more mobile foragers overlapped with those of the nonhuman primates.
“There are other things that could account for some of the differences between early agriculturalists and foragers,” Ryan says. “The amount of cultivated grains in the diet of the agriculturalists, in this case maize, as well as possible deficiencies in dietary calcium may also contribute to lower bone mass. It does seem, however, that the biomechanical aspects of foraging play a large part.”
“The findings of the present study have significant implications for understanding human skeletal form and its relationship to age-related bone loss in contemporary human populations,” the study reports.
“We need to understand the difference in bone structure caused by diet, behavior, and evolution,” Ryan says. “However, I think the key appears to be higher physical activity and mobility from a very young age that makes the bones of nonhuman primates and human foragers stronger.”
The National Science Foundation supported this work.
Source: Penn State