The comparatively light bone structure
of modern humans compared to early human species and other modern
primates may be due to the modern abandonment of the constant physical
activity that was inherent in the life of early hunter gathers.
The most plausible explanation, he said,
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.
Ryan, working with Colin N. Shaw of the
University of Cambridge, UK, looked at the hip joint to determine which
of the three possible explanations was likely.
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.
This knowledge might aid in prevention of osteoporosis and hip fracture in the elderly.
The study is published in the Proceedings of the National Academy of Sciences.
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