A new study published in the Proceedings of the National Academy of Sciences shows that modern human skeletons have become much lighter and more fragile since the invention of agriculture.
New research shows that, while human hunter-gatherers from around 7,000 years ago had bones comparable in strength to modern orangutans, farmers from the same area over 6,000 years later had significantly lighter and weaker bones that would have been more susceptible to breaking. Image credit: Alabama Cooperative Extension System / A&M University / Auburn University.
“We set out to test three potential explanations for modern human gracility and any one of them would have been interesting. What we found was the most interesting,” said study first author Dr Timothy Ryan from the Pennsylvania State University.
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 non-human 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.
“Contemporary humans live in a cultural and technological milieu incompatible with our evolutionary adaptations. There’s 7 million years of hominid evolution geared towards action and physical activity for survival, but it’s only in the last say 50 to 100 years that we’ve been so sedentary – dangerously so. Sitting in a car or in front of a desk is not what we have evolved to do,” said co-author Dr Colin Shaw of the University of Cambridge, UK.
Dr Ryan and Dr Shaw looked at the hip joint to determine which of the three possible explanations was likely.
They compared these joints in samples from foraging populations, early agriculturalists and comparably sized non-human primates.
They used noninvasive microcomputed tomography to scan the hip joint ends of the femurs. In all, the study included 59 adult humans and 229 primates.
The scientists compared the trabecular bone – the honeycomb-like bone that fills joint ends – among the three groups.
“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,” Dr Ryan and Dr Shaw said.
They 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 non-human primates.
“There are other things that could account for some of the differences between early agriculturalists and foragers. 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,” Dr Ryan said.
The scientists believe there are valuable lessons to be learnt from the skeletons of our prehistoric predecessors.
“You can absolutely morph even your bones so that they deal with stress and strain more effectively. Hip fractures, for example, don’t have to happen simply because you get older if you build your bone strength up earlier in life, so that as you age it never drops below that level where fractures can easily occur,” Dr Shaw said.
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Timothy M. Ryan & Colin N. Shaw. Gracility of the modern Homo sapiens skeleton is the result of decreased biomechanical loading. PNAS, published online December 22, 2014; doi: 10.1073/pnas.1418646112
