The evolutionary history of humans may explain why some people are more susceptible to a particular stress fracture known as spondylolysis that often affects athletes, according to study which may lead to new ways of preventing and managing back pain.
In the study, published in the journal Evolution, Medicine, and Public Health, scientists used advanced 3D shape analysis techniques to compare the final vertebrae of humans with and without spondylolysis, to the same bones in our closest living relatives -- the great apes.
The researchers, including those from the University of Sydney in Australia, said that the differences between human vertebrae with spondylolysis, and that of great apes were greater than those between healthy human and ape backbones.
"Because spondylolysis only occurs in humans and does not affect our great ape cousins, it has long been assumed to be the result of increased stress placed on our spine by our unique ability to walk upright on two legs," said Kimberly Plomp, study co-author from Simon Fraser University (SFU) in Canada.
"However, there have been few attempts to test this hypothesis," Plomp said.
According to the researchers, people who developed spondylolysis have vertebrae that are more wedge-shaped, where the front is taller than the back, in addition to other subtle shape differences.
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They said these differences are consistent with the vertebrae having "overshot" the optimum for walking on two legs, leaving the individual prone to developing spondylolysis.
The scientists said humans with intervertebral disc hernias have vertebrae that are more similar in shape to those of modern chimpanzees, and those of our fossil ancestors, than are humans with healthy spines.
"We can picture vertebral shape variation in humans as a spectrum with one end having vertebrae with an ancestral shape and the other end having vertebrae with exaggerated bipedal adaptations," said Mark Collard, study co-author from SFU.
"Where an individual's vertebrae lie within this distribution has a bearing on their spinal health," Collard said.
Based on the findings, the researchers said the evolutionary history of humans can have a direct bearing on current societal issues such as the prevention and management of back pain.
"For decades, scholars have assumed that the reason humans are so commonly afflicted with back problems is because we walk on two legs," Plomp said.
"Our studies are the first to show a clear link between the shape of your vertebrae, bipedalism, and the health of your spine," she added.