The cause of back pain can be linked to humanity's evolutionary past, according to new research from a team of bioarchaeologists at Simon Fraser University, the University of Liverpool, and the University of Sydney.
The study, published in Evolution, Medicine, and Public Health, examines why some people are more susceptible to a particular stress fracture known as spondylolysis - a condition that often affects athletes.
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. However, there have been few attempts to test this hypothesis."
Kimberly Plomp, SFU postdoctoral researcher
The researchers used advanced 3D shape analysis techniques to compare the final lumbar vertebrae of humans with and without spondylolysis to the same bones in our closest living relatives, the great apes.
The team found that the differences between human vertebrae with spondylolysis and great ape vertebrae were greater than those between healthy human vertebrae and great ape vertebrae. 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. The differences are consistent with the vertebrae having "overshot" the optimum for walking on two legs, leaving the individual prone to developing spondylolysis.
The latest research is the third study that the researchers have conducted linking vertebral shape and back pain to the evolutionary history of our lineage. Previously, they have demonstrated that 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. Where an individual's vertebrae lie within this distribution has a bearing on their spinal health," says Mark Collard, SFU archaeology professor and Canada Research Chair in Human Evolutionary Studies.
"For decades, scholars have assumed that the reason humans are so commonly afflicted with back problems is because we walk on two legs," says Plomp." Our studies are the first to show a clear link between the shape of your vertebrae, bipedalism, and the health of your spine."
Keith Dobney, professor of human palaeoecology at the University of Sydney and the University of Liverpool, adds: "This is an area requiring further study, but our data show that studying the past can have a direct bearing on current societal issues - in this case the prevention and management of back pain."
Plomp, K.A., et al. (2020) Spondylolysis and spinal adaptations for bipedalism: The overshoot hypothesis. Evolution, Medicine, and Public Health. doi.org/10.1093/emph/eoaa003.