A common spinal disease could be the result of some people's vertebrae, the bones that make up the spine, sharing similarities in shape to a non-human primate. The research, published in the open access journal BMC Evolutionary Biology, suggests that the relatively quick evolution of the ability to walk on two legs may have had a substantial impact on modern human health.
Humans are more commonly afflicted with spinal disease than non-human primates, and one widely discussed explanation for this is the stress placed on the spine by bipedal locomotion. This research backs up this theory.
A widespread cause of back pain, 'intervertebral disc herniation', has prevalence rates ranging from 20% to 78%, depending on the population. It is caused by the prolapse of a gelatinous substance inside the disc, and when the herniation is vertically directed it is often characterised by protrusions of cartilage called 'Schmorl's nodes'.
Researchers studied the vertebrae of humans, chimpanzees and orangutans to examine links between vertebral shape, locomotion, and the appearance of vertical disc herniation in humans.
Postdoctoral researcher Kimberly Plomp from Simon Fraser University, Canada, said: "Our study is the first to use quantitative methods to uncover why humans are so commonly afflicted with back problems compared to non-human primates. The findings have potential implications for clinical research, as they indicate why some individuals are more prone to back problems. This may help in preventative care by identifying individuals, such as athletes, who may be at risk of developing the condition."
The researchers compared 141 human vertebrae, 56 chimpanzee vertebrae (a knuckle-walking primate), and 27 orangutan vertebrae (a climbing primate that uses all four feet which are modified as hands) and found significant differences in their shape. They say this could be explained by the different modes of locomotion and contributes to the understanding of the human evolution of bipedalism.
Of the human vertebrae they studied, 54 had 'Schmorl's nodes', the skeletal indicators of vertical disc herniation. The researchers found that human vertebrae with Schmorl's nodes shared more similarities in shape with chimpanzee vertebrae than the healthy human vertebrae shared with those non-human primates.
This suggests that vertical disc herniation preferentially affects human individuals with vertebrae that are towards the ancestral end of the range of human shape variation. These individuals may therefore be less well-adapted for bipedalism and suffer more from load-related spinal disease.
The authors say their findings could be used for interpreting medical scans of spinal disease. This could help clinicians investigate an individual's vertebral shape and predict their susceptibility to intervertebral disc herniation.
The identification of an ancestral vertebral shape that influences the occurrence of a common spinal disease in humans also supports the idea that the relatively quick evolution of bipedalism may have had a substantial impact on modern human health.
The authors note several study limitations such as small sample sizes, and including humans that are derived from Medieval and Post-Medieval English populations. Future research will include larger sample sizes and multiple human populations from different ancestral backgrounds. It will also include the analysis of CT scans of living individuals in order to study horizontal herniations that do not leave evidence on the bone, and focus on capturing the 3D shape of human and non-human primate data to capture vertebral elements that have been missed in the present study.
The research team included Mark Collard from Simon Fraser University, Darlene Weston from the University of British Columbia, Una Strand Viðarsd?ttir from the University of Iceland, and Keith Dobney from the University of Aberdeen.
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Kimberly A Plomp, Una Strand Vidarsdottir, Darlene A Weston, Keith Dobney and Mark Collard, The ancestral shape hypothesis: an evolutionary explanation for the occurrence of intervertebral disc herniation in humans, BMC Evolutionary Biology 2015. DOI: 10.1186/s12862-015-0336-y