The relatively quick evolution pertaining to the ability to walk on two legs may have had a substantial impact on modern human health. Particularly, the evolution of human locomotion may explain why a particular back problem called intervertebral disk herniation is common.
Intervertebral disk herniation has a prevalence rate ranging from 20 to 78 percent. This condition results from the prolapse of a gelatinous matter found inside the disk. It is interesting to consider the fact that humans are more predisposed toward this condition than non-human primates. A widely accepted explanation for this is that the stress placed on the spine by bipedal locomotion results in lower back injuries. A new study supports this explanation and further hypothesises that the predisposition to this common back problem may be the consequence of human locomotion.
A team of researchers studied the vertebrae of humans, chimpanzees, and orangutans to examine the relationship among vertebral shape, locomotion, and the appearance of vertical disc herniation in humans using two-dimensional shape analyses. To be specific, the analyses involved comparing 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.
Of the examined human vertebrae, 54 had vertical disc herniation. Furthermore, the researchers found that human vertebrae with vertical disc herniation have more similarities in shape with chimpanzee vertebrae than the healthy human vertebrae.
The results suggest that vertical disc herniation is common in individuals with vertebrae shaped closely to the ancestral end of the range of human shape variation. In other words, humans with disc problems have vertebrae that are closer in shape to those of our closest ape relatives, the chimpanzee, than are the vertebrae of humans without disc problems. Therefore, these individuals suffer more from load-related spinal disease because they are less adapted for bipedalism.
According to Kimberly Plomp, postdoctoral researcher from Simon Fraser University in Canada, humans and chimpanzees split from a common ancestor about eight to nine million years ago, and at some point after that split it is thought that human lineage evolved to be bipedal, moving on two rear legs, while the chimpanzees evolved to be knuckle-walkers.
The researchers believe that their findings could be used as a reference for interpreting medical scans of spinal disease. This could help clinicians predict the susceptibility of an individual toward intervertebral disk herniation by investigating the shape of his or her spine.
“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,” said Plomp, “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.”
There are several limitations to the study according to the researchers. For example, small samples sizes and subjects derived from Medieval and Post-Medieval English population might not represent the entire situation. The researchers are planning to conduct a follow-up study that would 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 herniation 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.
Further details of the study are found in the article “The ancestral shape hypothesis: an evolutionary explanation for the occurrence of intervertebral disc herniation in humans” published in the journal BMC Evolutionary Biology. Other researchers include Mark Collard from Simon Fraser University, Darlene Weston from the University of British Columbia, Una Strand Viðarsdottir from the University of Iceland, and Keith Dobney from the University of Aberdeen.