Actually, the disc doesn't literally slip; the capsule surrounding it tears and some of the contents herniate out. This causes inflammation and swelling; the muscles spasm to protect the injury and the swelling can put pressure on the sciatic nerves coming out of the spine and going to the legs. This causes referred leg pain or sciatica.
The usual explanation for this problem is that we have not yet fully evolved a skeleton for bipedal locomotion. During our evolution, as we became upright, we needed to develop a curve in our lumbar spine to bring our upper body in line above our pelvis. Quadrupedal animals don't have this curve. We also needed other modifications to our neck, skull, face and pelvis but the one which seems to give us the most problems is the lumbar spine.
So, if the theory of evolution is true, and our evolutionary history is the reason we suffer back pain and slipped discs, we should be able to make a prediction. We should find that people with, for the want of a better term, more primitive vertebrae, in other words, vertebrae which are less different to those of the common ancestor we share with our closest relatives, should have more back problems. All we need do is to look at the evidence to validate or falsify this prediction.
Now a team led by Dr Kimberly Plomp, of Simon Fraser University in Canada has done just that. They have shown that people with vertebrae which are closer in shape to those of chimpanzees are much more prone to develop "Schmorl's nodes" - herniations of the intervertebral discs or, in layman's terms, slipped discs.
Background: Recent studies suggest there is a relationship between intervertebral disc herniation and vertebral shape. The nature of this relationship is unclear, however. Humans are more commonly afflicted with spinal disease than are non-human primates and one suggested explanation for this is the stress placed on the spine by bipedalism. With this in mind, we carried out a study of human, chimpanzee, and orangutan vertebrae to examine the links between vertebral shape, locomotion, and Schmorl’s nodes, which are bony indicators of vertical intervertebral disc herniation. We tested the hypothesis that vertical disc herniation preferentially affects individuals with vertebrae that are towards the ancestral end of the range of shape variation within Homo sapiens and therefore are less well adapted for bipedalism.
Results: The study employed geometric morphometric techniques. Two-dimensional landmarks were used to capture the shapes of the superior aspect of the body and posterior elements of the last thoracic and first lumbar vertebrae of chimpanzees, orangutans, and humans with and without Schmorl’s nodes. These data were subjected to multivariate statistical analyses.
Canonical Variates Analysis indicated that the last thoracic and first lumbar vertebrae of healthy humans, chimpanzees, and orangutans can be distinguished from each other (p<0.028), but vertebrae of pathological humans and chimpanzees cannot (p>0.4590). The Procrustes distance between pathological humans and chimpanzees was found to be smaller than the one between pathological and healthy humans. This was the case for both vertebrae. Pair-wise MANOVAs of Principal Component scores for both the thoracic and lumbar vertebrae found significant differences between all pairs of taxa (p<0.029), except pathological humans vs chimpanzees (p>0.367). Together, these results suggest that human vertebrae with Schmorl’s nodes are closer in shape to chimpanzee vertebrae than are healthy human vertebrae.
So, basically, for the measures chosen, the difference between the vertebrae of humans with Schmorl's nodes and those of healthy chimpanzees was statistically negligible and much smaller than the significant difference between the vertebrae of healthy humans and humans with Schmorl's nodes. This was based on an analysis of 114 human vertebrae (54 with Schmorl's nodules and 60 without), 56 chimpanzee vertebrae and 27 orangutan vertebrae.
For evolutionary biologists, these results come as no surprise and merely confirm what was generally suspected anyway. This is summed up in the conclusion to the paper which also points out the importance of an evolutionary perspective in problems of medicine and public health.
Conclusions: The results support the hypothesis that intervertebral disc herniation preferentially affects individuals with vertebrae that are towards the ancestral end of the range of shape variation within H. sapiens and therefore are less well adapted for bipedalism. This finding not only has clinical implications but also illustrates the benefits of bringing the tools of evolutionary biology to bear on problems in medicine and public health.
So much for the frequent creationist claim that evolution theory has no predictive powers and no practical application. Not only does this paper illustrate the lie in those claims but also confirms the close evolutionary relationship between humans and chimpanzees. It also shows that many of our health problems are not the result of intelligent design but the result of a utilitarian process which is still continuing and which has to settle for near enough is good enough when there isn't enough selection pressure to improve things.
Yet again, a perfectly straightforward piece of research not only utterly fails to support creationism or intelligent design but adds yet more evidence to the mass of supporting evidence for the science of evolution.
It must be a tough life being a creationist fraud - or at least it would be if the people they live off weren't so afraid to learn a little science.
The ancestral shape hypothesis: an evolutionary explanation for the occurrence of intervertebral disc herniation in humans - BMC Evolutionary Biology
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