Acquired Spinal Conditions in Evolutionary Perspective: Updating a Classic Hypothesis

In 1923, Sir Arthur Keith proposed that many common back problems are due to the stresses caused by our evolutionarily novel form of locomotion, bipedalism. In this article, we introduce an updated version of Keith’s hypothesis with a focus on acquired spinal conditions. We begin by outlining the main ways in which the human spine differs from those of our closest living relatives, the great apes. We then review evidence suggesting there is a link between spinal and vertebral shape on the one hand and acquired spinal conditions on the other. Next, we discuss recent studies that not only indicate that two common acquired spinal conditions—intervertebral disc herniation and spondylolysis—are associated with vertebral shape, but also suggest that the pathology-prone vertebral shapes can be understood in terms of the shift from quadrupedalism to bipedalism in the course of human evolution. Subsequently, we place the aforementioned findings under an umbrella hypothesis, which we call the “Evolutionary Shape Hypothesis.” This hypothesis contends that individuals differ in their propensity to develop different acquired spinal conditions because of differences in vertebral shape that relate to the evolutionary history of our species. We end the article with some possible directions for future research.