Spondylolysis and spinal adaptations for bipedalism: The overshoot hypothesis.

Background And Objectives: The study reported here focused on the aetiology of spondylolysis, a vertebral pathology usually caused by a fatigue fracture. The goal was to test the Overshoot Hypothesis, which proposes that people develop spondylolysis because their vertebral shape is at the highly derived end of the range of variation within .

Methodology: We recorded 3D data on the final lumbar vertebrae of and three great ape species, and performed three analyses. First, we compared vertebrae with and without spondylolysis. Second, we compared vertebrae with and without spondylolysis to great ape vertebrae. Lastly, we compared vertebrae with and without spondylolysis to great ape vertebrae and to vertebrae of with Schmorl's nodes, which previous studies have shown tend to be located at the ancestral end of the range of shape variation.

Results: We found that vertebrae with spondylolysis are significantly different in shape from healthy vertebrae. We also found that vertebrae with spondylolysis are more distant from great ape vertebrae than are healthy vertebrae. Lastly, we found that vertebrae with spondylolysis are at the opposite end of the range of shape variation than vertebrae with Schmorl's nodes.

Conclusions: Our findings indicate that vertebrae with spondylolysis tend to exhibit highly derived traits and therefore support the Overshoot Hypothesis. Spondylolysis, it appears, is linked to our lineage's evolutionary history, especially its shift from quadrupedalism to bipedalism.Lay summary: Spondylolysis is a relatively common vertebral pathology usually caused by a fatigue fracture. There is reason to think that it might be connected with our lineage's evolutionary shift from walking on all fours to walking on two legs. We tested this idea by comparing human vertebrae with and without spondylolysis to the vertebrae of great apes. Our results support the hypothesis. They suggest that people who experience spondylolysis have vertebrae with what are effectively exaggerated adaptations for bipedalism.