The relative contribution of intervertebral disc to the curvatures of the vertebral column

Habitual bipedality (BI), a human hallmark characteristic, is a defining specialization central to the evolutionary development of all our other uniquely human characters ‐ large brain, tool making, hunting, and language. Characters alleged to be exclusively associated to BI include disposition and orientation of foramen magnum, lumbar curvature (CU), and a non‐opposable adducted great toe, which have been used to argue for fossils to be bipeds and thus human ancestors. Regarding spinal CU, it is uncertain whether CU and thus BI can be inferred solely from fossilized remains. The non‐fossilizing soft‐tissue, the inter‐vertebral discs (IVD), may be largely responsible for the CU. To determine the validity of reconstructing CU based solely on bony remains this study examines the contribution of IVD and vertebral body joint (VBJ) surface orientation to the thoracic and lumbar CUs of the spine in cadavers and living humans. MR sagittal spine sections of seven healthy adult individuals, and seven cadaveric vertebral columns were measured for cervical, thoracic and lumbar CU using Cobb’s angle. Contribution of each IVD and VBJ surface orientation to CU was measured directly from the MR images and from tracings of the vertebral columns of cadavers. Results show that in the thoracic region VBJ surface orientation contributed 95% of the CU and IVD only 5%. The IVD between the last lumbar and 1 st sacral vertebrae, however, contributed as much as 14% towards lumbar CU. Our preliminary results indicate lumbar CU measures based solely on fossilized hard tissue are accurate except in the case of the lumbosacral joint.