Vertebral bodies or discs: which contributes more to human-like lumbar lordosis?

BACKGROUND: The attainment of upright posture, with its requisite lumbar lordosis, was a major turning point in human evolution. Nonhuman primates have small lordosis angles, whereas the human spine exhibits distinct lumbar lordosis (30 degrees -80 degrees ). We assume the lumbar spine of the pronograde ancestors of modern humans was like those of extant nonhuman primates, but which spinal components changed in the transition from small lordosis angles to large ones is not fully understood. QUESTIONS/PURPOSES: We wished to determine the relative contribution of vertebral bodies and intervertebral discs to lordosis angles in extant primates and humans.
METHODS: We measured the lordosis, intervertebral disc, and vertebral body angles of 100 modern humans (orthograde primates) and 56 macaques (pronograde primates) on lateral radiographs of the lumbar spine (humans-standing, macaques-side-lying).
RESULTS: The humans exhibited larger lordosis angles (51 degrees ) and vertebral body wedging (5 degrees ) than did the macaques (15 degrees and -25 degrees , respectively). The differences in wedging of the intervertebral discs, however, were much less pronounced (46 degrees versus 40 degrees ).
CONCLUSIONS: These observations suggest the transition from pronograde to orthograde posture (ie, the lordosis angle) resulted mainly from an increase in vertebral body wedging and only in small part from the increase in wedging of the intervertebral discs.