Great Apes and Humans Evolved from a Long-Back Ancestor

There is current debate whether the human/chimpanzee last common ancestor (LCA) had a short, stiff lumbar column like great apes or a longer, flexible column observed in generalized Miocene hominoids. Beyond reduction to four segments, three additional features contribute to lumbar stiffening: the position of the transitional vertebra (TV), orientation of the lumbar spinous processes, and entrapment of lumbar vertebrae between the iliac blades. For great apes, these features would be homologous from a short-back LCA, but likely functionally convergent though dissimilar phenotypes from a long-back LCA. We quantitatively and qualitatively analyzed hominoid, baboon, and colobine thoracic and lumbar vertebrae using 3D surface scanning and osteological measurements to compare spinous process morphology and sacral depth. We also used a large sample of hominoid vertebral counts to assess variation in the position of transitional vertebra and lumbosacral boundary. To determine the developmental independence of vertebral characters, we also analyzed thoracolumbar and lumbosacral transitions in Hox9 and Hox11 mutant mice. All extant hominoids modally place the TV at the ultimate thoracic. However, humans and orangutans place the TV at the 19th vertebral segment, while other apes place the TV at the 20th. Furthermore, chimpanzees, gorillas, and orangutans each have distinct patterns of spinous process angulation and morphology associated with lumbar stiffening. Finally, chimpanzees are unique compared to other hominoids with a greater sacral depth facilitating lumbar entrapment. These differences suggest that lumbar stiffening is convergent among great apes and that human bipedalism evolved from a more generalized long-back ancestor.