Department of Anthropology, Center for the Study of Human Origins, New York University
Thursday 8:00-8:15, Grand Ballroom II
Within- and between-species variation in vertebral formulae has complicated our understanding of hominin vertebral evolution. This study quantifies variation using diversity and similarity indices, which measure intraspecific and interspecific variation, respectively. These indices allow for testing the "short-backed" and "long-backed" models of hominin vertebral evolution, which call for disparate amounts of homoplasy, and by inference, different patterns of evolution. Under the long-back scenario, reduced variation (low diversity) in vertebral formulae is expected, resulting from relatively strong directional selection that is presumably associated with extreme homoplasy (independently and repeatedly reduced lumbar regions), particularly in closely related species that diverged relatively recently (e.g., chimpanzees and bonobos). Instead, high amounts of intraspecific variation are observed among all hominoids except humans and eastern gorillas, species that have likely experienced strong stabilizing selection on vertebral formulae associated with locomotor (i.e., bipedalism) and habitat specializations (i.e., terrestriality), respectively, or resulting from recent population bottlenecks. Interspecific variation is structured in such a way that vertebral formulae observed in western gorillas and chimpanzees represent a reasonable approximation of the ancestral condition for hominines, from which eastern gorillas, humans, and bonobos derived their unique vertebral profiles. Overall, these results support the short-back model and are consistent with a scenario of homology of reduced lumbar regions in hominoid primates. Therefore, hominins evolved five lumbar vertebrae from an "African ape-like" ancestor with a short (4-element) lumbar column via homeotic (border shifting), rather than meristic (numerical), change. Fossil hominin vertebral columns, including those of recently discovered Australopithecus sediba, support this model.
This study was funded by National Science Foundation Grant BCS-0925734