The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)


The number of vertebrae in early hominins: insights from Australopithecus sediba

SCOTT A. WILLIAMS1, STEVEN E. CHURCHILL2,3, KELLY R. OSTROFSKY2, PETER SCHMID3,4, NAKITA FRATER4 and LEE R. BERGER3,5.

1Center for the Study of Human Origins, Department of Anthropology, New York University, 2Department of Evolutionary Anthropology, Duke University, 3Institute for Human Evolution, University of the Witwatersrand, 4Anthropological Institute and Museum, University of Zurich, 5School of Geosciences, University of the Witwatersrand

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The vertebral column plays a central role in posture and locomotion, including bipedalism in the human lineage. Despite this, the evolution of its numerical composition (regional number of vertebrae expressed as a vertebral formula) is contentious and not well understood. Compared to our closest living relatives, chimpanzees and gorillas, modern humans have shortened thoracic and longer lumbar regions (12- versus 13-thoracic and 5- versus 4-lumbar vertebrae). Previously known early hominin partial vertebral columns (Australopithecus africanus and Homo erectus) are either transitional in appearance (Sts-14) or have fragmented, incomplete thoraco-lumbar transitions. New material attributed to the Nariokotome H. erectus skeleton demonstrate that this individual had the same pattern as modern humans (12T:5L), along with a cranially-placed transitional vertebra (the one in which zygapophyseal orientation changes) compared to modern humans. Here, we provide new data evidenced from two partial skeletons belonging to Australopithecus sediba (MH1 and MH2). Together, they preserve 25 vertebral elements, including the transitional and last thoracic vertebrae, which are complete and preserved in articulation. Based on multiple lines of evidence, we reconstruct the vertebral column of Au. sediba with 7-cervical, 12-thoracic, 5-lumbar, and 5-sacral vertebrae, along with a cranially-placed transitional vertebra. We argue that this pattern also characterized Au. africanus and African H. erectus. This contrasts with recent models of vertebral formula evolution (e.g., that implied in the interpretation of Ardipithecus ramidus). Finally, we speculate on the evolutionary history of the hominin vertebral column, including the LCA and earliest hominins, as well as functional implications of proposed vertebral patterns.

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