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

What 1st metatarsal cortical thickness distribution reveals about locomotion of Sterkfontein and Swartkran hominins


1Institute for Human Evolution, University of the Witwatersrand, South Africa, 2Statistics and Actuarial Science, University of the Witwatersrand, South Africa, 3NECSA, South African Nuclear Energy Corporation, South Africa

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Two first metatarsals from Sterkfontein (StW562, StW595) and two from Swartkrans (SKX5017, SK1813) are analyzed. Early work demonstrated a mosaic nature of morphological traits on these metatarsals, complicating functional interpretations: these hominins were unquestionably bipedal, but likely possessed a toe-off mechanism different from modern humans. Here we analyzed cortical thickness patterns in first metatarsal shafts of African apes. Fossil hominin metatarsals were analyzed similarly and patterns of thicknesses compared to the extant sample. We ask whether each of the four hominin metatarsals is most similar to one extant species, and if so, is it the same extant species in each case.

First metatarsals of chimpanzees, gorillas, modern humans, and fossils were CT scanned. For each metatarsal, we measured 17 cross sections from 25% to 65% of diaphyseal mechanical length. In each cross section, we measured cortical thickness along radii separated by one degree increments. Cortical thicknesses were size standardized, after which they were used in a penalised discriminant analysis (PDA) to assess species-specific or fossil patterns.

The first function of the PDA discriminates modern humans from chimpanzees and gorillas. The second function of the PDA discriminates chimpanzees from gorillas. SK1813 and StW562 metatarsals have patterns of thickness distribution most similar to gorillas. SKX5017 and StW595 metatarsals patterns occupy a space in the PDA between humans and chimpanzees. Thus, the fossils can be divided into two “morphs”, showing different diaphyseal structure, possibly the result of different locomotor repertoires. Interestingly, both Plio-Pleistocene sites contain the two “morphs”.

Founding was provided by SPARC, program of University of the Witwatersrand, South Africa

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