Analysis indicates Australopithecus sediba retained tree-climbing adaptations

Newswise — BOSTON—According to a new study, our Australopithecus ancestors may have used different approaches to getting around on two feet. The new findings, co-authored by Boston University researchers Jeremy DeSilva , assistant professor of anthropology, and Kenneth Holt, assistant professor of physical therapy, appear in the latest issue of the journal Science in an article titled “The Lower Limb and Mechanics of Walking in Australopithecus sediba” (http://www.sciencemag.org/lookup/doi/10.1126/science.1232999). The paper is one of six published this week in Science that represent the culmination of more than four years of research into the anatomy of Australopithecus sediba (Au. sediba). The two-million-year-old fossils, discovered in Malapa cave in South Africa in 2008, are some of the most complete early human ancestral remains ever discovered.

The locomotion findings are based on two Malapa Au. sediba skeletons. The relatively complete skeletons of an adult female and juvenile male made possible a detailed locomotor analysis, which was used to form a comprehensive picture of how this early human ancestor walked around its world.

The researchers hypothesize this species walked with a fully extended leg (like humans do), but with an inverted foot (like an ape), producing hyperpronation of the foot and excessive rotation of the knee and hip during bipedal walking. These bipedal mechanics are different from those often reconstructed for other australopiths and suggest that there may have been several forms of bipedalism throughout human evolution.

Australopithecus sediba has a combination of primitive and derived features in the hand, upper limb, thorax, spine, and foot. It also has a relatively small brain, a human-like pelvis, and a mosaic of Homo- and Australopithecus-like craniodental anatomy. The foot in particular possesses an anatomical mosaic not present in either Au. afarensis or Au. africanus, supporting the contention that there were multiple forms of bipedal locomotion in the Plio-Pleistocene. (The recent discovery of an Ardipithecus-like foot from 3.4-million-year-old deposits at Burtele, Ethiopia, further shows that at least two different forms of bipedalism coexisted in the Pliocene.)

“Our interpretation of the Malapa skeletal morphology extends the variation in Australopithecus locomotion,” says DeSilva. “As others have suggested, there were different kinematic solutions for being a bipedal hominin in the Plio-Pleistocene. The mode of locomotion suggested by the Malapa skeletons indicates a compromise between an animal that is adapted for extended knee bipedalism and one that either still had an arboreal component or had re-evolved a more arboreal lifestyle from a more terrestrial ancestor.” DeSilva adds that there is some evidence that the South African species Au. africanus may have been more arboreal than the east African Au. afarensis. “A hypothesized close relationship between Au. africanus and Au. sediba, along with features in the upper limbs of the latter thought to reflect adaptations to climbing and suspension, is consistent with a retained arboreal component in the locomotor repertoire of Au. sediba.”

Co-authors of this study are Kristian J. Carlson, Evolutionary Studies Institute, University of the Witwatersrand, South Africa and the Department of Anthropology, Indiana University, Bloomington, IN; Christopher S. Walker, Department of Evolutionary Anthropology, Duke University, Durham, NC; Bernhard Zipfel, Evolutionary Studies Institute, University of the Witwatersrand, South Africa and Bernard Price Institute for Palaeontological Research, School of Geosciences, University of the Witwatersrand, , South Africa; and Lee R. Berger, Evolutionary Studies Institute, University of the Witwatersrand, South Africa.

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