A Two-Million-Year-Old Puzzle

According to a new study co-authored by BU Sargent Assistant Professor of Physical Therapy Kenneth Holt in the journal Science, our Australopithecus ancestors may have used a different approach to walking than previously believed. Holt and colleagues including BU Assistant Professor of Anthropology Jeremy De Silva hypothesize that these early hominins 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.

The new findings appear in the latest issue of the journal Science in an article titled ” The Lower Limb and Mechanics of Walking in Australopithecus sediba“. The paper is one of six published in April in Science that represent the culmination of more than four years of research into the anatomy of Australopithecus sediba (Au. sediba). Two-million-year-old fossils of the species, 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.

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.)

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.

Post adapted from Boston University College of Arts & Sciences