The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)


Manual pressure distribution during stone tool use

ERIN MARIE S. WILLIAMS1 and BRIAN G. RICHMOND1,2.

1Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology, The George Washington University, 2Human Origins Program, National Museum of Natural History, Smithsonian Institution

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Modern humans possess a highly derived thumb that is robust and long relative to the other digits, with enhanced pollical musculature compared to extant apes. Researchers hypothesize that this condition was selected in Homo in part to withstand high forces acting on the thumb during stone tool production. However, our previous research demonstrated that during stone tool-making the thumb in fact experienced significantly lower pressures and normal forces compared with other regions of the hand. In this paper, we test an alternative hypothesis that stone tool use may have distinct pressure distribution patterns that place greater stress on the thumb.

We used a dynamic pressure sensor system to measure pressures and normal forces acting across the hand during a variety of stone tool behaviors hypothesized to have been practiced by early hominins. Tested behaviors included slicing animal muscle using a flake and a handaxe, acquiring marrow using a hammerstone and a chopper, and nut-cracking. Subjects included experienced and novice knappers.

Contrary to stone tool production, stone tool use placed similarly high or significantly greater pressures and/or normal forces on the thumb compared with digits II-III. During nut-cracking, pressures and normal forces tended to be lowest, but differences between the thumb and digits II-III also tended to be strongest.

These results suggest that stone tool use, rather than production, may have been a stronger selective pressure on pollical robusticity. They also raise questions about why pollical robusticity appears so late after the earliest records of stone tool use.

This material is based upon work supported by the Wenner-Gren Foundation’s Dissertation Fieldwork Grant (7995), The George Washington University’s Research Enhancement Fund and Selective Excellence Fund, and the National Science Foundation's Minority Postdoctoral Research Fellowship (SMA-1103470) and the Integrative Graduate Education and Research Traineeship (IGERT # DGE 0801634).

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