Biometric variables predict stone tool functional performance more effectively than tool-form attributes: a case study in handaxe loading capabilities

Both the form of a stone tool and the anatomy of the individual using it have potential to influence its cutting performance. To date, however, the selective pressures acting on stone-tool form and hominin biometric/biomechanical attributes have been investigated in isolation and their relative influence on performance have never been compared directly. This paper examines the influence of both tool-form attributes and biometric variation on the functional performance of Acheulean handaxes. Specifically, it investigates the impact of 13 tool attributes and eight biometric traits on the working forces applied through the edge of 457 replica tools. The relative contribution of tool-form and biometric attributes to handaxe loading levels were examined statistically. Results identify that both tool-form attributes and biometric traits are significantly related to loading; however, tool–user biometric variation has a substantially greater impact relative to tool-form attributes. This difference was demonstrated by up to a factor of 10. These results bear directly on the co-evolutionary relationships of stone tools and hominin anatomy, and the comparative strength of selective pressure acting on each. They also underline why handaxe forms may have been free to vary in form across time and space without necessarily incurring critical impacts on their functional capabilities.