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Over the 1.5-million-year duration of the Acheulean, there is considerable variation in biface finesse. It is not clear, however, if there is an improvement in biface knapping ability over time, or if variation between sites is largely unrelated to their age. The diversity and duration of the East African Acheulean presents an opportunity to examine this issue. Variables that reflect difficult aspects of biface knapping, and which were likely important goals for Acheulean hominins, were measured in order to assess skill. These variables—refinement (thinness), edge straightness, and symmetry—were compared across four East African Acheulean sites: Olduvai Gorge, Olorgesailie, Kariandusi, and Isinya. The influence of rock type, blank type, reduction intensity, aberrant scar terminations, and invasive flaking on these variables was assessed. Over relatively short timescales, confounding factors, including ones not possible to control for, tend to obscure any temporal signature in biface knapping skill. However, over the vast timespan of the Acheulean at Olduvai Gorge, a temporal trend was indeed apparent. Possible factors influencing this trend include the invention of new knapping techniques, the addition of adolescence as a life history stage, and evolving hominin cognition. [ABSTRACT FROM AUTHOR]