The influence of cognitive evolution on handaxe making skill in the Acheulean

Reconstructing the complex human behaviours that manifest in the Palaeolithic archaeological record remains an elusive but important challenge for capturing traces of how the human mind evolved. The roughly 1.5 million years of handaxe-making throughout the Acheulean provides a consistently preserved manifestation of hominin technological skills. Here, we analyse 1108 3D scans of handaxes from 12 sites in the Great Rift Valley of eastern Africa and the southern Levant spanning much of the Acheulean. We set out to chart the evolution of Acheulean toolmaking skill using a suite of computational 3D methods to quantify how well these handaxes have been thinned, shaped, and sharpened; traits which demand manual dexterity, planning, and hierarchical cognition. We find evidence that cognitive evolution likely played a role in improvements in handaxe-making skill, but only in the first half of the Acheulean. The most skilfully made handaxes among our sample begin to be made by about one million years ago. Any increases in cranial capacity, cognitive complexity, or manual dexterity after this point do not make a noticeable impact on how skilfully handaxes were made, and nor does raw material quality. Instead, the techno-morphology of handaxes appears to be relatively untethered from evolutionary constraints after about a million years ago, at which point we need to seek alternative explanations for handaxe variability. Above all, we show that the difficult pursuit of lowering bifacial asymmetry was more often achieved than any other skilful attribute, suggesting it was a fundamental goal of handaxe-making. Attempting to maintain 3D symmetry appears to be a common goal throughout the entire Acheulean and was routinely achieved by its second half. This capacity to consistently maintain bifacial balance while knapping was in place by at least 1.0 Ma, revealing the evolved technological abilities and hierarchical cognition of Acheulean hominins. • We analyse 1108 3D scans of Acheulean handaxes from 12 sites from the Great Rift Valley. • Computational 3D methods were used to estimate how skilfully each handaxe was made. • Hominins enacted handaxe reduction sequences with equivalent fidelity on raw materials of different qualities. • Early improvements in handaxe-making skill were likely governed by cognitive evolution, but not later improvements. • Acheulean knappers appear to be aiming to minimise bifacial asymmetry and routinely achieved this by one million years ago.