Early Acheulian & Developed OldowanB / h)peninj18
Biface/Handaxe cross-sections. Peninj, West Natron, Tanzania (c. 1.3-1.5 MYA). In such Early Acheulian biface cross-sections as these it is clear that the knappers worked only with 2-dimensional mirror symmetry.
Illustration © T. Wynn. T. Wynn. (1989). The Evolution of Spatial Competence. Urbana, IL: University of Illinois Press: fig. 18. (yellow highlight added)