There seems to be some consensus in the comments on McNabb et al.’s (CA 45:653–77) paper that Acheulean tools reveal a trend beginning with an imprecise kind of symmetry and leading to more defined regularities. Recent findings on how the brain functions can help to clarify why this sequence evolved. The visuo-spatial pathway (the “where/how” dorsal system), involving visually guided action, which would have been intimately associated with the making of tools, is thought to be “blind” (Ungerleider and Mishkin 1982, Goodale et al. 1991, Milner and Goodale 1995)—that is, more concerned with the parameters underpinning visuo-spatial guidance than with the overt identity of objects. In contrast, the “what” ventral pathway, which mediates explicit visual identification, may play down or even ignore visuo-spatial factors because these are largely irrelevant for a recognition system that strives to achieve object constancy (Turnbull, Carey, and McCarthy 1997, Turnbull, Beschin, and Della Sala 1997). The “where/how” pathway is thought to culminate in the superior parietal lobe. As this area is deemed to have undergone enlargement from Homo habilis through H. erectus to modern humans (Weaver 2002), the implications are obvious in the present context. The superior parietal lobe has been shown to be particularly active when an experienced stone knapper fashions a tool (Stout et al. 2000). Its enlargement from H. habilis to modern humans helps to explain the relative ineptness of chimps in manipulating tools. The makers of late, as opposed to early, Acheulean tools were therefore probably beginning to rely more on the “what” system than on the “where/how” pathway. Turnbull and collaborators believe that the inferior parietal lobe may supply some explicit form recognition to the visuo-spatial system, especially in non-optimal conditions and when mental rotation is required. Glover (2004) has proposed that enlargement of the phylogenetically newer inferior parietal lobe in humans is devoted to planning strategies for action, whereas on-line monitoring and control continue to be mediated by the superior parietal lobe. Tool and object use in particular required that human motor planning processes become integrated with some ventral pathway functions for object identification, which seems, in humans (in contrast