It is proposed that symmetry of stone tools may derive from perceptual similarity rather than from cognitively more complex awareness of symmetry. Although encodement of shapes necessarily involves symmetry (as evidenced by the confusability of enantiomorphs), it does not imply awareness of symmetry. Responses of relatively simple organisms, such as bees, support the notion that the processes involved are likely to be perceptual.

Wynn's claims are, in principle, entirely reasonable; although, as always, the devil is in the details. With respect to Wynn's discussion of the cultural evolution of artifactual symmetry, we provide a few more arguments for the utility of mirror symmetry and extend the enquiry into the tacit and explicit processing of natural and artifactual symmetry.

Cognition is readily seen to be connected to evolution through plots of the ratio of cranial capacity to body size of hominids which show two regions of sharply increasing ratios beginning at 2.5 and 0.5 million years ago – precisely the critical times inferred by the author from his study of tools. A similar correlation exists between current human brain growth spurts and the onsets of the Piagetian stages of reasoning development. The first goal of the author's target article is stated to be “to make a case for the relevance of archeological contributions to studies of the evolution of cognition” (sect. 1, Introduction). His analysis focuses on spatial cognition.

It appears that knappers intentionally produced symmetrical stones. Use of the dorsal pathways in knapping does not preclude shape perception, nor does it obviate use of ventral pathways in other tasks in Homo sapiens 400,000 years ago. Shape perception precedes production in present-day human infants, suggesting that symmetry perception was used by knappers of symmetrical stones.

I make two claims about cognitive archaeology. I question its role, seeing psychology as yet another contributor to the archaeological tool-kit rather than as something unique. I then suggest that cognitive archaeology is not in a position to provide evolutionary contexts without other disciplines. As a consequence it cannot deliver on the provision of evolutionary contexts for cognitive evolution.

Cognitive psychologists tend to treat intentionality as a control variable during experiments, yet ignore it when generating mechanistic descriptions of performance. Wynn's work brings this conflict into striking relief and, when considered in relation to recent neurophysiological findings, makes it clear that intentionality can be regarded mechanistically if one defines it as the planning of distal effects.

Wynn describes a revolution in cognitive abilities some 500,000 years ago, which added new sophistication to the curiosity of early man – the ability to form hypotheses. This derivative of archaic curiosity is a fundamental feature of learning, and it is our contention that the naive hypothesis testing behavior of early man will have left a distinctive trail in the archaeological record.

Handaxe morphology is thought to be the first example of the imposition of arbitrary form. Handaxes may thus inform researchers about shared mental templates and evolving cognitive abilities. However, many factors, not related to changes in cognition (e.g., material type, function, resharpening processes), influence handaxe shape over time and space. Archaeologists must control for these factors before making inferences concerning cognition.

This commentary contests Wynn's diagnosis of the cognitive implications of the earliest stone tools and Acheulian tools. I argue that the earliest stone tools imply greater cognitive abilities than those of great apes, and that Acheulian tools imply more than the preoperational cognitive abilities Wynn suggests. Finally, I suggest an alternative adaptive scenario for the evolution of hominid cognitive abilities.

Why did Homo erectus begin to craft symmetric tools? A parsimonious account assumes that preference for symmetry is inherent in all visual systems. This preference can be explained by a broader preference for perceptual fluency. The perceptual fluency account does not assume that selection for mate health or the production of symbolic art is a prerequisite for symmetry preference.

Reconstructing the evolution of cognition requires maximal extraction of information from very sparse data. The role that archaeology plays in this process is important, but strong empirical tests of plausible hypotheses are absolutely critical. Quantitative measures of symmetry must be devised, a much deeper understanding of nonhuman primate spatial cognition is needed, and a better understanding of brain/behavior relationships across species is necessary to properly ground these hypotheses.

Wynn's thesis that the acquisition of the rules of symmetry comprised the formative factor in the evolution of human spatial cognition is questioned on several grounds, including the ubiquity of symmetry across species and the apparent hard-wired nature of its evolution in both humans and animals.

I claim that the increase in complexity in the (known) trace of Paleolithic stone tools can be parsimoniously explained by postulating the emergence of effective mechanisms for the social transmission of representations. I propose that Paleolithic tools, similar to more contemporary tools, were subject to a process of evolution by artificial selection based on functionality.

Evolutionary theories of human cognition should refer to specific times in the primate or hominid past. Though alternative accounts of tool manufacture from Wynn's are possible (e.g., frontal lobe function), Wynn demonstrates the power of archaeology to guide cognitive theories. Many cognitive abilities evolved not in the “Pleistocene hunter-gatherer” context, but earlier, in the context of other patterns of social organization and foraging.

Wynn shows that intentionally standardized artifacts (handaxes) provide evidence of the ability to conceptualize form (symmetry). However, such conceptual ability is not sufficient for the actual production of these forms. Stone knapping is a concrete skill that is acquired in the real world. Appreciation of its perceptual-motor foundations and the broader issues surrounding skill acquisition may lead to further important insights into human cognitive evolution.

Implicitly, Wynn's target article starts from the transformational definition of symmetry. Unlike his suggestion, this traditional definition and the recent holographic definition are relevant to the discussion on the cognitive evolution of visual symmetries. These definitions reveal underlying properties and, thereby, they support the natural selection hypothesis. The holographic definition even agrees with an indirect test of this hypothesis.

Wynn suggests that the imposition of symmetry on stone tools is indicative of the evolutionary development of cognitive abilities of the tool makers, particularly that of creating mental images. I suggest that it is more likely indicative of the evolutionary development of the perceptual ability to detect resources for behavior of hand-held objects.

Wynn's model for the evolution of spatial cognition is well supported by fossil evidence from brain endocasts, and from neurological studies of the cerebellum and the posterior parietal region of the cerebral cortex. Wynn's intriguing hypothesis that the spatial skill reflected in artifacts is an index of navigational ability, could be further explored by an analysis of lithic transport patterns.

The underdetermination of intentional explanation by motor behavior complicates inferences drawn from preserved artifacts in the archaeological record to intentions in their production. Without knowledge of a producer's intentions, inferences drawn from those intentions to required cognitive abilities for having those intentions is also complicated.

Despite challenges on minimum necessary competence, intentionality, reliability, and context, the example of cognitive archaeology presented in the target article holds up well. The commentaries also present perspectives on cognition and symmetry that suggest an alternative to the target article's characterization of the cognitive abilities of early Homo erectus. However, the major conclusion of the initial argument – that the human ability to coordinate shape recognition and spatial cognition evolved hundreds of thousands of year ago in conditions unlike those of the modern world – remains intact.