Unlike all other lateral specializations, the necessity for unilateral dominance is clear only for the case of the motor control of the speech organs lying on the midline of the body and innervated from both hemispheres. All functional asymmetries are likely to be a consequence of this asymmetry of executive control.

Recent studies with human infants and nonhuman primates reveal that posture interacts with the expression and stability of handedness. Converging results demonstrate that quadrupedal locomotion hinders the expression of handedness, whereas bipedal posture enhances preferred hand use. From an evolutionary perspective, these findings suggest that right-handedness may have emerged first, following the adoption of bipedal locomotion, with speech emerging later.

Corballis's explanation for right-handedness in humans relies heavily on the gestural protolanguage hypothesis, which he argues for by a series of “intuition pumps.” Scrutinizing the mirror system hypothesis and modern gesture as components of the argument, we find that they do not provide the desired evidence of a gestural precursor to speech.

This commentary takes issue with Corballis's claim to have presented a falsifiable hypothesis. It argues that Corballis has instead presented a framework of weak inferences that, although unfalsifiable, might help to constrain future theory-building.

Corballis seems to have not considered two points: (1) the importance of direct selection pressures for the evolution of handedness; and (2) the evolutionary significance of the polymorphism of handedness. We provide arguments for the need to explain handedness in terms of adaptation and natural selection.

Right-hand preference for conversational gestures does not imply close connections between the neural systems controlling manual and vocal communication. Use of speech and gestures may dissociate in some cases of focal brain damages. Furthermore, there are limits in the ability to combine spoken words and concurrent hand movements. These findings suggest that discourse production depends on multiple components which probably have different evolutionary origins.

We propose that the fine discrete movements of the tongue as used in speech are what account for the extreme lateralization in humans, and that handedness is a mere byproduct of tongue use. With regard to syntax, we support the Armstrong et al. (1995) proposition that syntax derives directly from gestural motor movements as opposed to facial expressions.

Wittgenstein shifted from a picture theory of meaning to a use-based theory of meaning in his philosophical work on language. The latter picture is deeply congenial to the view that language and the use of our hands in practical activity are closely related. Wittgenstein's theory therefore offers philosophical support for Corballis's suggestion that the development of spoken language is the basis of dominance phenomena.

Given the primate propensity to make noise, it is unclear why a manual gestural stage would have been necessary in the development of either language or right-handedness. Cortical asymmetries are present in australopithecines but become clearly human-like with the appearance of Homo about two million years ago, including Broca's cap regions. Stone tool-making is still our only empirical entry into past cognitive processes.

The target article by Corballis presents an interesting and novel theoretical perspective on the evolution of language, speech, and handedness. There are two specific aspects of the article that will be addressed in this commentary: (a) the link between Broca's area and gestural communication in chimpanzees, and (b) the issue of population-level handedness in great apes, notably chimpanzees.

The evolutionary scenario described in this target article parallels developmental patterns observed in human infants. Early vocalizations are largely expressive, manual control develops more rapidly than intentional vocal articulation, and vocal and manual activity are linked. In ontogenetic development, language is strongly rooted in bodily action and gesture.

A premise of Corballis's theory is that speech arose when vocalization co-opted existing gestural functions in the left ventral premotor cortex. Yet, visuomotor functions in this region remain largely unchanged between humans and macaques and have no discernible connection to gestural communication. This functional continuity suggests that language production is not the result of modifying existing motor functions in this region.

The possibility that two forms of asymmetry underlie handedness is considered. Corballis has proposed that right-handedness developed when gesture encountered lateralized vocalization but may have been superimposed on a preexisting two-thirds dominance. Evidence is reviewed here which suggests that the baseline asymmetry is even more substantial than this, with possible implications for brain anatomy and genetic theories of handedness.

Anatomo-functional studies in humans point out that handedness and language-related functional laterality are not correlated – except during language production; and that the convergence of language and hand control is located in the precentral gyrus, whereas executive functions required by movement imitation and phonological and semantic processing converge onto Broca's area. Multiple domains are likely to be actors in language evolution.

This commentary points to the lack of sound data supporting Corballis's thesis that there is a general left-hemisphere dominance for nonverbal vocal production in mammals. I also point out that area F5 in the rhesus monkey, which Corballis considers as homologous to Broca's area, contains not only visual “mirror” neurons but also auditory “mirror” neurons. This weakens Corballis's thesis that language developed exclusively at the gestural level.

This commentary presents indirect support for Corballis's claim that language evolved out of a gestural system in our evolutionary past. Specifically, it presents behavioral and neurological evidence that present-day speech and gesture continue to be tightly integrated in language production and comprehension.

Human right-handedness does not originate in vocalisation as such but in selection pressures for structuring complex sequences of digital signals internally, as if in a vacuum. Cautious receivers cannot automatically accept signals in this way. Biological displays are subjected to contextual scrutiny on a signal-by-signal basis – a task requiring coordination of both hemispheres. In order to explain left cerebral dominance in human manual and vocal signalling, we must therefore ask why it became adaptive for receivers to abandon caution, processing zero-cost signals rapidly and on trust.

Corballis suggests that apes lack voluntary control over their vocal production. However, recent evidence implicates voluntary control of vocalizations in apes, which suggests that intentional control of vocal communication predates the hominid-pongid split. Furthermore, the ease with which apes in captivity manipulate the visual attention of observers implies a common cognitive basis for joint attention in humans and apes.

Corballis argues that language underwent two modality switches – from vocal to manual, then back to vocal. Speech has evolved a frame/content mode of organization whereby consonants and vowels (content) are placed into a syllable structure of frames (MacNeilage 1998). No homologue to this mode is present in sign language, raising doubt as to whether the proposed modality switches could have occurred.

Ontogenetic factors constrain the evolution of species-typical traits. Because human infants are born “prematurely” relative to other primates, the development of handedness during infancy can reveal important ontogenetic influences on handedness that may have contributed to the evolution of the human species-typical trait of a population-level right-hand dominance.