The strong predominance of right-handedness appears to be a uniquely human characteristic, whereas the left-cerebral dominance for vocalization occurs in many species, including frogs, birds, and mammals. Right-handedness may have arisen because of an association between manual gestures and vocalization in the evolution of language. I argue that language evolved from manual gestures, gradually incorporating vocal elements. The transition may be traced through changes in the function of Broca's area. Its homologue in monkeys has nothing to do with vocal control, but contains the so-called “mirror neurons,” the code for both the production of manual reaching movements and the perception of the same movements performed by others. This system is bilateral in monkeys, but predominantly left-hemispheric in humans, and in humans is involved with vocalization as well as manual actions. There is evidence that Broca's area is enlarged on the left side in Homo habilis, suggesting that a link between gesture and vocalization may go back at least two million years, although other evidence suggests that speech may not have become fully autonomous until Homo sapiens appeared some 170,000 years ago, or perhaps even later. The removal of manual gesture as a necessary component of language may explain the rapid advance of technology, allowing late migrations of Homo sapiens from Africa to replace all other hominids in other parts of the world, including the Neanderthals in Europe and Homo erectus in Asia. Nevertheless, the long association of vocalization with manual gesture left us a legacy of right-handedness.

http://radicalanthropologygroup.org/sites/default/files/pdf/class_text_092.pdf

From mouth to hand: gesture, speech, and the evolution of right-handedness.

Evolution of hemispheric specialization: advantages and disadvantages.

The human left and right hemispheres have distinct functions. Sun and Walsh discuss recent genetic, imaging and neurological studies in an attempt to unravel the molecular mechanisms of brain asymmetry and handedness and to understand their evolutionary underpinning.

Molecular approaches to brain asymmetry and handedness

Summary A pattern of hemispheric specialization common to all vertebrates has been presented (summarized in Fig. 6 ). It has been considered, as far as possible, in terms of behavior patterns that are important in the natural environment. Responses in foraging and prey capture are typical specializations of the left hemisphere and they are often apparent as rightward-directed responses. Responses to novel stimuli and potential predators are specializations of the right hemisphere and responses to these stimuli are directed preferentially to the animal's left side. The right hemisphere is also specialized for the expression of fear and aggression, and to process and memorize topographical information, essential in association with predator-escape responses. The baseline differences between the hemispheres are (1) rapid and species-typical responses being under the control of the right hemisphere and (2) responses requiring considered decision making being under control of the left hemisphere. Download : Download full-size image Fig. 6 . Summary of the general pattern of lateralization of the hemispheres. Side biases in responding to prey to the right side and agonistic responses delivered to conspecifics on the left side occur not only in species with eyes placed on the sides of their head and with only a small binocular field of vision, but also in species with smaller lateral fields and with a large binocular overlap. At least in one case, that of prey striking in toads, there is clear evidence that the side bias occurs within the binocular field and manifests itself as a hemifield neglect of prey to the left side of the toad's midline. Chicks also, provided they have been hatched from eggs incubated in the dark, have a bias within the binocular field: They show a preference to fixate with the right half of the binocular field before delivering an aggressive peck. It should be noted, however, that the majority of aggressive pecks made by chicks are delivered after use of the left, monocular field of vision in dark-incubated chicks as well as those exposed to light before hatching. Lateralized use of the visual hemifields deserves further investigation in a range of species. Side biases in responding are not easily associated with any evolutionary advantage but they may be balanced against other advantages of lateralization. Lateralization may enhance cognitive abilities and more efficient responding in some species and contexts. It may also be associated with specialized functioning of the immune system selected on the basis of past experience of the species in terms of physiological demand on the immune system, although this hypothesis has yet to be tested. Finally, population bias for the majority of individuals to be lateralized in the same direction is considered as a potential benefit to group cohesion, as in schooling fish, and a mechanism by which individuals can interact in predictable ways within a social group. For example, the bias for directing attack responses leftward may be used by an approaching animal to avoid an aggressive encounter; approach on the potential aggressor's right side would minimize the chances of attack. Until now, most examples of lateralization have come from experimentation in the laboratory or using captive animals. There are relatively few examples demonstrating lateralization in wild populations, despite the obvious implications of lateralization for survival of species and individuals. Those lateralizations found in species in their natural habitat have been mentioned. They highlight the promise of gaining a better understanding of behavior of animals in the wild if lateralization is taken into account as a characteristic of all vertebrates.

Lateralization in vertebrates: Its early evolution, general pattern, and development

Recent studies have revealed a variety of left–right asymmetries among vertebrates and invertebrates. In many species, left- and right-lateralized individuals coexist, but in unequal numbers (‘population-level’ lateralization). It has been argued that brain lateralization increases individual efficiency (e.g. avoiding unnecessary duplication of neural circuitry and reducing interference between functions), thus counteracting the ecological disadvantages of lateral biases in behaviour (making individual behaviour more predictable to other organisms). However, individual efficiency does not require a definite proportion of left- and right-lateralized individuals. Thus, such arguments do not explain population-level lateralization. We have previously shown that, in the context of prey–predator interactions, population-level lateralization can arise as an evolutionarily stable strategy when individually asymmetrical organisms must coordinate their behaviour with that of other asymmetrical organisms. Here, we extend our model showing that populations consisting of left- and right-lateralized individuals in unequal numbers can be evolutionarily stable, based solely on strategic factors arising from the balance between antagonistic (competitive) and synergistic (cooperative) interactions.

/pdf/intraspecific-competition-and-coordination-in-the-evolution-258tpqgawm.pdf

Intraspecific competition and coordination in the evolution of lateralization

Laterality in cats: paw preference and performance in a visuomotor activity.

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Laterality in cats: paw preference and performance in a visuomotor activity.