Evolution of the strongest vertebrate rightward action asymmetries: Marine mammal sidedness and human handedness.

Marine mammals and humans have the strongest manifestations of what is apparently a vertebrate-wide tendency toward a rightward action asymmetry associated with routine behavior. Marine mammal asymmetries usually involve whole-body actions associated with feeding. The human-like strength of these asymmetries may result from a problem of external aquatic support for the reactive component of the demanding lateral maneuvers of large marine mammals in daily pursuit of prey. Our asymmetrical primate heritage may also have begun with a rightward whole-body asymmetry, in prosimians, perhaps also resulting from problems of support for the reactive component of action; in this case arising from the arboreal habitat (and paradoxically including left-handedness). Monkeys and apes (simians) subsequently added right-sided adaptations for manipulation, bimanual coordination, bipedalism, throwing, and manual communication, most importantly by distal elaboration of limb function. The strength of human right-handedness may result partly from further elaboration of these simian action adaptations and partly from an evolving cognitive superstructure for tool use and language.