Lateral Asymmetries in Human Evolution

Abstract

Lateral asymmetries are not confined to humans. Palaeozoic trilobites and calcichordates are now known to have been asymmetrical; song control in passerines is vested in the left cerebral hemisphere; learning which is lateralized to the left forebrain of chicks includes imprinting, visual discrimination learning and auditory habituation, while responses to novelty, attack and copulation are activated by the right; in rats the right hemisphere is involved in emotional behavior and spatial discriminations, and there are numerous other behavioral, anatomical and pharmacological asymmetries; the left hemisphere of the female mouse is superior at processing its pups' calls, and there are reports of behavioral asymmetries in impala, cats and dogs. Anatomical asymmetries in the primate brain, from monkeys upwards, are matched by increasing evidence of behavioral asymmetries in visual pattern discrimination, discrimination of species-specific calls, and handedness. We discuss the interaction of preexisting behavioral and brain asymmetries with the evolution in hominids of an upright bipedal posture and tool use, and the origins of language, and conclude that there may be a continuity with earlier species of our two most obvious asymmetries, language lateralization and hand preferences. There may be an ancient left-brain specialization for sensory and motor discrimination learning, which is complemented by a relegation to the right of primitive spatial and emotional functions.Most of US are right handed (dextral) and left-hemisphere (LH) dominant for language (Bradshaw & Nettleton, 1983). The two sides of the face rarely are left-right symmetrical, as can be seen if we create photographic composites of the two left (LJ) or right (RH) halves. Moreover the left side of the face may express emotions more strongly than the right, due probably to its more direct access to the right hemisphere (RH), known to be involved in the mediation of emotions (Borod & Koff, 1984). Conversely the right side of the mouth may produce earlier, and larger, articulatory movements during speech than the left (Wolf & Goodale, 1987) again due to its more direct access to the LH. If male, the left testicle usually depends lower, as Greek sculptors knew 2,500 years ago (McManus, 1976). However we now know that many nonhuman species are also laterally asymmetrical, due ultimately perhaps to the chirality