Animal asymmetry and human heredity: dextrality, tool use and language in evolution--10 years after Walker (1980).

At the time of the last major review on this subject in the British Journal of Psychology (Walker, 1980) there was, with a few notable exceptions, little evidence of anatomical or behavioural asymmetries in non-human species at all comparable to our own. In the past decade the picture has changed dramatically, and now includes invertebrates from the Cambrian formations of half a billion years ago, many species of birds, rats and mice, antelopes, cats, dogs, whales, and primates. Asymmetries may differ as a function of sex (including hormone status-females of many species including humans appearing more lateralized at a motor level), developmental status and environmental influences. Effects may be manifest anatomically (limb or brain dimensions), or at a sensory, cognitive or motor level (paw preference, turning biases). A recurring observation across species is that the right hemisphere seems to be weakly specialized for spatial and emotional roles, and the left for learning, discriminatory and communicatory functions. Three million years ago our hominid ancestors were more or less fully bipedal, developed the first stone tools and were dextral in manipulating them. However, tool use seems not to have driven the evolution of language, our other strongly lateralized capacity. Evolutionary scenarios for bipedalism, tool use and language are discussed, together with the question of whether human language is continuous, in an evolutionary sense, with earlier primate call systems.