Animal asymmetry and human heredity: Dextrality, tool use and language in evolution–10 years after Walker (1980)

Abstract

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.

A gene-culture model of human handedness

“…This is consistent with findings from studies of handedness and footedness in nonhuman primate, mammalian, and avian species, which suggest that although individuals exhibit hand and foot preferences, there is no consistent bias within populations (Annett, 1985;Bradshaw, 1991). Comparative observations suggest that the events that shaped the evolution of human handedness must have taken place after the split between humans and chimpanzees (Corballis 1991).…”

Handedness in Captive Bonobos (Pan paniscus)

“…It has been argued that a bipedal posture is necessary for the manifestation of a hand preference since individuals are free to use both hands [Bradshaw, 1991]. An alternative explanation is that the type of grip used differs in different postures, and consequently it is the grip rather than posture that is influencing hand preference [Hopkins, 1993].…”

Structural Asymmetries of the Human Brain and Their Disturbance in Schizophrenia

“…However, as recently as 1929, a comparative study of endocranial casts (endocasts) of modern and prehistoric man and of anthropoid apes found no marked cerebral asymmetries (Weil 1929). Until relatively recently, it was usually held that only humans exhibited structural asymmetries, but increasing evidence indicates that structural lateralization is common among such disparate nonhuman species as birds, monkeys, and apes (for a review, see Bradshaw 1991). Furthermore, there is impressive evidence that structural asymmetries are to be found far back in evolution: studies of a 600-million-year-old Cambrian fossil, the calcichordate Placocystites forbesianus-a primitive forerunner of some vertebrates-appear to show clear evidence of caudal asymmetry (Jefferies and Lewis 1978).…”