Evolution and lateralization of the two great primate action systems
pp. 281-300
in: Jan Wind, Brunetto Chiarelli, Bernard Bichakjian, Alberto Nocentini, Abraham Jonker (eds), Language origin, Berlin, Springer, 1992Abstract
The assumption that hemispheric specializations for the vocal and manual action systems began in hominids, an assumption now known to be unwarranted, has unneccessarily prevented a serious comparative investigation of the evolution of these two systems. Such an investigation leads to the suggestion that the two systems share 4 major properties. 1. They are both overlaid functions. 2. Their most specialized properties evolved by elaboration of the most distal components of the anatomical substrates on which they were overlaid. 3. Much of their ability to produce variegated serially organized output results from use of a dual Frame/Content (F/C) mode of organization, the examples being, in the manual system, certain forms of bimanual coordination, and, in the vocal system, the insertion of vowels and consonants into syllable frames at the phonological level, and the insertion of content word stems into syntactic frames at the grammatical level. In addition, F/C modes of hand-mouth interaction made a contribution to the evolution of both systems. 4. They may have in common a postural origin for their shared left hemisphere specialization, in the form of an adaptation to demands of a (mostly) asymmetrical arboreal environment in early primates.These similarities presumably result from several factors. First, evolution commonly works by superimposing new structures and functions on older ones. Second, there are presumably similar selection pressures for the evolution of all purpose open systems, one for the social world and the other for the physical world. Third, choice of the distal site for elaboration may be due to the lesser disruption of existing functions involved in modifying their substrate at its distal extension than more proximally. A fourth common factor is the utility of the Frame/Content mode as a means of achieving openness while nevertheless remaining confined to a manageably small number of basic output configurations (frames). The main differences between the systems (which like the similarities can probably be explained on functional grounds) are in the ways in which the Frame/Content modes evolved, and in the fact that the F/C mode of grammatical organization differs from all the others by transcending the motor level.Major developments in hemispheric specialization for manual function may have paralleled major functional changes in hand use. A left hand/right hemisphere specialization for visual-spatial function related to unimanual predation may have evolved with the prehensile hand in the earliest primates. A right hand/left hemisphere specialization for invasive foraging may have evolved with internal hand control in higher primates. Together with the left hemisphere specialization for vocal communication in monkeys, these specializations constitute an indispensible context for the understanding of language origins.