Language lateralization to the dominant hemisphere: Tool use, gesture and language in hominid evolution

Shared tool use in our hominid ancestry is perhaps the most satisfactory explanation for human dextrality and left-hemisphere language lateralization. Recent palaeoarchaeological evidence suggests that brachiation preceded bipedalism, which in turn preceded advanced tool use, with all three preceding any dramatic increase in brain size and/or the development of speech-related neural structures. Shared tool use probably led to population dextrality, and then to the development of left-hemisphere centres for fine motor coordination and the mediation of serial, segmental, time-dependent and syntactic processes at sensory and more particularly motor levels, including the control of limbs, fingers and articulators. Such centres, initially developed for tool construction and use, would have occupied an intermediate position in the evolutionary sequence. Thus cortically-driven facial gestures may possibly have accompanied manual signing, modulating limbic vocalizations of affect, though a cortical-limbic distinction with respect to communication may be unwarranted. However, the uniqueness of human language is still a matter of debate both with respect to other primates and our own evolutionary ancestors.