Sympathetic mouth movements accompanying fine motor movements in chimpanzees (Pan troglodytes) with implications toward the evolution of language.

Some theories concerning the evolution of language include a gestural stage prior to glottogenesis. These theories propose that connections observed between fine motor movements of the hands and mouth may be responsible for the transfer of human language from one that was primarily gestural to one that is spoken. The fine motor manipulation of objects by five captive chimpanzees was examined to determine whether sympathetic mouth movements accompanied fine and gross motor movements. Sympathetic mouth movements were observed significantly more often during fine motor manipulation for all five participants. A significant increase in the presence of sympathetic mouth movements was observed in behaviors along a continuum of precision of manipulation from precision grip to gross motor manipulation without prehension. The results are discussed in regard to current theories of language evolution and neurological processes.