Neuroanatomical correlates of handedness for tool use in chimpanzees (Pan troglodytes): implication for theories on the evolution of language

Abstract

It has been hypothesized that cognitive mechanisms underlying lateralized complex motor actions associated with tool use in chimpanzees may have set the stage for the evolution of left-hemisphere specialization for language and speech in humans. Here we report evidence that asymmetries in the homologues to Broca's and Wernicke's areas are associated with handedness for tool use in chimpanzees. These results suggest that the neural substrates of tool use may have served as a preadaptation for the evolution of language and speech in modern humans.

Fig. 1

Depiction of the brain regions of interest in this study. The illustrations show (a) a three-dimensional reconstruction of a chimpanzee brain with the external sulci comprising the frontal orbital sulcus and the planum temporale highlighted in red and green, respectively; (b) a parasagittal view of a chimpanzee magnetic resonance imaging (MRI) scan with the fronto-orbital sulcus outlined in red; and (c) a coronal view of a chimpanzee MRI scan with the sylvian fissure highlighted in green. See the text for a description of the methods used to measure each brain region.

Fig. 2

Photographs of a chimpanzee engaged in (a) simulated termite fishing and (b) simulated nut cracking.

Fig. 3

Mean asymmetry-quotient (AQ) scores for non-right-handed and right-handed chimpanzees for each of the three handedness tasks. Error bars indicate standard errors.