The neural and cognitive correlates of aimed throwing in chimpanzees: a magnetic resonance image and behavioural study on a unique form of social tool use

Abstract

It has been hypothesized that neurological adaptations associated with evolutionary selection for throwing may have served as a precursor for the emergence of language and speech in early hominins. Although there are reports of individual differences in aimed throwing in wild and captive apes, to date there has not been a single study that has examined the potential neuroanatomical correlates of this very unique tool-use behaviour in non-human primates. In this study, we examined whether differences in the ratio of white (WM) to grey matter (GM) were evident in the homologue to Broca's area as well as the motor-hand area of the precentral gyrus (termed the KNOB) in chimpanzees that reliably throw compared with those that do not. We found that the proportion of WM in Broca's homologue and the KNOB was significantly higher in subjects that reliably throw compared with those that do not. We further found that asymmetries in WM within both brain regions were larger in the hemisphere contralateral to the chimpanzee's preferred throwing hand. We also found that chimpanzees that reliably throw show significantly better communication abilities than chimpanzees that do not. These results suggest that chimpanzees that have learned to throw have developed greater cortical connectivity between primary motor cortex and the Broca's area homologue. It is suggested that during hominin evolution, after the split between the lines leading to chimpanzees and humans, there was intense selection on increased motor skills associated with throwing and that this potentially formed the foundation for left hemisphere specialization associated with language and speech found in modern humans.

Target animacy influences chimpanzee handedness

“…In handedness for gesture, this manifests as the structured sequences of manual actions to achieve the manipulation of a social partner. This interpretation is consistent with recent characterizations of apes' gestures as a kind of social tool use (Bard 1990(Bard , 1992Gomez 2007;Hopkins et al 2012;Leavens et al 2005).…”

Evolution of the strongest vertebrate rightward action asymmetries: Marine mammal sidedness and human handedness.

“…With respect to throwing, although Hopkins, Russell, and Schaeffer (2012) found it to be rightwardly asymmetric in chimpanzees, they suggest that with hominin evolution there was "intense selection on increased motor skills associated with throwing and that this potentially formed the foundation for left hemisphere specialization associated with language and speech" (p. 37). If so, this presumably had a strengthening effect on handedness.…”

Functional mastery of percussive technology in nut-cracking and stone-flaking actions: experimental comparison and implications for the evolution of the human brain

“…Our behavioural results reviewed above can be seen as bridging the gap between work reviewed elsewhere in this issue on monkey tool use and social learning [85], and work on human tool use and on the evolution of increasing cognitive demands in hominin Palaeolithic stone tool traditions [86,87]. Such work does not directly address African ape -human cognitive and behavioural contrasts, nor do we yet have any brain imaging observations even for a human model of the circuits activated in a nutcracking as compared with a simple stone-flaking task (see also [88,89]). …”