Antonio Benítez-Burraco and Ljiljana Progovac

The emergence of modern languages resulted from several cognitive, behavioral and cultural human innovations, two of which we highlight here: cross-modality (necessary to support core aspects of language processing, e.g. metaphoricity) and reduced reactive aggression (necessary to enable more frequent and more diverse interactions, supporting language acquisition and ultimately, the complexification of languages through a cultural mechanism). Our proposal is that these two innovations were, partly, a side-effect of our self-domestication (SD) (e.g. Kruska, 1988; Hare, 2017), and more precisely, of the feedback loop between our increasing SD phenotype and the emerging grammars (Progovac and Benítez-Burraco, 2019; Benítez-Burraco and Progovac, 2020). We propose that the mechanism which unites these two dimensions in human evolution involves the modification of cortico-striatal networks.
SD primarily targeted reactive aggression, which is curtailed by cortico-subcortical networks (see e.g. research on Tourette’s syndrome, Mink, 2003; Ganos et al., 2013). However, these networks are also involved in cross-modality (e.g. Boeckx and Benítez-Burraco, 2014). Enhanced connectivity and plasticity in cortico-subcortical brain networks has been claimed to be a result of relatively recent evolution and selection (e.g. Enard et al., 2009; Lieberman, 2009; Hillert, 2014; Dediu, 2015; Ardila et al., 2016; Fisher 2017). As both reactive aggression and cross-modality processes demand a precise degree of (dis)inhibition in these brain circuits, our hypothesis is that selection for less aggressive phenotypes, i.e. for individuals who had better control over subcortical structures due to more enhanced brain connectivity, also facilitated the emergence of cross-modality, and ultimately, of more sophisticated forms of grammar. More precisely, our proposal characterizes language evolution as a process of fine-tuning of inhibition/disinhibition in the cortico-subcortical networks, through a continued feedback loop between biological forces, such as SD and selection, and cultural processes, such as verbal behavior and language sophistication. Accordingly, in the beginning, brain changes mostly associated with our less aggressive phenotype facilitated the entrenchment of the emerging forms of language/grammar, such as ideophones (e.g. Dingemanse 2009) and certain imageable compounds (e.g. Progovac 2016), exhibiting a high degree of raw metaphoricity and/or verbal aggression. In turn, these forms of early language also contributed to increasing brain connectivity, further reinforcing the control of cortical structures over the subcortical structures, and with it over reactive aggression. This ultimately contributed to sophistication of language forms and of metaphoricity. Importantly, because SD is considered to result from selected genetic (and epigenetic) changes (e.g. Wilkins et al., 2014), our proposal will enable us to identify specific new candidates for language evolution in the species, among the candidates for (self)-domestication, with an impact on these cortico-subcortical networks.
The significance of our proposal is not only that it identifies the underlying mechanism which unites different core dimensions in language evolution (cross-modality, (verbal) aggression, and language processing more generally), but also that it facilitates an answer to the question of why they cluster together, including in cognitive disorders. We will illustrate this link with the consideration of conditions such as synesthesia and Tourette’s syndrome, but also of schizophrenia and autism spectrum disorder.

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