The pivotal role of metaphor in the evolution of human language

There is broad agreement among evolutionary linguists that the emergence of human language, as opposed to other primate communication systems, is characterised by two key phenomena: the use of symbols, and the use of grammatical structure (Tomasello 2003). In this paper, we show that these two defining aspects of language actually emerge from the same set of underlying cognitive mechanisms within the context of ostensive-inferential communication. We take an avowedly cognitive approach to the role of metaphor in language change, setting out how general capacities such as the recognition of common ground, the inference of meaning from context, and the memorisation of language usage, can together lead to the conventionalisation of metaphors, and thence to systematic changes in language structure, including the development of grammatical linguistic units from formerly meaningful elements through grammaticalisation (Hoefler and Smith 2009). We show that the relevant cognitive competences are general-purpose mechanisms which are crucially not specific to language; they also underpin non-linguistic communication, where the same processes lead to the emergence of apparently arbitrary symbols.