Utterance evolution: the road to generative, combinatorial communicators.

Language has long been considered uniquely complex in the animal kingdom; however, animal research over the last decade has begun to challenge some long-standing premises about exactly which language capacities are uniquely human. The task of resolving why and how complex communication systems evolve, particularly human language, has manifested in a rich multi-disciplinary field, which relies on accurate empirical and comparative assessment of natural animal communicative capacities. For example, comparative models of relevant gene and brain functions will remain limited without cracking the animal communication code. To achieve this, empirical research needs an updated, quantitative and comparative framework that distinguishes combinatorial systems from simpler ones. We offer such a framework by integrating new work, demonstrating diverse meaning-bearing combinatorial signalling strategies in natural animal communication, into cross-disciplinary theoretical frameworks. We detail four major mechanistic transitions which we argue offer both evolutionary and ontogenetic routes from non-combinatorial communicators, that mainly rely on single signal utterances, to generative combinatorial signal users, that mainly rely on combinatorial, meaning-based utterances. These transitions are measurable and predictable: (i) signal combination versatility; (ii) signal combination learning and conventionalization; (iii) message expansion through combinatorial mechanism diversity and versatility; and (iv) message expansion through syntax involving linear or hierarchical rules. We offer testable hypotheses for each transition and propose that species demonstrating combinatorial Transitions (i) and (ii) will be prime candidates for testing Transitions (iii) and (iv), where generativity can emerge. We also point to future methodological tools to assess the meaning of signals and their combinations in signal utterance-to-context mapping analyses. Critically, we argue that simply having research on different species is not enough - testing evolution of language theories requires comparative research across species and ontogeny, based on relatable, quantitative measures. To date, only humans demonstrate meaning-based, generative communication. However, recent discoveries, particularly of combinatorial versatility in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus) vocal sequences, open the possibility of generative communication in other species. Whether such traits are human-unique can no longer be assumed but must be tested.