What sets human language apart has long looked a little less settled.
A new study suggests three non-human primate species may sometimes use supra-regular recursive grammar when calling to each other: the common marmoset, the olive colobus monkey, and the bonobo.
“We show for the first time that animals generate call sequences that can be captured by supra-regular recursive grammars and hence share more similarities with human language than previously thought,” University of Zurich linguist Remo Nitschke and his colleagues write.
The study has been posted on the preprint server bioRxiv and has not yet been peer-reviewed.
The researchers said human languages tend to follow specific patterns, with smaller grammatical units sitting inside larger sentence structures to form complex layers of syntax. They said those nested patterns, known as supra-regular recursive grammar, have long been thought to distinguish humans from other animals.
Because the components of primate calls are not understood, the team used mathematical models to analyse call sequences in audio recordings of human speech and primate calls.
They built two models to predict which call sequences would come next. One assumed the sequence of sounds was linear, also known as probabilistic regular grammar. The other included nested hierarchies, where the position of smaller chunks of sound within the sequence was used to predict what would come next, also known as probabilistic context-free grammar.
The researchers said these models reflect key differences between regular grammar, which some animals are known to be capable of, and supra-regular grammar, which has been considered unique to humans.
They first tested the approach on recordings of humans and said that confirmed the models’ reliability for the purposes of the study.
When they applied the models to primate calls, they found the calls of all three species appeared to contain elements of supra-regular grammar.
“These findings challenge syntax as the major transition in hominid evolution and opens up prospects for alternative evolutionary scenarios,” the authors write.
The researchers also said the results are based on probabilities and on models used to interpret communication sequences that are not actually understood. They said that means the calls likely contained supra-regular grammar, but this is not certain.
“Yet another possibility is that the groupings reflect a purely formal structure, similar to the syntax of music in humans or song in animals,” the authors note. “Disentangling these possibilities requires much further research.”
They also said the datasets were quite small, so more studies would be needed to test if the patterns hold across wider sets of calls.
“Despite these limitations, it is clear that the traditional view needs to be revised: some parts of call sequences of nonhuman primates are in fact most likely generated by supra-regular grammars,” the authors add.
“The absence of a supra-regular capacity has long been held to be the single-most important demarcation line in hominin evolution,” Nitschke and team conclude. “Our findings reset the agenda for research on the evolution of language: if not syntax, what is the critical step that enabled the evolution of language from a primate-like communication system?”
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