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Science Advances  21 Jul 2021:
Vol. 7, no. 30, eabj1517
DOI: 10.1126/sciadv.abj1517


Rawski et al. revisit our recent findings suggesting the latent ability to process nonadjacent dependencies (“Non-ADs”) in monkeys and apes. Specifically, the authors question the relevance of our findings for the evolution of human syntax. We argue that (i) these conclusions hinge upon an assumption that language processing is necessarily hierarchical, which remains an open question, and (ii) our goal was to probe the foundational cognitive mechanisms facilitating the processing of syntactic Non-ADs—namely, the ability to recognize predictive relationships in the input.

Rawski et al. (this issue) revisit our recent findings regarding adjacent and nonadjacent dependency (Non-AD) processing in marmosets and chimpanzees (1). Specifically, they take issue with our interpretation that such experiments are able to shed light on the cognitive building blocks underpinning human syntax. The crux of their argument relies on the assumption that human syntax (and, by extension, syntactic Non-ADs) necessarily involves “recursive, nested hierarchical relations”, which are absent from the artificial grammars presented in our experiment. Without denying the general importance of investigating the processing of nested, hierarchical structures in human and nonhuman cognition, we disagree with the idea that there is only one road to uncovering the evolutionary precursors to human syntax.

Whether humans obligatorily represent and process language hierarchically remains an open, empirical question. Language can be equally analyzed nonhierarchically rather invoking more linear-based sequential mechanisms (24). Since we remain agnostic on this issue, the purpose of our study was to determine whether apes and monkeys are capable of tracking predictive relationships between nonadjacent auditory stimuli, a necessary precondition for syntax regardless of the presence of hierarchy. Processing Non-ADs in hierarchical syntax would be impossible without this basic sensitivity to sequential input (5). We do not contest that this capacity is also fundamental to the production and processing of phonological Non-ADs but disagree that our findings are more demonstrative of phonological as opposed to syntactic-pattern learning, because tracking predictive relationships in the input is necessary for both.

Examining whether nonhuman animals are capable of processing more hierarchical, recursive structures as advocated by Rawski et al. is an extremely interesting topic. However, this is a different research program to that set about by Watson et al. (1), which rather takes a more “bottom-up” approach of examining the core capacities that are likely to scaffold the “higher” faculties related to hierarchical processing. These approaches are, of course, complimentary: Specifically, we argue that establishing this core capacity of auditory Non-AD processing, which had never been demonstrated before in chimpanzees, is a logical foundation toward probing the more complex, hierarchical structures that Rawski et al. are interested in. Having now established such a capacity, we anticipate that future research will expand in the direction suggested by Rawski et al. and determine the absolute limits of dependency processing in our primate cousins—thereby, perhaps, discovering where exactly the cognitive Rubicon separating language-ready minds from those of animals lies (6).

Our study might still be several steps away from that Rubicon, and primates may even share more complex computational capacities involved in syntax than those that were tested by our study. Yet, we hold that this does not mean that the ability that we demonstrated is irrelevant as an evolutionary precursor of syntax. One might draw an analogy to a different research field to illustrate our point. In language acquisition research, it has been shown that, from early on, infants are sensitive to syntactic categories (7, 8) and relations (9, 10), yet syntactic development is ongoing until early adolescence [see Skeide and Friederici (11) for review]. More precisely, early competence in infants’ processing of structural properties of language may often be strongly based on scaffolding by acoustic properties of the items that have to be related to each other and, thus, be surface-based rather than abstract and syntactic (12). Eventually, though, relations that are based on surface properties develop into relations that are more abstract and categorical in nature in children’s development (13). It would therefore be inappropriate to completely dismiss the earlier stages of development as irrelevant to syntactic emergence on the basis that they do not encompass the same level of complexity and abstraction as later stages of development. Rather, it makes sense to see them as important steps toward syntax, as is done by various bootstrapping approaches (14). Analogously, we regard the capacity for learning Non-ADs that we observed in our primate relatives as equally relevant first steps toward the evolution of human syntax. In short, we argue that a plurality of complimentary research programs is likely to be more productive in shedding light on the evolution of syntax than a singular focus on hierarchy.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.


Acknowledgments: We are grateful to J. Scwab, A. Bosshard, M. Rüdisühli, and A. Hervais-Adelman for feedback on this manuscript. Funding: Funding was provided by the Swiss National Science Foundation (PP00P3_163850 to SWT) and NCCR Evolving Language (agreement no.51NF40_180888). Competing interests: The authors declare that they have no competing interests. Data and materials availability: There are no data associated with this paper.

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