Ontogeny and phylogeny of language

Abstract

How did language evolve? A popular approach points to the similarities between the ontogeny and phylogeny of language. Young children's language and nonhuman primates' signing both appear formulaic with limited syntactic combinations, thereby suggesting a degree of continuity in their cognitive abilities. To evaluate the validity of this approach, as well as to develop a quantitative benchmark to assess children's language development, I propose a formal analysis that characterizes the statistical profile of grammatical rules. I show that very young children's language is consistent with a productive grammar rather than memorization of specific word combinations from caregivers' speech. Furthermore, I provide a statistically rigorous demonstration that the sign use of Nim Chimpsky, the chimpanzee who was taught American Sign Language, does not show the expected productivity of a rule-based grammar. Implications for theories of language acquisition and evolution are discussed.

Fig. 1.

Syntactic diversity in human language (A), memory-based learning model (B), and Nim Chimpsky (C) (details of the data are provided in SI Text). The diagonal line denotes identity; close clustering around it indicates strong agreement. For humans and Nim, the model predictions are made on the assumption that category combinations are independent. For the memory-based learner, the model prediction is based on frequency-dependent storage and retrieval. Only human data are consistent with a productive grammar (ρ_c = 0.977). Both the memory-based learning model (P < 0.002) and Nim (P < 0.004) show significantly lower diversity than expected under a grammar.