The Evolution of Hierarchical Structure in Language

0. Introduction Pattee (1973) has argued that all problems of biology are ultimately problems of hierarchical organization. Much the same claim can be made for problems of language, where hierarchical organization is central to grammar. We propose that the scaffolding for hierarchical structure in human language is physiologically based and exapted from an internal mapping of the vocal tract. Following Cruse (2003), we assume that the reorganization of a strictly reactive system into a cognitive system (which can characterize language evolution) often requires an internal mapping of the system body. Thus, an internal map of the vocal tract was created to fine-tune motor control of articulators like the lips, tongue and larynx; the hierarchical structures in that map were then exapted elsewhere in grammar. It has been argued that much of syntax and higher-order grammatical structure was exapted from the structure of the syllable (Carstairs-McCarthy 1999). This is a desirable approach since it relates various parts of human language through a shared structure, but it leaves unanswered where the syllable itself evolved from. We propose that there are two crucial parts to the syllable, the embedding and the headedness, and that each had a different evolutionary source.