From Sequence to Hierarchy:
Why Achieving Combinatoriality is Harder than We Think
W. Tecumseh Fitch
University of Vienna
The ability of nonhuman animals to structure both their actions and perception sequentially is well documented, and sequencing appears to rely on similar mechanisms in human phonology. However, syntax requires, in addition, hierarchical structure which appears to rely on different neural mechanisms that are more highly developed in humans than other species. But, although phonology is hypothesized to require computations limited to the finite state (regular) level, phonology too has an apparent hierarchical structure, with phonological phrases containing words that contain syllables that contain segments made of distinctive features. Furthermore, recent data from both birdsong and human babbling suggest that deriving free combinatoriality from the sequences that make up such vocalizations is far from trivial: an unexpected limitation in the flexibility of learned, complex signals. I will suggest that the fixed-depth hierarchical structure observed in phonology represents a middle term in the evolution of language, which is both powerful (granting free combinatoriality) and a crucial step towards the arbitrary depth hierarchicality seen in phrasal syntax, ending with some speculations about the brain basis for this ability and how it could evolve from sequential precursors.
