Mapping the body to the discourse hierarchy in sign language emergence

A common feature of discourse coherence is hierarchical organization: more generally, central relations (characterizing the overall topic or goal) dominate complementary or modifying relations. In this hierarchy, higher levels tend to be marked by stronger prosodic cues than lower levels. This study seeks to understand how such a system emerges in human communication - what is present at the outset, and what takes time to develop. Specifically, we investigate whether the conceptualization of hierarchical organization precedes overt linguistic structuring, and whether distinct types and strengths of prosodic marking at different hierarchical levels can be discerned in the process of emergence. The only empirical evidence for such an investigation comes from sign languages, because they can arise de novo at any time. Sign languages offer the additional advantage of directly linking instantiations of linguistic structure to articulations of different visually perceived bodily articulators. Our study of a young sign language, Israeli Sign Language (ISL), finds that conceptual hierarchical structuring of discourse arises very early. However, the organization of bodily articulators to linguistically mark hierarchical information takes time to emerge: Younger ISL signers use smaller, less salient articulators at lower levels of the hierarchy compared to older signers.