This special issue of JHL reconstructs the diachrony of a number of innovations in the coding of argument structure, particularly in the domain of verbal indexation, in four Amazonian language families (Chapacuran, Sáliban, Tukanoan and Tupi). It is one result of an international workshop on “Diachronic Morphosyntax in South American Languages” held in Lyon (France) in 2015, with financial support from the Collegium de Lyon (Institute for Advanced Study) and the LabEx ASLAN of the Université de Lyon. The goal was to encourage methodologically innovative (and more rigorous) historical studies of morphosyntactic patterns in languages or language families of South America. The five papers that comprise this collection all demonstrate the viability of syntactic reconstruction, even in languages with little or no written history.