Chitimacha is a language isolate formerly spoken in southern Louisiana, and is a part of the Southeast linguistic area. Using documentary materials recorded by Morris Swadesh in the 1930s, this talk examines the language-internal evidence for the diachrony of three features of Chitimacha grammar: positional auxiliary verbs, switch-reference, and agent-patient alignment. Each feature is shown to ... [Show full abstract] have a clear, language-internal diachronic pathway, wherein existing lexical and grammatical material were recruited for new functions. However, each of these features is shared by other unrelated languages of the Southeast, suggesting that they were in fact motivated by contact. How then did Chitimacha borrow these structural features without borrowing any lexical or grammatical material? The answer, I suggest, is that multilingual speakers in the Southeast carried over discourse-level patterns of managing information flow from other languages, and that as these discourse patterns became more frequent and routinized, they fundamentally reshaped the structure of Chitimacha grammar.