Grammaticalization is characterized by robust directional asymmetries (e.g., Kuteva et al. 2019). For instance, body-part nominals develop into spatial adpositions, minimizers develop into negation markers, and subject pronouns become agreement markers. Changes in the opposite direction are either rare or unattested (Garrett 2012:52). Such robust cross-linguistic asymmetries have led some scholars to reify grammaticalization trajectories as universal mechanistic forces (Heath 1998:729). One consequence of such a view is that the ambient morphosyntax of a language has little or even no relevance for grammaticalization. This paper uses Bayesian phylogenetic methods to demonstrate the critical role that pre-existing morphosyntax can play in grammaticalization. The empirical basis for this claim is the grammaticalization of definite and indefinite articles in the history of Indo-European: indefinite articles developed at a faster rate among languages in which a definite article had already emerged compared to those lacking a definite article. The two changes are thus correlated. The results of this case study suggest that there is much more to be learned about when and why grammaticalization occurs by investigating its relationship to the pre-existing linguistic system (cf. Reinöhl and Himmelmann 2017:381).