This paper presents a diachronic analysis of instrumental postpositions in Cahita languages (…Tehueco, Yaqui and Mayo; Uto-Aztecan). Based on historical data from the beginning of the 17th century (Buelna 1890) and hypotheses about the possible bridging contexts (Heine 2002) that may have favored the grammaticalization processes at the origin of the instrumental morphemes, this study proposes that the origin has to be found in the possessive noun phrase, an origin different from those proposed by Dedrick & Casad (1999) and Haugen (2008). This historical reconstruction also brings to light the connections within the domain of postpositions between instrumental and causal meanings, as well as the connections between postpositions and non-subject nominalizers, since the same marker was used to encode these different functions, exhibiting an interesting polysemy. This paper aims to explain this polyfunctionality from a diachronic perspective, trying to clarify the grammaticalization paths that may have originated such syncretism.