This paper shows that the verbal morphosyntax of Siriono, which is synchronically highly divergent from that of other Tupi-Guaranian (TG) languages, can be derived from the reconstructed proto-TG (PTG) system. Arguments will be presented to show that the S_A=A series of person markers (e.g. PTG 1sg *a-) has merged with the S_o=O series (e.g. PTG 1sg *če-) in the 1st and 2nd plural persons in Siriono. In spite of this partial merger of two series of person markers, morphological elements that appeared in PTG between the personal prefix and the stem of transitive verbs have been retained in Siriono with an identical distribution. The partial merger of the S_A=A series with the S_o=O series is explained by a combination of phonological and syntactic motivations. Additional evidence is drawn from Siriono's closely related sister language Yuki. Apart from this, the prefix k- that occurs on third-person forms of 'comitative-causative' verbs in Siriono has retained a trace of the PTG third-person prefix *o- in spite of the emergence of an innovated third-person prefix e-. These facts are taken as evidence of a PTG origin of the Siriono system.