AbstractChamacoco is a Zamucoan language of northern Paraguay that has considerably restructured its person reference system. Starting from the existing reconstruction of Proto-Zamucoan, I will analyze the evolution of person marking in free pronouns, verbs and possessable nouns. The verb lost the realis/irrealis distinction in speech act participants, while the third person underwent some allomorphic changes and introduced a distinction between third singular and plural, dependent on an innovative animacy hierarchy. The first person proved overall particularly unstable. In possessable nouns, it was replaced by a form for unspecified possessor, while a new exponent was created for the latter. In free pronouns, the first plural shifted to the first singular and then grammaticalized to the new verb prefix for the first singular. The most significant changes concern the introduction of clusivity in verbs and free pronouns, which was combined with an unusual number term: the greater plural. Besides, the verbal first-person exclusive is typologically unexpected, since it derives from the inclusive. I will discuss the reasons for these and other minor changes, which involve internal factors and language contact. Finally, I will show how recent contact with Spanish has affected the Chamacoco person system.