This paper discusses the diachrony of inalienable possessive constructions in East Bird's Head, a small Papuan family of the Bird's Head peninsula in northwest New Guinea. In particular, it focuses on a phenomenon known as Split Inalienable Coding, in which a language has two or more possessive coding strategies closely or exclusively associated with the expression of inalienable possession. Based on the available data, Split Inalienable Coding can be reconstructed to the proto-Meax branch of East Bird's Head, but not to proto-East Bird's Head itself. It is argued that Split Inalienable Coding was innovated in pre-proto-Meax, and had begun to erode in proto-Meax; after the divergence of the Meax branch, further changes in the daughter languages have obscured the original system of Split Inalienable Coding. As Split Inalienable Coding is found in other neighboring yet unrelated languages, the role of contact in the development of Split Inalienable Coding in pre-proto-Meax is also discussed. From the present-day distribution, it is inferred that Split Inalienable Coding first developed in an Austronesian/Papuan contact zone in the east of the Bird's Head, before spreading to other nearby Austronesian languages.