This paper applies the comparative method to two related languages of the southen Andaman Islands, Jarawa and Onge, leading to the reconstruction of a proto-language termed "Proto-Ongan" (PON). The same method is used to argue that Proto-Ongan may be related to Proto-Austronesian (PAN). Lexical and grammatical evidence suggests that Proto-Ongan and Proto-Austronesian are sisters, daughters of a Proto-Austronesian-Ongan (PAO). The implications of this discovery are wide-ranging, from potential solutions to problems in PAN grammar, to new hypotheses regarding ancient speaker migrations. While few of these implications are examined here, an extended Austronesian phylogeny is proposed in the hope that it will seed new avenues of research, and highlight the potential importance of Andamanese studies in understanding Austronesian prehistory.