[1][2][3][4] Proto-Philippine is not directly attested to in any written work, but linguistic reconstruction by the comparative method has found regular similarities among languages that cannot be explained by coincidence or word-borrowing.
While it was later found decades after (Himmelmann, 1990) that there are shared innovations between Philippine and Tomini languages, there are still uncertainties as to whether the latter do validly form one genetic group, or should be relegated as a mere geographic unit.
Lastly, there have been several proposals establishing southern Philippines as the boundary (Thomas & Healey, 1962; Dyen, 1965; Zorc, 1977; 1986) with the "Macro Meso-Philippine" and "Sangiric" as two primary branches.
[3] Due to issues in the validity of a Philippine genetic group, and thus the existence of an ancestral Proto-Philippine language, most of its features particularly its phonology remain as proposals.
[1][4] Proto-Philippine's schwa *ə often merged with other vowels (e.g. /u/ in Cebuano, Hiligaynon, Waray; /a/ in Ibanag, /i/ in Tagalog), but is retained in a diverse range of Philippine languages (e.g. Gaddang, Kinaray-a, Maranao, Maguindanao, Rinconada Bikol, Palawano), and in southern dialects of Ilokano.