The merger of PAN *S1 and *S2 is an innovation of eastern Walu-Siwaish
Working in the tradition of Dyen (1971), Tsuchida (1976:250, 308) distinguished two major PAN sibilant phonemes, *S1 and *S2. Examples of the former are many; Tsuchida (1976:250 and index) examplified the latter with seven lexical items:
*kaS2iw ‘tree, wood’,
*CuS2uR ‘to thread’,
*Cuma[S2H1] ‘body louse’,
*guS2am ‘skin disease’.
The ending in *Cuma[S2H1] is ambiguous with *H1 if the evidence is limited to Saisiyat and Atayalic (as in Tsuchida 1976:202), but when Kavalan tumes and Amis tomes are included, *H1 is excluded. More examples can be added. By my count at least 15 reconstructable Austronesian words include S2.
See this table.
15 words is not a negligible number. S2 occurs in all positions: initial, medial, final.
The distinction between *S1 and *S2 is consistently observed in Saisiyat (S ≠ h), Pazeh (s ≠ h), Atayal (s ≠ h), Seediq (s ≠ h), Thao (ʃ ≠ 0), Siraya (x ≠ 0), Bunun (s ≠ 0), Tsou (s ≠ 0), Kanakanabu (s ≠ 0) and Rukai (Maga dialect: s ≠ 0; Mantauran dialect ʔ ≠ 0). It is lost in Saaroa (0 = 0), Kavalan (s = s), Amis (s = s), Paiwan (s = s), Puyuma (0 = 0), PMP (0 = 0), Proto-Kra (word-medially: s = s), Proto-Hlai (word-finally: -t = -t), Proto-Tai (word-medially: s = s). Th evidence for the merger in Kra-Dai will be detailed below.
Treatment of *S1 and *S2 in extinct Taokas, Favorlang/Babuza, Hoanya, Papora, Basay and Trobiawan cannot be determined due to the fragmentary character of the evidence.
Geographically the languages that merge *S1 and *S2 are spoken along the eastern and southern coasts of Taiwan. Those that keep them distinct are found on the west coast and in the central regions. Saaroa, a central Taiwan language that merges *S1 and *S2 is special: its distance from the eastern coast, and its affiliation with Tsouic argue strongly that the same merger took place independently on the east coast and in Saaroa.
I now present the evidence for the merger of *S1 and *S2 in the Kra-Dai branches. The evidence is as limited as the examples of *S2 are. To my knowledge, only *CumeS2 ‘louse’, *kaS2iw ‘tree, wood’, and *qaluS2ip-an ‘centipede’ are reflected in Kra-Dai.
In the Kra branch, Proto-Kra *sui A ‘firewood’ (Ostapirat 2000) is the most probable reflex of *kaS2iw ‘tree, wood’—several Formosan languages reflect *kaS2uy instead of *kaS2iw. This shows that the PK reflex of *S2 in word-medial position is *s-. *s- is also the reflex of *S1 in the same position: PAN *duS1a ‘two’ = PK *sa A.
In the Hlai branch (Norquest 2015) we have *CumeS2 ‘louse’ reflected by Proto-Hlai *hməːt ‘flea’ and ‘to winnow’ *tapeS1 by *wet, showing the merged reflex of *S1 and *S2 is -t in word-final position.
In the Tai branch, Proto-Tai *q.sip D (Pittayaporn 2009) ‘centipede’ corresponds to PAN *qaluS2ip-an ‘centipede’ (I take final *-an to be suffixal, compare *waNiS-an ‘boar’, *RiNaS-an ‘pheasant). PAN medial *S1 also goes to s- in Proto-Tai: PAN *qaS1aN ‘grain before husking’, PT *sal A ‘husked rice’ (Pittayaporn 2009).
Dyen, I. (1971) The Austronesian languages and Proto-Austronesian. In T. Sebeok (ed.) Current Trends in Linguistics, Vol. 8:5-54. The Hague and Paris: Mouton.
Norquest, Peter. 2015. A Phonological Reconstruction of Proto-Hlai. Brill.
Ostapirat, Weera. 2000. Proto-Kra. Linguistics of the Tibeto-Burman Area 23.1:1-251.
Pittayaporn, Pittayawat (2009) The phonology of proto-Tai. Unpublished Cornell U. dissertation.
Tsuchida, S. (1976) Reconstruction of Proto-Tsouic phonology. Tokyo: Study of languages and cultures of Asia and Africa monograph series N° 5.