Emergence and evolution of free variation in Central Pame prefixes

Many word forms from different classes in Central Pame (cent2145, Otomanguean) allow two synonymous forms, one containing a prefix with the vowel /a/ and another one with /u/ (e.g. wa-ttsáuʔ ~ wu-ttsáuʔ ‘(s)he feels’). I conduct historical corpus research and elicitation to throw light on the diachronic origin and contemporary profile of this unusual phenomenon. Evidence suggests that it started as a sound change /a/ > /u/ between bilabial consonants. However, paradigmatic pressures have largely dismantled the original distribution of allomorphy synchronically, generalizing free variation (i.e. overabundance). In addition, other phonological and morphosyntactic cues have emerged for the probabilistic prediction of these allomorphies. The case provides an extraordinary window into the cognitive underpinnings of sound change, allomorphy, and the paradigm in a highly-inflecting and understudied language.