Is there a language known to have no minimal pairs separating vowels, or in which only one vowel exists phonemically in the language, or whose speakers don't detect a difference between any two vowels when they appear in words in the language? (My knowledge of linguistics is poor enough that I'm not sure whether those three criteria are identical.)
There's a famous 1960 book by Aert Kuipers called Phoneme and Morpheme in Kabardian (Eastern Adyghe), in which he argues that there is only one phonemic vowel in Kabardian, because it has so many distinctive phonemic consonants that their features determine the features of adjoining vowels. This produces a fairly normal profile of phonetic vowel variation ("normal", at least, for a NW Caucasian language), but only requires that the positions of the vowels be marked phonemically, since all their phonetic features are predictable from their position among surrounding consonants.
This is an extreme case, and there was a big debate on the subject; the final outcome seems to be that there are now three phonemic vowels recognized in Kabardian, sort of like PIE laryngeals. But there are still some afterthoughts.
Ergo this entry, from Jim McCawley's Days in the Month of May that are of Interest to Linguists:
- May 13. Vowel Day. (Public holiday in Kabardian Autonomous Region). The ceremonial vowel is pronounced by all Kabardians as a symbol of brotherhood with all speakers of human languages.