Kalam is a Trans New Guinea language of Papua New Guinea. Kalam has two distinct vowel types: full vowels /a e o/, which are of relatively long duration and stressed, and reduced central vowels, which are shorter and often unstressed, and occur predictably within word-internal consonant clusters and in monoconsonantal utterances. The predictable nature of the reduced vowels has led earlier researchers, e.g. Biggs (1963) and Pawley (1966), to suggest that they are a non-phonemic 'consonant release' feature, leading to lexical representations with long consonant strings and vowelless words. Here we compare Kalam to other languages with similar sound patterns and assess the implications for phonological theory in the context of Hall's (2006) typology of inserted vowels. We suggest that future work on predictable vowels should explore the extent to which clusters of properties are explained by evolutionary pathways.