1. Typological analysis of known human languages has convincingly
demonstrated that human language minimally requires three vowels.
Human language minimally requires two vowels. A good brief
cross-linguistic overview is:
Maddieson, Ian. 2005. “Vowel Quality Inventories.” Pages 14-17 in The
World Atlas of Language Structures. Edited by Martin Haspelmath, Matthew
S. Dryer, David Gil, and Bernard Comrie. Oxford: Oxford University Press.
Interestingly, languages with small vowel inventories (ie 2-4 vowels)
cluster in the Americas and Australia.