title_body: What reasons would there be for not having a human conlang with only vowels? All natlangs have both consonants and vowels, but it's not immediately obvious why a human language couldn't be made from only vowels.
  
  Has anyone theorised about why natlangs always have consonants?
  
  Have there been any attempts to create conlangs with only vowels, and were they considerably harder to learn than languages with both vowels and consonants?
upvoted_answer: A vowel-only language is surely constructable (and I think, learnable, too), but I am afraid that it will be instable against evolutionary pressure.
  
  Vowel sequences like /aua/ or /aia/ tend to develop into glides /awa/ and /aja/ giving raise to the first consonants in the language, and at the hiat between two vowel a third consonant, the glottal stop, may materialise.
  
  Efficiency of the coding is also a concern, maybe tones can take over the role of consonants in creating more different syllables from the vowels.