This is a piece of what goes on in my head... If you think about it. Cave men are known for making weird sounds like oooh, ahhh eee. And language was created to communicate understandable sounds amongst each other. If you think about it further, those earlier sounds I mentioned are vowels O,A,E: all the vowels are A,E,I,O,U FYI. Now user, I want you to pronounce those sounds outloud to sound like a caveman A! Eee! Iii! Oooo! Uuu!. With that being said, of course only so many combos of those can work before you run out of combinations to describe certain things and thats when they started adding little sounds to those vowels which snowballed into full on words, then came refinement of those words. Example O sound "pin" became O sound "pen" which is a full on word "Open".