What does a letter do?

Basic communication can be done with vowels alone.

We need to separate territory, family, and other things. Language studies have indicated that counting has been adequately covered by indicators for 'one','two', and 'many'. Vowels alone can handle that task.

So we have vowels to use within our comfort zone, vowels to use to indicate things outside of that zone, and simple counting can be done using this spatial differentiation between 'them' and 'us', between 'outside our zone' and 'inside our zone'.

Within our comfort zone, we 'slide' vowel sounds within a range to do another kind of measurement indicating rank within a family grouping.
Another set of vowels are used to measure things outside of our comfort zone, and again, there is a sliding vocal range to gauge the nature of the external phenomenon.

Therefore, it may be concluded that an adequate indicative language can be achieved by vowels alone. Modulation of tone and 'roundness' of vocalization differentiate the different languages that arise as a population learn to distinguish the 'song' inspired by any given event within or without the comfort zone.