There are a large number of people that have no clue how to write but are perfectly fluent in a language.
I think that you can say that letters/writing is an artifact to capture either syllables (most alphabet based languages and Hangul) or word sounds (pictograph based languages).
By the same token, the stream of syllables to create the elaborate alert, mating, and dominance calls (the basis of language) that animals use for signaling is an artifact of our biological sound production hardware.
If you are looking to create biologically based systems and are trying to decide on the correct level to match up with the human neural hardware I would offer that our human semantic grounding in the speaking-hearing axis along the arcuate fasciculus pathway may be the best place to start. (Syllables)
Hangul: possibly the best match between spoken syllables and writing?