I am quite convinced that phosphene maps, or protoglyphs, are the First Graphical Language used in visual expression. I think there is a vocabulary, and there is a syntax. The rules that comprise the syntax have evolved through human experience and human perception of the natural environment.
Palaeolithic humans lived directly in nature, as relatively small beings in a much larger world. It is difficult for contemporary humans to understand the relationship of early humans with nature, because for such a long time now we have lived in human-made environments. But just as contemporary humans quite voraciously 'want to know' what's happening 'out there' and watch the news or talk to neighbours or friends, I think early humans also had this curiousity and shared their knowledge of the greater world with their clan. Graphical depiction is a way to share information either as an aid to a verbal description or as a standalone communication when the subject is either unable to speak for some reason, either because they need to be quiet, perhaps aren't physically present, or may be incapacitated or even disceased.