Building on my previous post, it could be argued that that the recent statistical analyses of Pictish Symbol Stones, shows that literacy comes from the need to record events, and that by implication proto literacy precedes complex list making.
It doesn't. It shows that a society developed a controlled vocabulary of symbols that conveyed certain meanings, just like the signs for the exit, trains and buses at airports. Yes, they contain meaning, but unless you know the conventions they are meaningless and contextless. Roadsigns are another example of a set of universally understood conventional symbols.
List making literacy happened when someone had the bright idea of combining tally counts with these symbols so that instead of knowing there were 'cattle' in fact there were 8 cows, and then from this one can add modifiers so we have '7 female dun cows', '1 male black cow' etc.
And literacy really happens when one person can read the list and know what it refers to without having to know other contextual information.
Of course somewhere in the middle it's a halfway house, like my previous flashcard analogy, but literacy happens when we can say more than 'men fought here, cattle were taken', ie when we can say how many people fought, who died and how many cattle were taken.
Literacy adds detail and detail enables the development of narrative ...