Heh, that's a fun point. Maybe even a deep point. They don't have to leave a long trail of artifacts of incremental groping toward the concept of written language, starting with seal icons and tally marks and then account ledgers and then complaint letters. Instead, somebody could just have the idea, all of a sudden, if conditions are right to suggest it. Or several people could. But this raises the question of how big an idea it's possible for one person to have all at once, without handing it between multiple people in evolutionary stages. I guess there's no real limit on that, it's just that excellent ideas require excellent zeitgeist conditions (like the availability of paper that you mention).