IIRC, the oldest Sumerian tablets (around 3300 B.C. - older than the Egyptian hieroglyphs which began around 3100-3000) are in an unknown language, but they are lists, with quantities, of repetitive characters, which sounds an awful lot like an inventory, which means . . . taxes!
IIRC, Manes' stela is ca. 3200 BC, but I might be wrong.