Writing and reading had to have developed simultaneously, otherwise the person writing was just drawing. To be writing you have to be able to interpret what you are writing.

Its a chicken and egg situation. The chicken has to come from the egg but the egg has to come from the chicken. In reality, both the chicken and the egg had to come from something that was neither recognizable as a chicken nor an egg. The proto chicken if you will. In the case of writing that would probably be using symbols to indicate numbers of things. Or hieroglyphs. Pictures that are representative of ideas like words, even though they are not words themselves