The interrelationship of, speaking, reading and writing is based on a system of codependence. Each one relates to the other, and expands upon the previous. The most basic would have to have been speech. Since, by default we all make some sort of sound. Whether it be nasal or by the mouth. This probably led to the idea of writing or self expression, of writing.
Reading would have to come last, because it would be necessarily to teach what each word or symbol means. Also comprehension would play its own role by giving the person ability to speak. With this in mind when language grows or speech. They would each affect each other hence creating dependence. Without one or the other things would not make sense.