I'm reading "Introducing Phonetic Science" by Michael Ashby and John Maidment and they say:
Speech is the original channel for which human language evolved and all written languages have (or once had) a spoken form.
This doesn't make much sense to me. Isn't non verbal communication, like those used by other animal species, more original in the evolution of our language that pure sound speech? Is he just being lax in his words?