Did spoken language evolve from sign language?
Some writers have suggested that the earliest human language was sign language. Evidence they put forward in support includes “iconicity” in sign languages as opposed to spoken languages. That is to say, in brief, that there is a similarity between the form of a sign and its meaning. There is also the observation that new sign languages can be created in ways that spoken languages cannot.
However the differences between sign and spoken languages in modern humans cannot be used as evidence for evolution of human language, since all modern humans have “language-ready” brains, and there are no “ancestral” sign languages older than spoken languages. More relevant for the evolution of language is the observation that all human communication is multi-modal. That is to say speakers always use vocal and manual gestures when speaking, and signers always use facial and body gestures when signing.