Possible stages in the evolution of the language capacity.

Much current discussion of the evolution of language has concerned the emergence of a stage in which single vocal or gestural signals were used symbolically. Assuming the existence of such a stage, the present review decomposes the emergence of modern language into nine partially ordered steps, each of which contributes to precision and variety of expression. Bickerton's proposed 'protolanguage' falls somewhere in the middle of this succession. In addition to the by-now accepted evidence from language learning, language disorders, and ape language experiments, modern languages provide evidence of these stages of evolution through the presence of detectable 'fossils' in vocabulary and grammar.