Although related to the more general problem of the origin of language, the evolution of distinctively human speech capacities has become a abstract the development of speech in young children and in many ways separate area of scientific research. Uncontroversially, monkeys, apes and humans, like many other animals, have evolved specialised mechanisms for producing sound for purposes of social communication. On the other hand, no monkey or ape uses its tongue for such purposes. The term modality means the chosen representational format for encoding and transmitting information.
A striking feature of language is that it is modality-independent. Should an impaired child be prevented from hearing or producing sound, its innate capacity to master a language may equally find expression in signing. Marc Hauser, Noam Chomsky and W. The Faculty of Language: What Is It, Who Has It, and How Did It Evolve? Figure 18 from Charles Darwin’s The Expression of the Emotions in Man and Animals. Caption reads “Chimpanzee disappointed and sulky. Animal communication systems routinely combine visible with audible properties and effects, but not one is modality-independent.
No vocally impaired whale, dolphin or songbird, for example, could express its song repertoire equally in visual display. Indeed, in the case of animal communication, message and modality are not capable of being disentangled. Whatever message is being conveyed stems from intrinsic properties of the signal. Modality independence should not be confused with the ordinary phenomenon of multimodality. Monkeys and apes rely on a repertoire of species-specific “gesture-calls” — emotionally expressive vocalisations inseparable from the visual displays which accompany them. The human linguistic property of “modality independence” is conceptually distinct from this.
It allows the speaker to encode the informational content of a message in a single channel, while switching between channels as necessary. Modern city-dwellers switch effortlessly between the spoken word and writing in its various forms — handwriting, typing, e-mail and so forth. Whichever modality is chosen, it can reliably transmit the full message content without external assistance of any kind. Speaking is the default modality for language in all cultures.