The only natural languages we know of are human. In addition to such human languages as English, Spanish, Russian, and Chinese, with which we are all familiar, there are many less well-known languages, many of them spoken by hundreds of people. The more marginalized languages are dying out at an alarming rate. Owing to lack of evidence, our information about their origin is limited, but it seems likely that they evolved out of communication systems similar to those used by animals for communication. Living human languages are learned as first languages by infants and are used for face-to-face communication and many other purposes.

Natural languages are influenced by a mixture of unconscious evolutionary factors and conscious innovation and policy making. In most cases, the historical record does not allow us to tell what role these factors played in the development of a...