From manual gesture to speech: a gradual transition.

There are a number of reasons to suppose that language evolved from manual gestures. We review evidence that the transition from primarily manual to primarily vocal language was a gradual process, and is best understood if it is supposed that speech itself a gestural system rather than an acoustic system, an idea captured by the motor theory of speech perception and articulatory phonology. Studies of primate premotor cortex, and, in particular, of the so-called "mirror system" suggest a double hand/mouth command system that may have evolved initially in the context of ingestion, and later formed a platform for combined manual and vocal communication. In humans, speech is typically accompanied by manual gesture, speech production itself is influenced by executing or observing hand movements, and manual actions also play an important role in the development of speech, from the babbling stage onwards. The final stage at which speech became relatively autonomous may have occurred late in hominid evolution, perhaps with a mutation of the FOXP2 gene around 100,000 years ago.