It is often said that speech is what distinguishes us from other animals. But are we all talk? What if language was bequeathed to us not by word of mouth, but as a hand-me-down?

The notion that language evolved not from animal cries but from manual and facial gestures--that, for most of human history, actions have spoken louder than words--has been around since Condillac. But never before has anyone developed a full-fledged theory of how, why, and with what effects language evolved from a gestural system to the spoken word. Marshaling far-flung evidence from anthropology, animal...