Humans have language and other animals don’t. That’s obvious, but how it happened is not. Since Darwin’s time, scientists have puzzled over the evolution of language. They can observe the present-day product: People today have the capacity for language, whether it be spoken, signed or written. And they can infer the starting state: The communication systems of other apes suggest abilities present in our shared ancestor.

But the million-dollar question is what happened in between. How did