Why Does Man Have Language?

I've just begun reading Tom Wolfe's new book The Kingdom of Speech. In his first chapter he raises a very important question for which there is a very important answer.

Wolfe:

Speech is not one of man's several unique attributes -- speech is the attribute of all attributes!... Darwin had [a big] problem: a huge gap in evidence when he came to language, which set humans far apart from any animal ancestors. It gnawed at him. He could explain man's opposable thumb, upright stature, and huge cra...