Why has language only evolved once?

I'm reading a book by Terrence W. Deacon called THE SYMBOLIC SPECIES: The Co-evolution of Language and the Brain that I pulled off the philosophy shelf at Barnes & Noble and then borrowed from the library, and I'm loving it. The author is unknown to me (which doesn't tell you anything about how well-regarded he might be; but a book that's on store shelves a decade after it was published is sure doing better than my book), I don't have the background to evaluate whether he's doing right by the information he's drawing on, and I don't know the linguistics discourse he's responding to so am only provisionally assuming he's right in his presentation of other people's ideas.

What I like is that he asks questions, and they seem like good questions, and it seems to make sense that he's asking them. Here's something that needs to be explained, he's saying, and he's saying that he doesn't like the explanations given so far, so he's going to come up with his own.

Deacon was talking to an elementary school class, and an eight-year-old asked him, "But don't other animals have their own languages?"

Deacon responded by explaining how human language is different from any other form of communication in its precision and its capacity to combine elements logically but with open-ended possibilities.

"Do animals just have SIMPLE languages?" the kid continued.

Deacon said that though animals have calls and such they have nothing like words, nouns, verbs, or sentences.

Another child asked, "Why not?"

And this stumped Deacon, who had no idea how to answer. So he poses a question in his preface: "Why are there no simple languages with simple forms of nouns, verbs, and sentences?"

( Why has language only evolved once? )