In his discussion of why only humans have language, science writer Tom Siegfried gets a lot right, but he misses the crucial reason:

It’s true that humans, and humans alone, evolved the complex set of voice, hearing and brain-processing skills enabling full-scale sophisticated vocal communication. Yet animals can make complicated sounds; parrots can mimic human speech and cats can clearly convey that it’s time for a treat. Many animals possess an acute sense of hearing and are able to distinguish random noises from intentional communication. So even though only humans possess the complete linguistic package, the components of language ability “have very deep evolutionary roots,” says Fitch, of the University of Vienna. In fact, he suggests, just a handful of changes in the communication repertoire of humankind’s ancestors endowed people with the full faculty of language.

Much of the physiological apparatus for hearing and speaking is found in all land-dwelling vertebrates — the tetrapods — including mammals, birds, amphibians and reptiles. “Humans share a significant proportion of our basic machinery of hearing and vocal production with other tetrapods,” Fitch writes in the Annual Review of Linguistics.

Life-forms occupying numerous branches of the tree of life possess anatomical tools for producing and perceiving vocal communication. Where human ability exceeds our predecessors, Fitch says, is the sophistication of the brain circuitry adapted to the uniquely human capacity for complex linguistic expression.

Tom Siegfried, “Why Speech is a Human Innovation” at Real Clear Science

Siegfried is right that many non-human animals have the physiological apparatus needed to form words. Yet they have no language. They can make and respond to signs—gestures, grunts and the like. A dog, for example, can respond appropriately to simple words directed at him (“Sit!” “Fetch!”). But all animal communication is symbols, that is, signals that point directly to an object. In this case, the object is a simple expected action the animal is to perform immediately.

What animals cannot do is communicate using abstractions. They cannot use designators,—words employed abstractly as language. For example, a dog can be trained, by reward and punishment, to stay when told, “Stay!” He associates the sound “s-t-a-y” with a behavior and performs the behavior. But he doesn’t know what you mean when you say “Let’s stay a bit longer on the beach,” “He extended his stay in Peru,” or “The judge issued a stay of the eviction order.”

Animals can only think concretely. Their thought is of particulars—the particular bowl of food, thrown stick, or warm bed. They don’t contemplate nutrition, exercise, or rest. Humans can think abstractly, without any particular physical object in mind. For example, a vet might tell her client during an office visit, “Tuffy here needs to lose about 1.5 kg. I suggest a lower calorie kibble and more exercise—if possible, before bedtime.” She can explain it to her client but not to the dog because it’s all abstractions about times, places, things, and concepts. Of course, he might recognize his name, “Tuffy,” and raise his ears slightly to see if he is being told to do something concrete.

Concrete thought needs no language because the concrete thinker focuses on a perceived object. Tuffy thinks of his bowl of food. If he were to think of nutrition, an abstract concept, he would need abstract designators as objects, not only to express his thought but even to think it. In short, animals don’t have language because they don’t have abstract thought and thus have neither the capacity nor the need for abstract designators—words as language.

The difference between human language and animal symbols or signals is the difference between abstract thought and concrete thought. Language is not primarily a tool for communication. Signals (gestures, grunts and the like) work just fine for communication, and animals use them robustly. Language is a tool for abstract thinking—a necessary tool for abstraction—and humans are the only animals who think abstractly.

Siegfried again:

Nonhuman primates can learn the meaning of individual words… but aren’t capable of combining words into meaningful sequences of any substantial length. That ability also depends on circuitry connecting different parts of the brain… Understanding that circuitry depends on comparing the cellular architecture and nerve fiber tracts of the human brain with the brain of animals with lesser linguistic power.

Tom Siegfried, “Why Speech is a Human Innovation” at Real Clear Science

It isn’t brain circuitry that renders humans capable of language and animals incapable. As Aristotle pointed out two millennia ago, abstract thought is inherently an immaterial power—it is the immaterial aspect of the human soul. Animals have material souls, without an immaterial aspect. It is the nature of, and the differences between, animal and human souls that provide animals with symbolic communication and grace man with language.

Note: W. Tecumseh Fitch’s paper, The Biology and Evolution of Speech: A Comparative Analysis, is open access.

Also by Michael Egnor: How is human language different from animal signals?

and

Does brain stimulation research challenge free will? (a classical understanding of how the mind works)

Also: Can genes predict which birds can learn to talk? (Denyse O’Leary)