At first, these efforts to teach chimpanzees and apes to "talk" seemed to be bearing results. As philosopher David Berlinski notes in a recent blogpost at evolutionnews.org:
...For a time in the 1970s, a number of biologists were [sic] actually convinced that they had taught chimpanzees and great apes to talk; many of them reported long conversations, chiefly about bananas...that they held with their charges. Their research was no sooner published than it was accepted and believed, largely, I think, because a crude Darwinian theory--there is no other--made it difficult to imagine that profound and ineradicable differences exist between human beings and the rest of the animal world. Penny Peterson at Stanford, Herbert Terrace at MIT, and David Premack at the University of Pennsylvania all convinced themselves that somehow the great apes had sat in stony silence throughout the vast reaches of biological time only because they lacked human conversational companionship.
However, this apparent success in teaching apes and chimpanzees to communicate using human language was ultimately exposed as an illusion, as Berlinski points out:
The inevitable, skeptical reaction soon set in. Videotapes taken of chimpanzees revealed, when carefully analyzed, that what had passed for chimpanzee conversation was nothing more than prompted signings in the best of cases--a record of the beast's pathetic endeavor to say whatever it was that his trainer wished him to say; in the worst of cases, the beast simple babbled...his signs utterly devoid of meaning. Herbert Terrace, who had wasted years in browbeating the poor creatures, examined videotapes of his own encounters with his animals and came away shaken.
Perhaps these researchers would not have been taken in by the animals' behavior had they realized that their "long conversations, chiefly about bananas" with "their charges" had not really been actual conversations about bananas! What I mean here is explained by philosopher J.P. Moreland:
...the question of animal language cannot be adequately discussed without drawing a distinction between a sign and a symbol. A sign is a sense-perceptible object, usually shaped like the characters "BANANA" or a sound (the utterance of "BANANA"). Now if an animal...comes to experience repeatedly the simultaneous presence of a sign (the visual presentation of BANANA) and the presence of a real banana, a habitual association will be set up such that the animal will anticipate the sense perception of a real banana shortly after seeing this shape: BANANA. In the case of the animal, BANANA does not represent or mean a banana, so it is not a symbol. Rather, BANANA is merely a certain geometrically perceived shape that comes to be associated with a banana in such a way that the latter is anticipated when the former is observed.
By contrast, real language requires symbols and not mere signs. When language users use the word banana, it is used to represent, mean, and refer to actual bananas. Now the evidence suggests that animals have certain abilities to manipulate and behaviorally respond to signs, but it is far from clear that they have a concept of symbols. One reason for this claim is the lack in animals of grammatical creativity and logical thought about language itself that is present in real language users [i.e., humans]. (The Soul: How We Know It's Real and Why It Matters, p. 144).
In other words, in their "long conversations, chiefly about bananas," the human researchers were using "banana" as a symbol, while their animal subjects perceived it as being a mere sign. Consequently, the huamsn and primates were not having a real conversation about bananas because they lacked a shared understanding of "banana."
However, not all researchers have given up on the effort to teach animals to use language. Consequently, according to Berlinski, "Ever credulous, scientists now report that they have engaged the dolphin in stimulating conversation. Next year, no doubt, it will be the turkey."
Berlinski ends his commentary thus: "Seventeenth-century Jesuits wondered why dogs do not talk. Their conclusion bears repeating. They have nothing to say." The reason dogs--and other animals--"have nothing to say" is that they are incapable of language! (As much as we may wish otherwise.) Language is a unique gift of the Creator to humanity.
Image of chimpanzee from Wikimedia Commons