For his part, Klinghoffer seems underwhelmed by the evidence. He suggests that Rocky's performance falls short of that of a family dog whom his mother claimed could "sing" the song "I'm Forever Blowing Bubbles." He also compares Rocky unfavorably with Clover, supposedly the world's best talking parrot. Indeed, parrots and other birds have demonstrated the clearest ability to imitate the sounds of human language among all animals. The problem, from an evolutionary perspective, is that birds are not as closely related to humans as are the great apes.
I think Klinghoffer makes some valid points about the significance of Rocky's performance. I would add, however, that regardless of whether Rocky can imitate the sounds of human language, that does not get us any closer to explaining the origin of human language itself. As the famed linguist Noam Chomsky and his colleagues point out in a 2014 article (see here) on the evolution of human language:
[It is a] misconception is that language is coextensive with speech and that the evolution of vocalization or auditory-vocal learning can therefore inform us about the evolution of language... [Speech] and speech perception, while functioning as possible external interfaces for the language system, are not identical to it. An alternative externalization of language is in the visual domain, as sign language; even haptic externalization by touch seems possible in deaf and blind individuals. Thus, while the evolution of auditory-vocal learning may be relevant for the evolution of speech, it is not for the language faculty per se.
According to Chomsky and his colleagues, language is "a computational cognitive mechanism that has hierarchical syntactic structure at its core." Speech is merely the externalized expression of this "cognitive mechanism." In fact, it could be argued that speech is not essential to language: after all, as Chomsky et al. point out in the quotation above, deaf and blind individuals are able to externalize language without using speech. Thus, even if it could be proven that the (supposed) common ancestors of humans and modern primates had an ability to produce the sounds used in speech, we would still need to explain how the "cognitive mechanism" (language) itself evolved and how it became connected to those particular sounds.
In short, Rocky's performance--even if as impressive as claimed--does not really get us any closer to understanding how humans acquired the sophisticated cognitive ability that language is. This is the great challenge faced by those who attempt to explain the origin of language in evolutionary terms.
Image of orangutan from commons.wikimedia.org