Explanation:
I don’t think that animals like Koko should be considered ‘proof’ of evolution at all. It is certainly consistent with evolution, but is not really proof. There are multiple possibilities as to why gorillas and humans can learn a gensture language.
For instance, Koko and his relatives could be merely descendents of Adam, just like ‘humans’. Maybe the Great Designer separately designed humans and gorillas to use gesture language. Maybe those organic molecules that Fred Hoyle postulated infected both human and gorilla zygotes separately.
Animals like Koko are more like a disproof of theological ideas that humans are qualitatively different from other. The evidence that dogs can understand audible words is perhaps even more evidence that mammals have a common ancestor.
Some of the new work correlating communication skills with parts of the brain seem to me convincing EVIDENCE (not proof) that animals and humans have a common ancestor.
I also think that the similarity in emotional expressions seen in all mammals is evidence for a common ancestor. Darwin correlated different emotional expressions in ‘The Expression of Emotion in Man and Animals’. If Koko’s gesture language is added to the vast amount of data presented in ‘Expressions’, then maybe it could be considered additional evidence.
Humans baring canines that they don’t really have is evidence of common ancestry with apes. Goose bumps in human skin corresponding to absent hair is also evidence for common ancestry. In the context of this data, maybe a gesture language that the gorilla does not use is evidence for a common ancestry. However, calling all this ‘proof’ is overselling the theory.
If you put all this evidence together, then one gets a strong case for evolution. However, one should avoid overselling even the best argument
