A few observations to throw into the pot.
Homo Habilis had language of some kind, as evidenced by the existance of Wernike's Area and Broca's Area in endocasts.
Koko the gorilla has evidenced a very broad vocabulary and a very high capacity for understanding. However, she is unable to master more than basic grammar (2-3 word constructions of the noun-verb/noun-verb-noun variety). In a blind assessment she scored about the same level as a 3-4 year old human child. Her emotional and abstract capacity is quite high.
Koko is not the only pongid that has given such results. There is no reason to deny that at least the great apes (with which we share a common ancestor) do not have incipient language capability. Watch koko in action. She initiates conversations, responds appropriately, plays with words, etc. She is using language. Is it human level? No, but it's damn close.
™ Version 4.2
