From a 2016 paper by University College’s Chris Knight:
Highlights
• Language emerged in only one species, H. sapiens.
• Such a system cannot evolve in a Darwinian social world. • Language emerged for reasons which no currently accepted theoretical framework can explain.
Abstract: Language evolved in no species other than humans, suggesting a deep-going obstacle to its evolution. Could it be that language simply cannot evolve in a Darwinian world? Reviewing the insights of Noam Chomsky, Amotz Zahavi and Dan Sperber, this article shows how and why each apparently depicts language’s emergence as theoretically impossible. Chomsky shuns evolutionary arguments, asserting simply that language was instantaneously installed. Zahavi argues that language entails reliance on low cost conventional signals whose evolutionary emergence would contradict basic Darwinian theory. Sperber argues that a symbolic expression is, literally, a falsehood, adding to the difficulty of explaining how – in a Darwinan world – systematic reliance on language could ever have evolved. It is concluded that language exists, but for reasons which no currently accepted theoretical paradigm can explain. (paywall), Chris Knight, Puzzles and mysteries in the origins of language, Language & Communication Volume 50, September 2016, Pages 12-21 More.
Knight is described as radical (Times Higher). Is that why he is permitted to ask: “Could it be that language simply cannot evolve in a Darwinian world?” It’s not like that is a normal question these days.
The obvious reason language evolved only in humans is that only humans have ever had anything to say that requires it. Human consciousness is symbiotic with language. But, what, specifically, are we conscious of that requires language?
See also: Linguist Daniel Everett: Homo erectus must have been able to speak, to get to Flores. Others dispute his claims, to be sure. But, you know, those hominins get smarter every time we research them.
and
Can we talk? Language as the business end of consciousness