From an evolutionary point of view, why did humans develop language?

I was listening to a TED talk about language. The speaker said that we developed language because because we have different work specializations and we needed language for bartering. I am not convinced of that. Hunter-gatherers all have language and they are pretty much equal in what they do.

All groups of humans have language. There is strong evidence that we are hard-wired for it. Groups of people in isolation have been known to spontaneously create a language, complete with rules for grammar.

Is the function of language primarily social? Single individuals in isolation do not develop language. Wittgenstein stated that there is no such thing as a private language.

It has been suggested that language is inseparable from thought. Maybe language is just a byproduct of the ability to do abstract thinking.