Terrence Deacon and Ursula Goodenough have written a great article on the evolution of symbolic language. I’m mentioning it because they make two particularly interesting points. First point:
Language is in effect an emergent function, not some prior function that just required fine-tuning. Our inherited (“instinctive”) vocalizations, such as laughter, shrieks of fright, and cries of anguish, are under localized, mostly subcortical, neurological control, as are analogous instinctive vocalizations in other animals. By contrast, language depends on a widely dispersed constellation of cortical systems. Each system is also found in other primate brains, where they engage in other functions; their collective recruitment for language was apparently driven by the fact that their previously evolved functions overlapped with particular processing demands necessitated by language. Old structures came to perform unprecedented new tricks.
Using their own interpretations of previous research into birdsong, they also claim a relaxation of selection pressures may have played a role in the emergence of human language:
This reduction of emotional and contextual constraint on sound production opens the door for numerous other influences to play a role, allowing many more brain systems to participate in vocal behavior, including socially acquired auditory experience. In fact, such freedom from constraint is an essential precondition for being able to correlate learned vocal behaviors with the wide diversity of objects, events, properties, and relationships that language is capable of referring to. Hence an evolutionary de-differentiation process, while clearly not the whole story, may be a part of the story for symbolic language evolution.