How about a wild thought following up on my last post about the integration hypothesis of language’s origin that posits two ‘layers’ combined to produce human language, the expressive layer exemplified in bird song interacting with the lexical layer exemplified by the silver gibbon’s songs (as well as communication about the world by other mammals). If you understand these two layers as the result of structures changing in the evolutionary stream due to genetic shift, you can conjecture that the hemispheric development so different in birds and mammals reflects these two streams or layers. So how could they merge into one confluent stream?
Their embryos develop quite differently. Bird embryos first form as masculine and then some are feminized by hormones. Their songs, mostly by males, function in mating and territorial behaviors. Mammalian embryos form as female and then some are masculinized by hormones. Their calls function in broader social behaviors, expressing emotions, organizing the group, and having some referential properties, e.g., specific calls for specific dangers. On the face of it, beginning with a female brain looks to have more advantages.
Mammalian brains show more lateralization. This is in part because testosterone, the masculinizing hormone, slows the development of the left hemisphere (which plays a role into why males seem to develop more slowly and have more language disabilities). Mammalian brains, being at base female, show an essential proclivity for social interaction, especially through the empathic flow of relationships. Then, as the left side comes on line later, so to speak, the right side has developed its capabilities for the current moment and interaction leaving the left to process information displaced in time and space along with its inherent tendency to communicate. And without current referents evident in the ambient, lexical organization becomes very important. So, maybe here are the two streams, two roots of our linguistic abilities, developed and joined as sexual dimorphism effected embryological development. Just supposing.