> Other species are intelligent, but none of them have our innate ability to communicate using complex language.

That isn't proven. There is plenty of evidence, that "complex" is relative and humans have limited sensory organs, compared to other species. Naturally our vocalizations have had to diversify.

It's important to remember that the vast majority of all interspecies communication is non-vocal (this includes non-written). The ability to write is unique to humans, as far as I know. There are more, but they aren't related to vocalization (or communication at large).