The origin of human language is a big problem, since it is far in advance over what our closest relatives have, and because it does not produce easily fossilized evidence. We have an adaptation for producing language sounds, the top of our windpipe being well below the back of the mouth, In our closest relatives, it extends up into the back of the mouth, and in human babies, it sticks up that high, then descends. We pay a price for increasing the variety of sounds that we can make: we are vulnerable to choking.
Teaching human language members of other species has had the most success in chimpanzees, our closest relatives, but that success has been *very* limited. Chimps can't make many of the speech sounds that we can make, and the most success has been had with sign language. Chimps can learn lots of individual signs, but their ability to string those signs together is *very* limited - mostly two-sign phrases, like "drink fruit" for watermelon.
I've found an interesting theory:
That human speech emerged from singing, and that singing was originally much like birdsong, without much semantic content.
That gets around the problem of how several things can evolve together -- only a few at a time need to evolve. Birdsong requires vocal control and sequencing to generate, and it is often learned. So it's been studied in the hope of providing clues about the origin of human language.
But singing birds are *very* far from humanity in evolutionary terms, and convergent evolution often has lots of differences in detail.
Scientists Sequence Genomes of 48 Bird Species, Unveil Avian Family Tree | Genetics | Sci-News.com
and
Ornithologists Publish Most Comprehensive Avian Tree of Life | Sci-News.com
The singing birds are a subgroup of Passeriformes, the taxonomic order of sparrow-like birds. These birds' closest relatives are the Psittacoformes, the taxonomic order of parrot species, and parrots are well-known for their ability to make imitations of sound sequences. Some passerines also have a parrot-like ability to imitate sounds, and that's how mockingbirds got their name.
But the most recent human - parrot common ancestor was an early reptile in the Late Carboniferous, a little over 300 million years ago, and neither chimps nor most other birds share their vocal abilities.