In The Atlantic today, a summary of a new article in Science Advances this week about speech evolution:
https://www.theatlantic.com/science/archive/2019/12/when-did-ancient-humans-start-speak/603484/?utm_source=feed
https://advances.sciencemag.org/content/5/12/eaaw3916
I think Greg Hickok had the most trenchant comment, that people are hoping “that there was one thing that had to happen and that released the linguistic abilities.” And John Locke had the best bumper sticker, “Motor control rots when you die.”
As the authors say in the article, recent work has shown that primate vocal tracts are capable of producing some vowel sounds:
https://advances.sciencemag.org/content/2/12/e1600723
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0169321
This is certainly interesting from a comparative physiological perspective, and the article has a great summary of tube models for vowels. But I don't think that "producing vowel sounds" should be equated with "having speech" in the sense of "having a phonological system". My own feeling is that we should be looking for a couple of things. First, the ability to pair non-trivial sound sequences (phonological representations) with meanings in long term memory. Some nonhuman animals (including dogs) do have this ability, or something like it, so this isn't the lynch pin.
http://science.sciencemag.org/content/304/5677/1682.short
Second, the emergence of speech sound sequencing abilities in both the motor and perceptual systems. That is, the ability to perform computations over sequences; to compose, decompose and manipulate sequences of speech sounds, which includes concatenation, reduplication, phonotactic patterning, phonological processes and so on. The findings closest to showing this for nonhuman animals (birds in this case) that I am aware of are in:
https://www.nature.com/articles/ncomms10986
https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2006532
In those papers the debate is framed in terms of syntax, which I think is misguided. But the experiments do show some sound sequencing abilities in the birds which might coincide with some aspects of human phonological abilities. But, of course, this would be an example of convergent evolution, so it tells us almost nothing about the evolutionary history in primates.
