But you're focusing on humans and the human languages, and I'm honestly curious about the "step change" hypothesis!
I think I've heard about it before, but my stint at university in linguistics is > 15 yrs ago, so I don't know much about it.
I remember recognozing Chomsky when later studying CS and formal language basics came up, bringing the Chomsky hierarchy.
Do you have some favorite links to share about the "step change" hypothesis? I remember reading some odd info-news article comparing brains among different species of homo; and and maybe also this particular hypothesis, but I'm not really informed.
That aside, I don't understand why this "onomatopoeia" couldn't be shared among different animals and humans:
> I think objects with these shapes make similar sounds in the real world. Sharp shapes make percussive sounds. Round shapes make softer sounds.
Claiming that animals had no language at all would obviously be false. But we still have reason to think (?) that the human languages are intertwined with the intelligence gap we observe between humans and other mammals.