Most research on human vs chimp misses the point. Most research focuses on physical differences like skull shape and jaw shape to explain why humans speak and apes don’t. Feedback ‘factors out’ such differences. Within modern humans there are similarly large differences in size and shape of bones, and we all speak.
Here’s a physical difference that seems much more plausible. Most other primates have an extra flap of tissue on the edge of the vocal fold, which sometimes vibrates in resonance with the fold itself and sometimes vibrates with its own unrelated resonance.
Human speech production obeys the same acoustic principles as vocal production in other animals but has distinctive features: A stable vocal source is filtered by rapidly changing formant frequencies. To understand speech evolution, we examined a wide range of primates, combining observations of phonation with mathematical modeling. We found that source stability relies upon simplifications in laryngeal anatomy, specifically the loss of air sacs and vocal membranes. We conclude that the evolutionary loss of vocal membranes allows human speech to mostly avoid the spontaneous nonlinear phenomena and acoustic chaos common in other primate vocalizations. This loss allows our larynx to produce stable, harmonic-rich phonation, ideally highlighting formant changes that convey most phonetic information. Paradoxically, the increased complexity of human spoken language thus followed simplification of our laryngeal anatomy.
This explanation focuses on the feedback control of SALIENT formants. SALIENT is the key. When the larynx itself is producing a set of frequencies that are varying randomly in their relative phase, the ear can’t reliably spot the CHANGES in formants that signal a change in phonemic vowels and consonants.
Decreasing complexity is how innovation works, in biology and technology and language. The first creation is hugely complex, with all possible features available. As it goes into production, then expands into different situations and markets and cultures, it loses features.