I don't know but I think that the ability for human speech diminished Tim Burton's ape saga,much to dark from a cinamatic point of view

The original script for the first sequel to "Apes" which was proposed by Pierre Boulle required the humans to regain the power of speech, but it was deemed uncinematic by the producers. I always thought the TV series lost a bit through having the humans speak, because they are at a halfway point in their evolution. And as such, aren't very interesting. They're a little, well, dumb! Added to this is that the gorillas aren't very bright either (I was always VERY annoyed by Mark Lennards portrayal of Urko, because he plays him like such a simpleton! He's a banana short of a thickshake). So you constantly have the astronauts negotiating their way through a bunch of morons. That's why my favorite episodes of the TV series center on adventure, such as the one with the shark, and the hang glider one - Up Above the World So High (there was also one with some nice touches of warmth, where a blind chimp girl falls for Burke, thinking him an ape). As for the new 2001 movie, echhh! All that stuff about the human uprising didn't really work for me. Where the shit did all this quasi "patriotic" crap come into the Apes series? The only thing missing was a Ron Howard music score.

With speech humans should also have the power to organize and share collective discoveries and knowledge. Hard to keep a human race at that level with speech in check for long I would think. So the later speaking ability by humans in the TV series and Battle for the Planet of the Apes does lack a certain realism.

The Human characters speaking was more just used to allow the stories to be told I imagine. Its just like how the Apes speak English. That should tip off Charlton Heston's Taylor character in the first movie that maybe he is still on Earth. But that is done more for storytelling so to nitpick such is kind of silly.