Rod Serling really changed Pierre Boulle's PLANET OF THE APES. I remember sometime later reading Boulle's novel and thinking to myself what was used were just the title and the nouns "planet" with "apes". Everything else was Serling's creation in the movie.
With everything I've read, my contention is that Serling took ideas from his Twilight Zone series and his Twilight Zone-ish 1964 "A Carol for Another Christmas" with its nuclear annihilation theme and applied that to the premise of a planet with apes. I would even go so far to say that the casting of both Peter Sellers and Sterling Hayden reinforces the meme of their Dr. Strangelove characters; hip and subliminal.
For those not familiar with the Boulle novel, his apes had nothing to do with the movie apes as Boulle's were highly technological, space-age even. Budget constrictures, and I'm sure Heston's hefty salary, forced the movie apes to be demoted from a scientifically advanced culture to an agrarian one. PLANET OF THE APES essentially revisits Serling's favorite end-of-the-world-by-nuclear-destruction theme, which was used many times in The Twilight Zone. One can read the movie as a polemic.