Yeah The Butterfly Effect was a real mind bender.
With Planet of the Apes, I think the idea was relativity with time, in that these people went into outer space and the planet aged at a rapid rate while they aged barely at all.
So everyone they knew, and generations after were long gone and the apes had taken over somehow... I can't remember the exact details there. Maybe it was mankind just kind of destroying themselves, in a Orwellian fashion? War? Not sure, maybe I should re-watch it. It's been a while.
The apes with the "wisdom" wanted to keep man's history a secret, because they knew that man was a more evolved creature than themselves. That's why they would round them up in the wild and search for the smart one's to lobotomize.
So they changed/tried to wipe history, and there's a scene where they find relics in a cave, proving that man had existed first, before the apes, and was more technologically advanced.
I think that's basic gist of it.
Speaking of the original anyway. I'm not too familiar with the newer ones.
https://en.wikipedia.org/wiki/Planet_of_the_Apes_(1968_film)
RE: Here are some random thoughts relating to time-travel