Paralysis during REM sleep likely evolved as a safety mechanism for primates sleeping on the ground.

“i think it has to do with the paralysis that happens during rem sleep which is obviously if you're living 20 30 50 100 feet up in the trees and you're lying on a branch and all of a sudden you lose all muscle tone which is what happens when you go into rem sleep bad things are probably going to happen you're probably going to be popped out of the gene pool quite quickly that way so when we became safe on the ground from a sheer newtonian physics standpoint then all of a sudden rem sleep blossomed and with it i think a whole constellation of new benefits for the hominid species”