The ultimate origin of Palpatine isn't exactly cleared up in the PT, anyway. Was he human and got scarred, or is his "scarred" skin his true appearance, unveiled? Screen canon doesn't say.
Either way he's most likely from human stock, with the monstrous look being due to the effects of the dark side.
It's all a bit confusing at times, and it's not helped by the lack of advanced prosthetics and CGI creatures in the original film (not counting the Special Edition additions made in 1997) that make the prequels seem more abundant with non-human life forms than the Original Trilogy could afford to show.
Even in the PT there seems to be an abundance of human Jedi ( or at least near-humans who look human from a distance ).