They are 'cavemen' (but not offically human) who have stayed underground. They've evolved in this environment over thousands of years.
They've adapted perfectly to thrive in the cave. They've lost their eyesight, they have acute hearing and smell and function perfectly in the pitch black. They're expert climbers, so they can go up any rock face and that is their world.They never left the caves
They were never reffered as 'Crawlers' at all in both films, only reffered as 'Things'.
During the making of the first film the director barred the film's all-female cast from seeing the actors in full crawler make-up until their first appearance on screen. Actress Natalie Mendoza said of the effect, "When the moment came, I nearly wet my pants! I was running around afterwards, laughing in this hysterical way and trying to hide the fact that I was pretty freaked out. Even after that scene, we never really felt comfortable with them."