I don't know how that developed.
He is..., isn't he?
You are ..., aren't you?
But NOT: I am... , amn't I? -- It's what you'd expect, though, not "I am..., aren't I"?
But all of those can take the emphatic:
He is the best choice for the position, is he not?
You are the manager and therefore able to fix this, are you not?
I am the mom here, am I not?