USAGE: Aren't/Amn't

>Nik Taylor wrote:
>> "Dr. David E. Bell" wrote:
>> > What about -n't in couldn't?
>>
>> That's not a clitic, IMO, because it can't be moved.  -'s can detached
>> from the possessor, as in the standard example "The Queen of England's
>> crown", but not *"I could go-n't there" or anything like that.  -n't is
>> a suffix which can only be added to auxiliaries.
Tom Wier:
>That's true.  In my syntax class, we also treat "aren't" as
>in "Aren't I supposed to go?" as a single grammaticalized
>form, not as "are" + the reduced form of "not", "n't".
Can anybody tell me how this developed in English? Why is it grammatically=20
correct to say "aren't I" instead of "amn't I"?=20
After all, we have:
He/she/it     is
Isn't she supposed to go?
Is she not supposed to go?
She isn't supposed to go.
She is not supposed to go.

They    are
Aren't they supposed to go?
Are they not supposed to go?
They aren't supposed to go.
They are not supposed to go.

But:
I    am
*Amn't I supposed to go? (English: Aren't I supposed to go?)
Am I not supposed to go?
*I amn't supposed to go.  *I aren't supposed to go. (English: I'm not=20
supposed to go.)
I am not supposed to go.