Now, I have heard that the reason we may
>need chiropractic is because we haven't yet adapted completely to
>walking upright -- our back is a modified version of one for a
>tree-dweller.  But how long would this really take?

I see many reasons that can explain our back situations.

neolithic revolution brought us the experience of hard work bending over the
crops for hours .
Something that hunter gatherers don't need to do .

Our modern way of living where sitting on chair is the norm , sitting in a
vibrating car, rarely squatting, sleeping on soft matress etc... by itself
can be the reason for our weak backs.

lack of exercises

Not experiencing the constant movment of the 1st year of infancy, while
carried by the mother ( good to make muscular backs).

Learning to walk with a bulky diaper between the legs. ( try it that is a
performance)

wearing shooes with heels

eating an inflamatory diet.

Hunter gatherers squatt  or sit on the floor with their back strait,  sleep
on hard floor generally ( some in hamac),don't wear diapers ,are carried by
their parents a long time, walk barefeet, eat better etc....

 jean-claude