Question

what would happen if the backbone was made up of one single bone

Answer 1

Then there would be no movement.
To bend to rest and nothing would be possible

Answer 2

$\huge\purple{Ello \: ♡}$

The structural composition of back bone allows us to bend, crouch, stand and lie down on the ground. The vertebral units promote this movement. If back bone is made up of one long bone then we would never be able to sit, crouch or lie. One single bone means no flexibility in the structure of a bone.