Question

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

Answer 1

If the backbone is made up of only one bone than it will not be much strong to stretch the back of the human.Human will not be able to down and move his/her neck too.You can't even sit and walk.

Hope it will help you.
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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.