What would have happened if our backbone was made of one single bone?

Hint: The backbone or Vertebral column is made up of numerous interlocked small bones called vertebrae. It is with the backbone, we are able to stand erect. The vertebral column provides a stable stature to the body. The pectoral and pelvic girdle is attached to the vertebral column.

Complete answer:
The backbone also known as spine is a series of $33$ vertebrae that are interconnected to make a column. The vertebrae are classified into five types based on their location on the body. They are classified as follows:
Cervical: There are $7$ vertebrae in the cervical region numbered from $C1$ to $C7$.
Thoracic: There are $12$ vertebrae in the thoracic region numbered from $T1$ to $T12$.
Lumbar: There are $5$ vertebrae in this region numbered from $L1$ to $L5$.
Sacrum: There are $5$ vertebrae in this region and they are fused together.
Coccyx: There are $4$ coccyx vertebrae that are fused with one another in our body.
The vertebrae located at different sites serve a specific function. If there were no vertebrae and if the backbone was made up of one single bone, many normal body movements would be impossible.

Additional information:
By the presence or absence of a vertebral column, animals can be classified as vertebrates and invertebrates. Vertebrates are animals with a true vertebral column and it consists of five phyla. Invertebrates are animals without vertebral columns and they are grouped into nine phyla.

Note:
In addition to providing body movements and erect posture, backbone is also important for other functions. The vertebral column protects the spinal cord which is an integral part of the nervous system from external injury and shock. The backbone supports our weight and acts as a central axis of our body.