Question

How did vertebrates evolve?

Answer 1

Probably to protect the spinal cord of the central nervous system

Explanation:

Starting from radial organism , organism starts to possess bilateral symmetry (symmetrical to the right and left). This is where vertebrates and invertebrates evolve from. The easiest way to observe bilateral animal is by looking at their embryo. They generally form head-to-tail-axis where the eye is located in the front and the anus is located at the back.

Read: https://en.wikipedia.org/wiki/Embryological_origins_of_the_mouth_and_anus

Read: http://www.mhhe.com/biosci/genbio/raven6b/graphics/raven06b/other/raven06_60.pdf

But before vertebrates build vertebrates, their progenitor organism produce a lot more exoskeleton. This includes armored jawless fish. .

Vertebrate tend to use bone, cartilage and dentine as exoskeleton material. Meanwhile, invertebrate tend to rely on chitin to which they form cuticle. that makes the material for their exoskeleton. (unlike bone which can grow, cuticle have to be replaced for every molting cycle.)

Especially the formation of hard exoskeleton around the skull region would help create an ideal condition for the development of brain. The hard exoskeleton around the skull region is displayed in craniates . Read: cephalization

After cephalization, the next involves the development of notochord or spinal cord which can connect the nerve cells from the main "brain" to the muscles and other peripheral tissues.

The development of vertebrates help protects this spinal cord.

But notice that vertebral column is not the only feature that constitute vertebrates. The evolution is closely related to the development jaws, gills, lungs, tetrapod (four feet), blood vessel
you can read on them here:
https://www.learner.org/courses/essential/life/session6/closer3.html
https://en.wikipedia.org/wiki/Vertebrate#Evolutionary_history
https://en.wikipedia.org/wiki/Evolution_of_fish
https://en.wikipedia.org/wiki/Evolution_of_tetrapods
https://www.quora.com/How-did-fish-evolve-lungs
https://www.quora.com/What-path-do-red-blood-cells-follow-What-is-their-purpose
https://www.quora.com/Why-are-lobsters-invertebrates