The first vertebrates to evolve were conodonts. They were a group of extinct, eel-like animals that lived in the oceans during the Cambrian period (541 to 485 million years ago).

Here's why they are considered the first vertebrates:

* Conodonts had a notochord: This is a flexible rod that runs along the back of the body, providing support and giving the animals their characteristic eel-like shape. The presence of a notochord is a defining characteristic of all vertebrates.

* Conodonts had mineralized teeth: These teeth were actually small, pointed structures called "conodont elements" that they used for feeding. They are one of the earliest signs of mineralized tissues in the vertebrate lineage.

While conodonts are considered the first vertebrates, they were not the first animals to evolve. They evolved from an earlier group of animals called chordates. Chordates are characterized by having a notochord at some point in their development.