The first fish were glorified aquatic worms. They arose 530 MYA, with a distinct head and tail, gills for breathing, camera-like eyes, and a notochord for support. The term notochord is broadly used as a flexible rod-shaped body, but more specifically refers to cartilage back support, which evolved into a backbone.
The earliest fish were jawless and boneless. Vertebrate backbones appeared ~500 MYA, while jawed fish emerged over 420 MYA.