Fish come from worm-like animals that lived more than a half a billion years ago during the Cambrian period. These animals had a primitive spinal cord, a distinct head and tail and V-shaped muscles.
Some of these animals were more advanced and had prototypical fins and gills. They also had bilateral symmetry, which means that their left and right sides more or less mirrored each other. These early fish then evolved into the jawless fish that dominated the Ordovician period. They not only lacked jaws but were notable for the bony plates on their heads. Because they lacked jaws, these fish couldn't handle large prey and so were bottom feeders and scavengers.
About 420 million years ago, these fish split into three branches. They developed jaws and also grew much larger. One branch had plated skin and was given the name "placoderms." These fish died out. However, fish with bony and cartilaginous skeletons survived and flourished. The cartilaginous fish became sharks, rays and skates. The bony fish bifucated into fish that remained in the water and lobe-finned fish, whose robust fins eventually allowed them to leave the water and live on land. These lobe-finned fish are the ancestors of all vertebrates.