Cold-blooded, scaled aquatic vertebrates, fish were the first animals known to develop bones. The first fossil, Anaspis, dates from more than 500 million years ago and is though to have been armored and jawless. Subsequent jaw formation, a little over 400 million years back, may have resulted in the proliferation of fish: jawless fishes left very few ancestors. Next came the Placoderms, which had both jaws and scales as well as armored plates. Not a single Placoderm survived a mass extinction event about 360 million years ago, but luckily ray-finned fishes and lobe-finned fishes evolved afterward and spawned ancestors we know today. Ray-finned fishes now comprise about 30,000 species throughout freshwater and ocean habitats all over the world.

Globally, 1,201 species of fish — including lampreys and hag fish, sharks, skates, rays and chimaeras, bony fishes, coelacanths, sea urchins and starfish — or four percent of a total 30,000 described fish species were deemed at risk of extinction by IUCN's 2007 Red List, 166 of them in the United States.