Fish are incredibly successful, they are numerous and have a huge variety of adaptations.
They are successful in every water filled habitat. Fish are complicated, diverse, and have a long evolutionary history. In order to understand the place of fish as both a transitory group and as a modern one it is important to understand the times in which and from which they evolved. Fish are the first known true chordates. Fish did most of their evolving between five million and three and a half million years ago.