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. These two periods were known as the Silurian and the Devonian periods. In yhe middle Silurian, the jawless fishes had diversified, but it was not until the Devonian that the true variety of fishes really flourished. Fish are in no way simply a stepping stone to amphibians, they are a much more significant than this. They have evolved to be masters of their domain, the water.