These huge tetrapods may have began to use land in a very gradual fashion, for breeding or to escape predators. In conclusion, there is much to be learned from fish. They have evolved to fill every niche that includes water and have an incredible diversity of adaptations. Their evolutionary history is complicated and complex. They began as jawless, bottom feeders and evolved into sharks, rays, tunas, and many extinct species including the placoderms.
Fish are also important as an evolutionary step.
They are the first known true vertebrates and also provide a very key stepping stone to the evolution of all land living vertebrates. This evolutionary step from bony fish to amphibians is still not fully understood, and the research is very current. The multiple theories all have merit and a crucial debate is whether limbs evolved out of a push to be land dwellers or as an adaptation that made certain fish able to manipulate the water of the Devonian period. Many more specimens need to be found in order for this debate to lead to one side or the other. Vertebrate Evolution , by Thomas Hebert, University of Miami 2.