Fish
Lamprey
About 510 million years ago, some of the worms on Earth evolved into the earliest fish. Fish were different from worms because they had spinal cords, nerves running all along their backs that carried messages to their other end. None of these earliest fish survive today, but lampreys are a lot like them.
Soon after developing spinal cords, most fish also developed vertebrae to protect their spinal cords and keep them from getting broken.
Great white shark
These early fish weren't very successful, and there don't seem to have been very many of them. But then, about 420 million years ago, some fish evolved to have jaws. The earliest kind of fish that had jaws were sharks. Jaws turned out to be very useful - we don't know whether this was because they helped with eating or with breathing - and fish with jaws quickly became very common in Earth's oceans and rivers. Possibly the jaws and teeth helped to pre-digest the food, so that the fish's cells didn't have to do so much of the work, and could save their energy for other things. Because fish with jaws and teeth had more energy, they could swim faster and catch more food than the older kind of fish. Some fish even have two sets of teeth - one in their jaw like yours, and another set in their throat.
By about 395 million years ago, there were fish in the rivers and not just in the oceans. These river fish were the earliest modern-type fishes with bones. Some of these river fish became the ancestors of all modern fish (even the ocean fish). One early example that's still around today is the sturgeon, which doesn't even have vertebrae. By around fifty million years ago, some of these river fishes re-adapted to live in the oceans, or partly in the oceans like salmon.
Coelacanth
A different kind of early river fish, related to coelocanths, became the ancestors of amphibians, reptiles, and all land animals.
To find out more about fish, check out these books from Amazon.com or from your library:
