How fish evolved

The first animals to evolve backbones were fish, which appeared on Earth almost 500 million years ago. Fish are aquatic creatures that are adapted to live in the world's oceans, lakes, and rivers. They can breathe underwater, and many have streamlined shapes that allow them to swim quickly through the water.

How fish work

Half of the world's vertebrates are fish. Some species live in the saltwater of oceans, and others, in the freshwater of rivers, ponds, and lakes, but all are adapted to a life spent swimming. Most have a streamlined, scaly body with fins to control their movement and gills for breathing underwater.