What is this passage mainly about?

Scientists say sharks have lived in the world’s oceans for millions of years. Scientists say there are more than three-hundred-fifty different kinds of sharks.
   Sharks do not have bones, and a shark has an extremely good sense of smell. It can find small amounts of substances in the water, such as blood, body liquids and chemicals produced by animals. Sharks also sense electrical and magnetic power linked to nerves and muscles of living animals. These powerful senses help them find their food. Some sharks will eat just about anything. Many unusual things have been found in the stomachs of some tiger sharks. They include shoes, dogs, a cow’s foot and metal protective clothing.
   About forty percent of the different kinds of sharks lay eggs. The others give birth to live young. Some sharks carry their young inside their bodies like humans do.
   Scientists are beginning to understand the importance of sharks to humans.  Medical researchers want to learn more about the shark’s body defense system against disease. They know that sharks recover quickly from injuries. Sharks appear never to suffer infections, cancer or heart diseases.