Sharks have existed on Earth for centuries, even thousands of years with very little change to their structure. Why do you think that is? Defend your answer.

Sharks, in their basal form have been on Earth for hundreds of millions of years. They were the first to develop a stream-like, torpedo shaped body and created a niche for themselves. Their aerodynamic body shape has evolved to allow for efficient and dynamic movement in water. Having a cartilage instead of bones allowed them to grow large and still be relatively light and buoyant. After they've evolved flexible, protruding jaws, there wasn't much to change. They were able to adapt and exploit all the water habitats as the world was changing.