Sharks have been around since the Upper Devonian, approximately 409 million years ago. The subclass Elasmobranchii, to which the sharks belong, have contained three branches through time. The original and most primitive branch is the cladodontoids branch, which is now extinct. The second branch is the hybodonts, who are also now extinct, but who eventually gave rise to the modern sharks and rays contained in a grouping called the elasmobranchs. There are still a few shark species alive today that represent a transitional position between the hybodonts and today's modern shark.
Sharks have many uses to humans.
Parts of the shark have medicinal value as well. And since sharks are very resistant to cancer, cancer researchers are studying their immune system.