Fossil records indicate that the first sharks lived some 300 million years ago, and by about 100 million to 70 million years ago, most of the modern sharks had evolved. Sharks are among the oldest living things, and they have remained essentially the same since the modern sharks first appeared. The approximately 200 to 250 species of sharks live in all the oceans of the world. The great majority live in temperate and tropical regions, though the Greenland shark lives in the cold Arctic waters and the huge basking shark is at home in the seas around Antarctica.
Nurse sharks spend most of their time at the bottom of shallow water.
The Portuguese and recently discovered megamouth sharks live in the deepest parts of the ocean. Despite the shark's reputation for viciousness, only a few species of sharks are known to attack humans. Some sharks, including the thresher, brown, and lemon sharks, are fished commercially for food. The mako is a highly prized sport fish. Description and Habits Sharks have skeletons composed of cartilage rather than of bone.