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. Many people think of sharks as mean and dangerous but only a few are a danger to people. There are approximately 200 to 250 species of sharks 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.
Sharks have skeletons composed of cartilage rather than of bone.
With a few exceptions, sharks have torpedoshaped bodies, an efficient, streamlined design for fastswimming predators. Dwarf shark is the smallest The shape of a shark's tail suits its way of life. Many sharks have tail fins with an upper lobe that is larger than the lower, which tends to push the head down. Sharks are known for their speed and maneuverability in the water. Sharks have five to seven gill slits and many sharks must keep moving in order to breathe.