Sharks have wandered the oceans since well before the days of the dinosaurs and remain among the most effective hunters in the world.
Sharks originated about 450 million years ago, and many species have changed little in the last 100 million years.
Unlike most fish, they do not have a gas bladder to keep them afloat, so many species must move constantly to avoid sinking.
Their skeletons are made of light and sturdy cartilage instead of bone, and many have large livers filled with oil that make them more floating.
The mouths of sharks are covered with rows of teeth that are constantly replaced by new ones when they break or wear out.
The first vertebrates to develop an immune system, sharks may have greater immunity to cancer than humans, and are being studied for possible new cancer drugs and antibiotics.
Most sharks are no more dangerous than other fish. Humans, however, have proven to be extremely dangerous to sharks.
Of the approximately 340 species of sharks, most never attack people. Less than 100 people a year are bitten by sharks, and less than 15 percent of these attacks are fatal.