Sharks play an important ecological role in their communities by acting as scavengers and eating weak organisms. Only about 30 species of shark are truly dangerous to humans. Most people killed by shark attacks have died because of excessive blood loss, not because of being eaten. The first sharks appeared on our planet about 300 to 400 million years ago, during the Age of Fishes. One of the most primitive creatures alive today, sharks have outlasted dinosaurs, Neanderthals and many other now-extinct life forms.
Scientists credit the evolutionary success of sharks to several factors.
They reproduce in a variety of ways, some laying eggs and others giving birth to live young. In addition, many sharks are resistant to disease. Only humans, who kill them for food and sport, and contaminate their oceans. Fishing industries harvest large numbers of sharks, finding uses for many parts of their bodies. Their oil is used in skin creams, lipsticks and other cosmetics, and as a lubricant for delicate scientific instruments.