Simple Science Site: 10 facts about sharks

According to the ReefQuest Centre for Shark Research, sharks date back between 455 million and 425 million years in the fossil records.
Despite rumors to the contrary, most species of sharks actually have good eyesight, at least according to the Florida Museum of Natural History. Some species of sharks have better eyesight than others, but generally speaking sharks have eyesight at least as good as humans. Some scientists suggest sharks can see objects better at a distance, which could help with hunting prey. Also, the eyes of sharks are more sensitive to light than the eyes of humans, which gives sharks an advantage in darker waters.
Do sharks sleep? This question continues to stump scientists across the globe. R. Aidan Martin, former Director of the ReefQuest Centre for Shark Research, offers no direct answers, but there is some evidence that some species of sharks sleep from time to time while others never sleep. Generally speaking, most species of sharks need to keep on the move to flood their gills with water, thus providing them with live-giving oxygen. But all animals need rest, including sharks and other fishes. Witnesses have given reports of what they believed were sleeping sharks, but at the same time the eyes of the sharks appeared to follow movement. Martin reports that the part of the nervous system that co-ordinates swimming in sharks is not found in the brain, but in the spinal column, which theoretically might make it possible for sleeping sharks to continue swimming. The truth to the question about sleeping sharks is that sciences does not know. At least not yet.
Science generally accepts the whale shark as the largest fish on the planet. Whale sharks have been known to grow as long as 42 feet and weigh nearly 80,000 pounds.
What is the smallest shark? That's somewhat debatable. Why? Because several species of sharks are quite small. The Dwarf Lanternfish shark is usually between six and eight inches in length. The pygmy ribbontail catshark is usually between six and seven and a half inches in length. Also, the spined pygmy shark is often between seven and eight inches in length.
The shortfin make shark is considered the fastest shark on the planet. It has been recorded swimming at a swimming speed of 31 miles per hour. More impressive is the fact these sharks have been recorded going as fast as 46 miles per hour for short durations.
According to National Geographic, there is an average between 50 and 70 shark attacks worldwide each year. Of those attacks, an average between five and 15 are fatal.
How long do sharks live? There is no definitive answer. According to sharks-world.com, sharks average living between 20 and 30 years, but scientists suspect some species of sharks live as long as a hundred years, and some species might live even longer.
How many different kinds of sharks are there? Science generally accepts there are about 440 different species of sharks currently swimming in Earth's waters.
The megalodon is an extinct species of shark that supposedly existed between one and a half million and nearly 30 million years ago. It is believed to be the largest shark species to ever exist, having been more than 50 feet in length.