The earliest sharks swam through Earth's oceans around 420 to 450 million years ago, a time when there were no true trees on land. Paleontologists can trace the history of sharks this far back by examining fossilized shark teeth and scales. About 370 million years ago, sharks appeared in forms that we would find familiar today, though some had spines and bizarre, flat-topped fins.
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Key Facts In This Video
Sharks have existed for about 200 times longer than modern humans. 00:35
Sharks have survived five major mass extinction events on Earth. 00:56
The Megalodon shark could have reached 20 meters in length. 01:43