In the 4.5 billion year history of Earth, a mere 10 million years seems rather insignificant--the equivalent of two months in the life of a 75 year-old man. Yet, during a 10 to 20 million year stretch of time, beginning about 540 million years ago, life evolved at an explosive rate. National Geographic reveals theories and findings about the rapid appearance of new animals during what scientists call the "Cambrian Explosion."