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. Scientists call the period the "Cambrian Explosion." Paleontologists believe that before this explosion began, the only animals on Earth were sponges, cnidarians and ancestral bilateral worms. Yet by the end of the Cambrian explosion, all of the eight major animals body plans in existence today, along with 27 minor ones, had emerged. And no new body plans have developed since. A group of animals called annelid worms developed during the Cambrian Explosion. Today, about 15,000 species of annelids exist including earthworms, marine bristle worms, and leeches. Scientists believe that burrowing worms play a vital role in maintaining life on Earth by recycling plant and animal remains into carbon dioxide gas. This gas helps modify the climate of the biosphere. Before active burrowers appeared, organic remains became buried in sediments and depleted the atmosphere of carbon dioxide. Actively feeding worms however, recycle buried organic material in a timely basis releasing carbon dioxide back into the atmosphere. With sufficient carbon dioxide in the air, land plants can thrive and the oceans remain free of ice across much of the planet.
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