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The Precambrian Period: The Evolution Of Hard-Shelled Animals

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The formation of Earth, 4.5 billion years ago till the evolution of hard-shelled animals, about 500 million years ago, is called the Precambrian period. There are many events during this time that are specific and affect the Precambrian life. The earth was formed and the first tectonic plate arose and began to move which caused many variations on the earth conditions during Precambrian that affected atmospheric composition and oceans. The level of oxygen arose in the atmosphere which enabled the enrichment of atmosphere with oxygen reaching its peak about 600 million years ago which enabled the first appearance of animal life that required oxygen for the production of collagen and the formation of skeleton. Also, during this period the ozone
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