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From fossil evidence, it appears that life may have existed on Earth as early as 3.5 billion years ago. This suggests that life must have evolved sometime during Earth's tumultuous first billion years.
Setting The Stage for Complex Life. The Precambrian is the first time period of the earth. The earth took form about 4.5 billion years ago. For the first 4 billion years of that time, the Earth was growing and changing. The only living things during this time were one-celled organisms.