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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.
The earliest evidence for life found so far is in a 3.8 billion-year-old rock, the Isua sediments, found in western Greenland. The evidence for life in these rocks does not come from fossilized remains, but from a peculiar chemical signature of living organisms.