Interval of geologic time from c. 4 billion years ago, the age of the oldest known rocks, to 542 million years ago, the beginning of the Cambrian Period. This interval represents more than 80% of the geologic record and thus provides important evidence of how the continents evolved. The Precambrian is divided into the Archean and Proterozoic eons, with the boundary between them at 2.5 billion years ago. It was originally defined as the era that predated the emergence of life in the Cambrian Period. It is now known, however, that life on Earth had begun by the early Archean Eon. Soft-bodied organisms without skeletons began to appear toward the end of the Precambrian during the Ediacaran Age.
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