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The Precambrian is an informal name for the eons of the geologic timescale that came before the current Phanerozoic eon.

It spans from the formation of Earth around 4500 Ma (million years ago) to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 Ma.

Remarkably little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what little is known has largely been discovered in the past four or five decades.

The Precambrian fossil record is poor, and what fossils are present are of little use for biostratigraphic work.

Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.

Details of plate motions and such are only hazily known in the Precambrian.

Note:   The above text is excerpted from the Wikipedia article "Precambrian", which has been released under the GNU Free Documentation License.
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