The Precambrian (Pre-Cambrian) is an informal name for the supereon comprising 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 animals, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 Ma. It is named after the Roman name for Wales, Cambria, where rocks from this age were first studied.

The Precambrian has sometimes been erroneously referred to as an "era", which is a subdivision of an eon

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 those fossils present (e.g. stromatolites) are of limited biostratigraphic use. Many Precambrian rocks are heavily metamorphosed, obscuring their origins, while others have either been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata. It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500 Ma and may have been struck by a very large (Mars-sized) planetesimal shortly after it formed, splitting off material that came together to form the Moon (see Giant impact theory). A stable crust was apparently in place by 4400 Ma, since zircon crystals from Western Australia have been dated at 4404 Ma. The term Precambrian is somewhat out-moded, but is still in common use among geologists and paleontologists. It was briefly also called the Cryptozoic eon. It seems likely that it will eventually be replaced by the preferred terms Proterozoic, Archaean, and Hadean, and become a deprecated term. (See geologic time scale.)

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