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Concept
Precambrian
Summary
The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of the Phanerozoic Eon, which is named after Cambria, the Latinised name for Wales, where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time.
The Precambrian is an informal unit of geologic time, subdivided into three eons (Hadean, Archean, Proterozoic) of the geologic time scale. It spans from the formation of Earth about 4.6 billion years ago (Ga) to the beginning of the Cambrian Period, about million years ago (Ma), when hard-shelled creatures first appeared in abundance.
Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history, and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic, and fossils from the Precambrian (e.g. stromatolites) are of limited biostratigraphic use. This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.
It is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet called Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma.
The term "Precambrian" is used by geologists and paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey and the International Commission on Stratigraphy regard the term as informal.
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