The Precambrian (Pre-Cambrian) is an informal name for the span of time before the current Phanerozoic Eon, and is divided into several eons of the geologic time scale. It spans from the formation of Earth around 4500 Ma (million years ago) to the beginning of the Cambrian Period, when macroscopic hard-shelled animals first appeared in abundance about 542 Ma.
Very 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 fifty years. 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.