The Precambrian (colored green in the timeline figure) is a supereon that is 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 541 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 that time (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 a very large (Mars-sized) planetesimal 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 recognized by the International Commission on Stratigraphy as the only "supereon" in geologic time; it is so called because it includes the Hadean (~4.6—4 billion), Archean (4—2.5 billion), and Proterozoic (2.5 billion—541 million) eons. (There is only one other eon: the Phanerozoic, 541 million-present.) "Precambrian" is still used by geologists and paleontologists for general discussions not requiring the more specific eon names.
A specific date for the origin of life has not been determined. Carbon found in 3.8-billion-year-old rocks (Archean eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia. Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder (Proterozoic eon) of the Precambrian.