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
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 538.8 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.
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. However, there is evidence that life could have evolved over 4.280 billion years ago. There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian.
Complex multicellular organisms may have appeared as early as 2100 Ma. However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers." Other possible early complex multicellular organisms include a possible 2450 Ma red alga from the Kola Peninsula, 1650 Ma carbonaceous biosignatures in north China, the 1600 Ma Rafatazmia, and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic. The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period. A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as
Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the
Burgess Shale, including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the
Cambrian explosion of life.
While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic
mats that covered terrestrial areas.
Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.
Evidence of the details of
plate motions and other
tectonic activity in the Precambrian has been poorly preserved. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a single
supercontinent around 1130 Ma. The supercontinent, known as
Rodinia, broke up around 750 Ma. A number of
glacial periods have been identified going as far back as the
Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the
Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "
atmosphere of the early Earth is not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and was lacking in free
oxygen. There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean.
At present, it is still believed that
molecularoxygen was not a significant fraction of Earth's atmosphere until after
photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism. This radical shift from a chemically inert to an oxidizing atmosphere caused an
ecological crisis, sometimes called the
oxygen catastrophe. At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive
banded iron formations that were laid down as iron oxides.
Proterozoic: this eon refers to the time from the lower
Cambrian boundary, 538.8 Ma, back through 2500 Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary. The Proterozoic Eon is divided into three eras: the
Neoproterozoic: The youngest
geologic era of the Proterozoic Eon, from the
Cambrian Period lower boundary (538.8 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
Hadean Eon: 4000–4567.3 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although some
zircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from the
It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by
GSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows:
Kenorland supercontinent 2.5 billion years ago
Map of Kenorland breaking up 2.3 billion years ago
Columbia about 1.6 billion years ago
Proposed reconstruction of
Rodinia for 750 million years ago
Landmass positions near the end of the Precambrian
The movement of Earth's
plates has caused the formation and break-up of continents over time, including occasional formation of a
supercontinent containing most or all of the landmass. The earliest known supercontinent was
Vaalbara. It formed from proto-continents and was a supercontinent 3.636 billion years ago.
Vaalbara broke up c. 2.845–2.803
Ga ago. The supercontinent
Kenorland was formed c. 2.72
Ga ago and then broke sometime after 2.45–2.1
Ga into the proto-continent
Yilgarn craton and
Kalahari. The supercontinent
Columbia, or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago. The supercontinent
Rodinia is thought to have formed about 1300-900 Ma, to have embodied most or all of Earth's continents and to have broken up into eight continents around 750–600 million years ago.
Phanerozoic – Fourth and current eon of the geological timescale
Paleozoic – First era of the Phanerozoic Eon 539–252 million years ago
Mesozoic – Second era of the Phanerozoic Eon: ~252–66 million years ago
Cenozoic – Third era of the Phanerozoic Eon (66 million years ago to present)
^Cavosie, Aaron J.; Valley, John W.; Wilde, Simon A. (2007). "Chapter 2.5 The Oldest Terrestrial Mineral Record: A Review of 4400 to 4000 Ma Detrital Zircons from Jack Hills, Western Australia". Developments in Precambrian Geology. 15: 91–111.
^Hitchcock, C. H. (1874).
The Geology of New Hampshire. p. 511. The name Eozoic seems to have been proposed by Dr.
J.W. Dawson, of Montreal, in 1865. He did not fully define the limits of its application at that time; but it seems to have been generally understood by geologists to embrace all the obscurely fossiliferous rocks older than the Cambrian.
^Bulletin. Vol. 767. U.S. Government Printing Office. 1925. p. 3.  Sir
J. W. Dawson prefers the term "Eozoic" [to Archean], and would have it include all the Pre-Cambrian strata.
Valley, John W., William H. Peck, Elizabeth M. King (1999) Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-Madison
Wgeology.wisc.eduArchived 2012-03-16 at the
Wayback Machine – Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago Accessed Jan. 10, 2006
Wyche, S.; Nelson, D. R.; Riganti, A. (2004). "4350–3130 Ma detrital zircons in the Southern Cross Granite–Greenstone Terrane, Western Australia: implications for the early evolution of the Yilgarn Craton". Australian Journal of Earth Sciences. 51 (1): 31–45.