The Precambrian is an informal unit of geologic time,[3] subdivided into threeeons (Hadean,Archean,Proterozoic) of thegeologic 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 theEarth's history, and what is known has largely been discovered from the 1960s onwards. The Precambrian fossil record is poorer than that of the succeedingPhanerozoic, and fossils from the Precambrian (e.g.stromatolites) are of limitedbiostratigraphic use.[4] This is because many Precambrian rocks have been heavilymetamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.[4][5][6]
It is thought thatthe Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet calledTheia shortly after it formed, splitting off material that formed theMoon (seeGiant-impact hypothesis). A stable crust was apparently in place by 4,433 Ma, sincezircon crystals fromWestern Australia have beendated at 4,404 ± 8 Ma.[7][8]
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 westernGreenland may be of organic origin. Well-preserved microscopic fossils ofbacteria older than 3.46 billion years have been found inWestern Australia.[17] 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.[18][19][20][21] 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.[22] However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers."[23] Other possible early complex multicellular organisms include a possible 2450 Ma red alga from theKola Peninsula,[24] 1650 Ma carbonaceous biosignatures in north China,[25] the 1600 MaRafatazmia,[26] and a possible 1047 MaBangiomorpha red alga from the Canadian Arctic.[27] The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period.[28][29] 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 asEdiacaran 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 theBurgess Shale, including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called theCambrian explosion of life.[30][31]
While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryoticmats that covered terrestrial areas.[32]
Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.[33][34]
TheRNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes.[35] During the Hadean Eon (4,567–4,031 Ma) abundantgeothermalmicroenvironments were present that may have had the potential to support the synthesis and replication ofRNA and thus possibly the evolution of a primitive life form.[36] It was shown that porous rock systems comprising heated air-water interfaces could allowribozyme-catalyzed RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes.[36] This primitive RNA replicative system also may have been able to undergo template strand switching during replication (genetic recombination) as is known to occur during the RNA replication of extantcoronaviruses.[37]
Evidence of the details ofplate motions and othertectonic activity in the Precambrian is difficult to interpret. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a singlesupercontinent around 1130 Ma. The supercontinent, known asRodinia, broke up around 750 Ma. A number ofglacial periods have been identified going as far back as theHuronian epoch, roughly 2400–2100 Ma. One of the best studied is theSturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a "Snowball Earth".[38][39]
Theatmosphere 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 freeoxygen. There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean.[40]
At present, it is still believed thatmolecular oxygen was not a significant fraction of Earth's atmosphere until afterphotosynthetic life forms evolved and began to produce it in large quantities as a byproduct of theirmetabolism. This radical shift from a chemically inert to an oxidizing atmosphere caused anecological crisis, sometimes called theoxygen 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 massivebanded iron formations that were laid down as iron oxides.[41][42]
A terminology has evolved covering the early years of the Earth's existence, asradiometric dating has allowed absolute dates to be assigned to specific formations and features.[43] The Precambrian is divided into three eons: theHadean (4567–4031 Ma),Archean (4031-2500 Ma) andProterozoic (2500-538.8 Ma). SeeTimetable of the Precambrian.
Proterozoic: this eon refers to the time from the lowerCambrian 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.[44] The Proterozoic Eon is divided into three eras: theNeoproterozoic,Mesoproterozoic andPaleoproterozoic.
Neoproterozoic: The youngestgeologic era of the Proterozoic Eon, from theCambrian Period lower boundary (538.8 Ma) back to 1000 Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
Ediacaran: The youngestgeologic period within the Neoproterozoic Era. The "2012 Geologic Time Scale" dates it from 538.8 to 635 Ma. In this period theEdiacaran biota appeared.
Hadean Eon: 4031–4567 Ma. This term was intended originally to cover the time before any preserved rocks were deposited, although somezircon crystals from about 4400 Ma demonstrate the existence of crust in the Hadean Eon. Other records from Hadean time come from theMoon andmeteorites.[45][46]
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 byGSSPs. The Precambrian could be divided into five "natural" eons, characterized as follows:[47]
Archean: a period defined by the first crustal formations (theIsua greenstone belt) until the deposition ofbanded iron formations due to increasing atmospheric oxygen content.
Transition: a period of continued banded iron formation until the first continentalred beds.
The movement of Earth'splates has caused the formation and break-up of continents over time, including occasional formation of asupercontinent containing most or all of the landmass. The earliest known supercontinent wasVaalbara. It formed from proto-continents and was a supercontinent 3.636 billion years ago.Vaalbara broke up c. 2.845–2.803Ga ago. The supercontinentKenorland was formed c. 2.72Ga ago and then broke sometime after 2.45–2.1Ga into the proto-continentcratons calledLaurentia,Baltica,Yilgarn craton andKalahari. The supercontinentColumbia, or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago.[48][49] The supercontinentRodinia is thought to have formed about 1300-900 Ma, to have included most or all of Earth's continents and to have broken up into eight continents around 750–600 million years ago.[50]
^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.Bibcode:2007DevPG..15...91C.doi:10.1016/S0166-2635(07)15025-8.ISBN9780444528100.
^Senter, Phil (1 April 2013). "The Age of the Earth & Its Importance to Biology".The American Biology Teacher.75 (4):251–256.doi:10.1525/abt.2013.75.4.5.S2CID85652369.
^"Stratigraphic Guide".International Commission on Stratigraphy. Table 3. Retrieved9 December 2020.
^Hitchcock, C. H. (1874).The Geology of New Hampshire. p. 511.The nameEozoic 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.[1888] SirJ. W. Dawson prefers the term "Eozoic" [to Archean], and would have it include all the Pre-Cambrian strata.
^Rozanov, A. Yu.; Astafieva, M. M. (1 March 2013). "A unique find of the earliest multicellular algae in the Lower Proterozoic (2.45 Ga) of the Kola Peninsula".Doklady Biological Sciences.449 (1):96–98.doi:10.1134/S0012496613020051.PMID23652437.S2CID15774804.
^Qu, Yuangao; Zhu, Shixing; Whitehouse, Martin; Engdahl, Anders; McLoughlin, Nicola (1 January 2018). "Carbonaceous biosignatures of the earliest putative macroscopic multicellular eukaryotes from 1630 Ma Tuanshanzi Formation, north China".Precambrian Research.304:99–109.Bibcode:2018PreR..304...99Q.doi:10.1016/j.precamres.2017.11.004.
^Kump, Lee R.; Fallick, Anthony E.; Melezhik, Victor A.; Strauss, Harald; Lepland, Aivo (2013), Melezhik, Victor A.; Prave, Anthony R.; Hanski, Eero J.; Fallick, Anthony E. (eds.),"8.1 The Great Oxidation Event",Reading the Archive of Earth’s Oxygenation: Volume 3: Global Events and the Fennoscandian Arctic Russia - Drilling Early Earth Project, Berlin, Heidelberg: Springer, pp. 1517–1533,doi:10.1007/978-3-642-29670-3_11,ISBN978-3-642-29670-3, retrieved2025-01-27
^Zhao, Guochun; Cawood, Peter A.; Wilde, Simon A.; Sun, M. (2002). "Review of global 2.1–1.8 Ga orogens: implications for a pre-Rodinia super-continent".Earth-Science Reviews.59 (1):125–162.Bibcode:2002ESRv...59..125Z.doi:10.1016/S0012-8252(02)00073-9.
Valley, John W., William H. Peck, Elizabeth M. King (1999)Zircons Are Forever, The Outcrop for 1999, University of Wisconsin-MadisonWgeology.wisc.eduArchived 2012-03-16 at theWayback 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.Bibcode:2004AuJES..51...31W.doi:10.1046/j.1400-0952.2003.01042.x.
![Map of Kenorland supercontinent 2.5 billion years ago[citation needed]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aKenorland.jpg)
![Map of Kenorland breaking up 2.3 billion years ago[citation needed]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aKenorland_breaking_up.jpg)
![Landmass positions near the end of the Precambrian[citation needed]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPositions_of_ancient_continents%2c_550_million_years_ago.jpg)
