Precambrian

Template:Short description Template:For Template:Redirect Template:Infobox geologic timespan The Precambrian (Template:IPAc-en Template:Respell;Template:Refn or pre-Cambrian, sometimes abbreviated pC, 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 Latinized 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,<ref name="Gradstein_et-al_2012">Template:Cite book</ref> 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 Template:Period end million years ago (Ma), when hard-shelled creatures first appeared in abundance.

OverviewEdit

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.<ref name=Monroe>Template:Cite book</ref> 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.<ref name=Monroe/><ref>Template:Cite book Outlined in {{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite book</ref>

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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>Template:Cite journal</ref>

Template:AnchorThe 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<ref>Template:Citation</ref> and the International Commission on Stratigraphy regard the term as informal.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Because the span of time falling under the Precambrian consists of three eons (the Hadean, the Archean, and the Proterozoic), it is sometimes described as a supereon,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> but this is also an informal term, not defined by the ICS in its chronostratigraphic guide.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Template:Vanchor (from Template:Wikt-lang "earliest") was a synonym for pre-Cambrian and Precambrian,<ref>Template:Cite book</ref><ref>Template:Cite book</ref> or more specifically Archean.<ref>Template:Cite book</ref>

Life formsEdit

Template:Further 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.<ref>Template:Cite book</ref> 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.<ref name="NAT-20170301">Template:Cite journal</ref><ref name="NYT-20170301">Template:Cite news</ref><ref name="BBC-20170301">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="4.3b oldest">Template:Cite news</ref> 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.<ref>Template:Cite journal</ref> However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers."<ref>Template:Cite journal</ref> Other possible early complex multicellular organisms include a possible 2450 Ma red alga from the Kola Peninsula,<ref>Template:Cite journal</ref> 1650 Ma carbonaceous biosignatures in north China,<ref>Template:Cite journal</ref> the 1600 Ma Rafatazmia,<ref>Template:Cite journal</ref> and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic.<ref>Template:Cite journal</ref> The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> 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.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>

While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas.<ref>Template:Cite encyclopedia</ref>

Tracks from an animal with leg-like appendages have been found in what was mud 551 million years ago.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }} The Independent</ref><ref>Template:Cite journal</ref>

Emergence of lifeEdit

The RNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes.<ref>Template:Cite journal</ref> During the Hadean Eon (4,567–4,031 Ma) abundant geothermal microenvironments were present that may have had the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form.<ref name = Salditt2023>Template:Cite journal</ref> It was shown that porous rock systems comprising heated air-water interfaces could allow ribozyme-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.<ref name = Salditt2023/> 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 extant coronaviruses.<ref>Template:Cite journal</ref>

Planetary environment and the oxygen catastropheEdit

File:Temagami greenstone belt pillow lava.jpg

Weathered Precambrian pillow lava in the Temagami Greenstone Belt of the Canadian Shield

Evidence of the details of plate motions and other tectonic 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 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 "Snowball Earth".<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

It is believed that molecular oxygen 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.<ref>Template:Citation</ref><ref>Template:Cite journal</ref>

SubdivisionsEdit

{{#invoke:Labelled list hatnote|labelledList|Main article|Main articles|Main page|Main pages}} Template:Life timeline A terminology has evolved covering the early years of the Earth's existence, as radiometric dating has allowed absolute dates to be assigned to specific formations and features.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The Precambrian is divided into three eons: the Hadean (Template:Period start–Template:Period start Ma), Archean (Template:Period start-Template:Period start Ma) and Proterozoic (Template:Period start-Template:Period start Ma). See Timetable of the Precambrian.

Proterozoic: this eon refers to the time from the lower Cambrian boundary, Template:Period start Ma, back through Template:Period start Ma. As originally used, it was a synonym for "Precambrian" and hence included everything prior to the Cambrian boundary.<ref>Template:Cite journal</ref> The Proterozoic Eon is divided into three eras: the Neoproterozoic, Mesoproterozoic and Paleoproterozoic.
- Neoproterozoic: The youngest geologic era of the Proterozoic Eon, from the Cambrian Period lower boundary (Template:Period start Ma) back to Template:Period start Ma. The Neoproterozoic corresponds to Precambrian Z rocks of older North American stratigraphy.
  - Ediacaran: The youngest geologic period within the Neoproterozoic Era. The "2012 Geologic Time Scale" dates it from Template:Period start to Template:Period start Ma. In this period the Ediacaran biota appeared.
  - Cryogenian: The middle period in the Neoproterozoic Era: Template:Period start-Template:Period start Ma.
  - Tonian: the earliest period of the Neoproterozoic Era: Template:Period start-Template:Period start Ma.
- Mesoproterozoic: the middle era of the Proterozoic Eon, Template:Period start-Template:Period start Ma. Corresponds to "Precambrian Y" rocks of older North American stratigraphy.
- Paleoproterozoic: oldest era of the Proterozoic Eon, Template:Period start-Template:Period start Ma. Corresponds to "Precambrian X" rocks of older North American stratigraphy.
Archean Eon: Template:Period start-Template:Period start Ma.
Hadean Eon: Template:Period start–Template:Period start 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 Moon and meteorites.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

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:<ref>Template:Cite book also available at Stratigraphy.org: Precambrian subcommission</ref>

Accretion and differentiation: a period of planetary formation until giant Moon-forming impact event.
Hadean: dominated by heavy bombardment from about 4.51 Ga (possibly including a cool early Earth period) to the end of the Late Heavy Bombardment period.
Archean: a period defined by the first crustal formations (the Isua greenstone belt) until the deposition of banded iron formations due to increasing atmospheric oxygen content.
Transition: a period of continued banded iron formation until the first continental red beds.
Proterozoic: a period of modern plate tectonics until the first animals.

Precambrian supercontinentsEdit

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 cratons called Laurentia, Baltica, 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.<ref name="Zhao1">Template:Cite journal</ref><ref name="Zhao2">Template:Cite journal</ref> The supercontinent Rodinia 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.<ref>Template:Cite journal</ref>

Kenorland.jpg

Map of Kenorland supercontinent 2.5 billion years agoTemplate:Citation needed
Kenorland breaking up.jpg

Map of Kenorland breaking up 2.3 billion years agoTemplate:Citation needed
Paleoglobe NO 1590 mya-vector-colors.svg

The supercontinent Columbia about 1.6 billion years ago
Positions of ancient continents, 550 million years ago.jpg

Landmass positions near the end of the PrecambrianTemplate:Citation needed