Precambrian Eon

4600541 million years ago

The geological eons and eras
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Hadean
Archean

Protero
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Paleo
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Neo
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Ceno
Scale:
Millions of years

Precambrian
From Wikipedia, the free encyclopedia

For the album by German band The Ocean, see Precambrian (album).
"Cryptozoic" redirects here. For the games publisher, see Cryptozoic Entertainment.
The Precambrian or Pre-Cambrian, sometimes abbreviated p, is the largest span of time
in Earth's history before the current Phanerozoic Eon, and is a supereon divided into several eons 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. The Precambrian is so named because it preceded the Cambrian, the
first period of the Phanerozoic Eon, which is named after Cambria, the classical name for Wales,
where rocks from this age were first studied. The Precambrian accounts for 88% of geologic time.
Contents
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1Overview

2Life forms

3Planetary environment and the oxygen catastrophe

4Subdivisions

5Precambrian super-continents

6See also

7References

8Further reading

9External links

Overview[edit]
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 succeedingPhanerozoic, and those fossils
present (e.g. stromatolites) are of limited biostratigraphicuse.[1] 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. [1][2]
It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500 Ma, or
4.5 billion years ago (Ga), and may have been struck by a very large (Marssized) planetesimal shortly after it formed, splitting off material that formed the Moon (see Giant
impact hypothesis). A stable crust was apparently in place by 4400 Ma, since zircon crystals
from Western Australia have been dated at 4404 Ma.[3]
The term Precambrian is recognized by the International Commission on Stratigraphy as a general
term including the Archean and Proterozoic eons.[4] It is still used
by geologists and paleontologists for general discussions not requiring the more specific eon names.
It was briefly also called the Cryptozoic eon.

Life forms[edit]
For more details on this topic, see Origin of life.

Landmass positions near the end of the Precambrian

A specific date for the origin of life is not available. Carbon in 3.8 billion year old rocks from islands
off western Greenland may be of organic origin. Well-preserved bacteriaolder than 3.46 billion years
have been found in Western Australia.[5] 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 of the
Precambrian.
Excluding a few contested reports of much older forms from North America and India, the first
complex multicellular life forms seem to have appeared roughly 600 Ma. The oldest fossil evidence
of complex life comes from the Lantian formation, at least 580 million years ago. A quite diverse
collection of soft-bodied forms is known from a variety of locations worldwide between 542 and 600
Ma. These are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward
the end of that time span. By the middle of the later Cambrian period a very diverse fauna is
recorded in the Burgess Shale, including some which may represent stem groups of modern taxa.
The rapid radiation of lifeforms during the early Cambrian is called the Cambrian explosion of life.[6][7]
While land seems to have been devoid of plants and animals, cyanobacteria and other microbes
formed prokaryotic mats that covered terrestrial areas.[8]

Planetary environment and the oxygen catastrophe[edit]

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

Evidence illuminating the details of plate motions and other tectonic functions in the Precambrian
has been poorly preserved. It is generally believed that small proto-continents existed prior to 3000
Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1000 Ma.
The supercontinent, known as Rodinia, broke up around 600 Ma. A number of glacial periods have
been identified going as far back as the Huronian epoch, roughly 2200 Ma. One of the most delved
into is theSturtian-Varangian glaciation, around 600 Ma, which may have brought glacial conditions
all the way to the equator, resulting in a "Snowball Earth".
The 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, lacking in free oxygen. This has
been disputed with evidence in support of an oxygen-rich atmosphere since the early Archean. [9]
Molecular oxygen was not present as 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 an inert to an oxidizing atmosphere caused an ecological
crisis sometimes called the oxygen catastrophe. At first, oxygen would quickly combine with other
elements in Earth's crust, primarily iron, as it was produced. After the supply of oxidizable surfaces
ran out, oxygen began to accumulate in the atmosphere, and the modern high-oxygen atmosphere
developed. Evidence for this lies in older rocks that contain massive banded iron formations, laid
down as iron and oxygen first combined.

Subdivisions[edit]
Main article: Timetable of the Precambrian
Life timeline
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0
water

Simple life
photosynthesis

Eukaryotes
Multicellular organisms
Land life
Dinosaurs
Mammals
Flowers

Earliest Earth (-4540)

Earliest water

Earliest life
(-4100)

Meteorite bombardment

Earliest oxygen

Atmospheric oxygen

Oxygen Crisis

Earliest sexual reproduction

Cambrian explosion

Earliest humans
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Axis scale: millions of years.

also see {{Human timeline}} and {{Nature timeline}}

An established terminology has evolved covering the early years of the Earth's existence,
as radiometric dating allows plausible real dates to be assigned to specific formations and features.
[10]
The Precambrian Supereon is divided into three eons: the Hadean (45003950
Ma),Archean (4000-2500 Ma) and Proterozoic (2500-541 Ma). See Timetable of the Precambrian.

Proterozoic: this eon refers to the time from the lower Cambrianboundary, 541 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:
theNeoproterozoic, Mesoproterozoic and Paleoproterozoic.

Neoproterozoic: The youngest geologic era of the Proterozoic Eon, from

the Cambrian Period lower boundary (541 Ma) back to 1000 Ma. The Neoproterozoic
corresponds to Precambrian Z rocks of older North American geology.

Ediacaran: The youngest geologic period within the Neoproterozoic Era. The
"2012 Geologic Time Scale" dates it from 541 to 635 Ma. In this period the Ediacaran
faunaappeared.

Cryogenian: The middle period in the Neoproterozoic Era: 635-720 Ma.

Tonian: the earliest period of the Neoproterozoic Era: 720-1000 Ma.

Mesoproterozoic: the middle era of the Proterozoic Eon, 1000-1600 Ma.

Corresponds to "Precambrian Y" rocks of older North American geology.

Paleoproterozoic: oldest era of the Proterozoic Eon, 1600-2500 Ma. Corresponds to

"Precambrian X" rocks of older North American geology.
Archean Eon: 2500-4000 Ma.

Hadean Eon: 39504500 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.[11]

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: [12]
1. Accretion and differentiation: a period of planetary formation until giant Moon-forming impact
event.
2. 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.
3. 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.
4. Transition: a period of continued iron banded formation until the first continental red beds.
5. Proterozoic: a period of modern plate tectonics until the first animals.

Precambrian super-continents[edit]
The movement of plates has caused the formation and break-up of continents over time, including
occasional formation of a super-continent containing most or all of the continents. The earliest
known super-continent was Vaalbara. It formed from proto-continents and was a super-continent by
3.1 billion years ago (3.1 Ga). Vaalbara broke up c. 2.8 Ga ago. The super-continent Kenorland was
formed c. 2.7 Ga ago and then broke sometime after 2.5 Ga into the protocontinent cratons calledLaurentia, Baltica, Australia, and Kalahari. The super-continent Columbia or
Nuna formed during a period of 2.01.8 billion years and broke up about 1.51.3 billion years ago. [13]
[14]
The super-continent Rodinia is thought to have formed about 1 billion years ago, to have
embodied most or all of Earth's continents and to have broken up into eight continents around 600
million years ago.