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PRECAMBRIAN
Precambrian
The Precambrian (Pre-Cambrian) is the name which describes the large 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 4570 Ma (million years) ago to
the beginning of the Cambrian Period, about 542 Ma, when macroscopic hard-shelled animals
first appeared in abundance. The Precambrian is so named because it precedes 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.
Overview
Not much is known about the Precambrian, despite its making up roughly seven-eighths of the
Earth’s history, and what little is known has largely been discovered in the past 50 years. The
Precambrian fossil record is poor, and those fossils present (e.g. stromatolites) are of limited
biostratigraphic use.1 This is because many Precambrian rocks are heavily metamorphosed,
obscuring their origins, while others have either been destroyed by erosion, or remain deeply
buried beneath Phanerozoic strata.12
It is thought that the Earth itself coalesced from material in orbit around the Sun roughly 4500
Ma (4.5 Ga) and may have been struck by a very large (Mars-sized) planetesimal shortly after it
formed, splitting off material that came together to form 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 before the Cambrian
Main articles: Origin Of Life
It is not known when life originated, but carbon in 3.8 billion year old rocks from islands off
western Greenland may be of organic origin. Well-preserved bacteria older 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.
Excepting a few contested reports of much older forms from USA and India, the first complex
multicelled life forms seem to have appeared roughly 600 Ma. A quite diverse collection of softbodied forms is known from a variety of locations worldwide between 542 and 600 Ma. These
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are referred to as Ediacaran or Vendian biota. Hard-shelled creatures appeared toward the end
of that timespan. The oldest fossil evidence of complex life comes from the Lantian formation, at
least 580 million years ago.
A very diverse collection of life forms appeared around 544 Ma, starting in the latest
Precambrian with a poorly understood small shelly fauna and ending in the very early Cambrian
with a very diverse, and quite modern Burgess fauna, the rapid radiation of forms called the
Cambrian explosion of life.
Planetary environment and the oxygen catastrophe
This section does not cite any references or sources. (September 2012)
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. The best studiedneeded is the Sturtian-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
comprised 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.6
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
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.7 The Precambrian Supereon is divided into three Precambrian eons: the Hadean
(4500-3950 Ma), Archean (3950-2500 Ma) and Proterozoic (2500-542 Ma). See Timetable of
the Precambrian.
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Proterozoic: this eon refers to the time from the lower Cambrian boundary, 542 Ma,
back through 2500 Ma. The boundary has been placed at various times by various
authors, but has now been settled at 542 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, Mesoproterozoic and
Paleoproterozoic.
Neoproterozoic: The youngest geologic era of the Proterozoic Eon, from the
Cambrian Period lower boundary (542 Ma) back to 1000 Ma. The
Neoproterozoic corresponds to Precambrian Z rocks of older North American
geology.
Precambrian supercontinents
The movement of plates has caused the formation and break-up of continents over time,
including occasional formation of a supercontinent that contains most or all of the continents.
The earliest known supercontinent was Vaalbara. It formed from proto-continents and was a
supercontinent by 3.1 billion years ago (3.1 Ga). Vaalbara broke up ~2.8 Ga ago. The
supercontinent Kenorland was formed ~2.7 Ga ago and then broke sometime after 2.5 Ga into
the proto-continent Cratons called Laurentia, Baltica, Australia, and Kalahari. The
supercontinent Columbia or Nuna formed during a period of 2.0–1.8 billion years and broke up
about 1.5–1.3 billion years ago1011 The supercontinent Rodinia is thought to have formed
about 1 billion years ago and to have embodied most or all of Earth’s continents, and broken up
into eight continents around 600 million years ago.
See also
Mesozoic
Cenozoic
References
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