Precambrian
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The Precambrian Precambrian Time
Hadean Eon Archean Eon Proterozoic Eon
Phanerozoic Eon
Paleozoic Era Mesozoic Era Cenozoic Era The Precambrian
History Divisions of the Precambrian Precambrian Evolution of Life Summary of the three eras of Precambrian time Links
The Precambrian is sometimes referred to as an "eon." However, it actually has no rank. It is simply Precambrian time. The Precambrian is that stretch of geological time from the formation of the Earth itself to the start of the Cambrian period. This immensely long stretch of time - some four billion years or more - saw the formation of the Earth as a planetary body, including geosphere, atmosphere, and hydrosphere, as well as the appearance of the biosphere and hence the transformation of the Earth from a dead planet to a living one.
History During the eighteenth century geologists first began mapping the strata of the earth's crust. In doing so they frequently found a "basement complex" of igneous and metamorphic rocks beneath the lowest sedimentary layers. These were called the "Primitive" or "Primary", although the term "Primary Era' later came to be applied to the oldest sedimentary stage (later to be called the Paleozoic). In 1835 the English geologist Adam Sedgwick used the name "Cambrian" for the oldest sedimentary strata. Thereafter the underlying rocks were term Precambrian - "before the Cambrian". During the twentieth century the term "Cryptozoic" - age of hidden life" - was used to designate this period, whilst Phanerozoic - "age of obvious (or revealed) life" - was used for those periods from which fossils of multicellular organisms are known (i.e. the Cambrian period to the present-day). Although the latter term is still in use, "Cryptozoic" pretty much disappeared in favor of the older and well established Precambrian.
Divisions of the Precambrian Because Precambrian time is so long, it is useful to divide it into stages or eons. Two divisions are in use. One option divided the Precambrian into a more recent Proterozoic ("age of first life") a preceding Archeozoic ("first life") and an even earlier Azoic (lifeless) era. Currently, Archeozoic and Azoic have been replaced by Archean ("first", "primary") and Hadean the latter term referring to the hellish conditions of the very early Earth. More recently, the term Priscoan, has been used to refer to the period where the geosphere was still forming an life had not yet come into being.
These three eras, the Hadean, Archean and Proterozoic have recently been promoted to the status of eons, although both the Geological Society of America and the International Commission on Stratigraphy have chosen to ignore the Hadean. The Archean and Proterozoic are both divided into various eras.
Following are two tables - one (by the ICS) ignoring the earliest stage of Earth's formation (from which there are no surviving terrestrial rocks) and presenting a simple division of Archean and Proterozoic Eons into eras, and the other (based on Harland et al) having a larger number of Precambrian eras and periods. We follow the ICS system, but recognize a separate Hadean eon.
Disclaimer: We didn't make up this horrendous color scheme. Except for the Hadean colors, all of the color coding for time periods is the official scheme used by the ICS.
The Precambrian Time-Scale eon era period end - began (Mya)
Proterozoic PR Neoproterozoic NP Ediacaran NP3 540 - 650
Cryogenian NP2 650 - 850 Tonian NP1 850 - 1000 Mesoproterozoic MP Stenian MP3 1000 - 1200 Ectasian MP2 1200 - 1400 Calymmian MP1 1400 - 1600 Paleoproterozoic PP Statherian PP4 1600 - 1800 Orosirian PP3 1800 - 2050 Rhyacian PP2 2050 - 2300 Siderian PP1 2300 - 2500
Archean AR Neoarchean NA 2500 - 2800
Mesoarchean MA 2800 - 3200 Paleoarchean PA 3200 - 3600 Eoarchean EA 3600 -3800? Hadean Early Imbrian 3800-3850 Nectarian 3850-3950 Basin Groups 1-9 3950-4150 Cryptic 4150- c. 4560
International Stratigraphic Chart, International Union of Geological Sciences: International Commission on Stratigraphy, 2001, published by Micropress
era period when began
myrs ago duration
myrs
Proterozoic Sinian
(late Proterozoic) Ediacaran 610 40
Sturtian 800 190
Riphean
(middle Proterozoic) Karatau 1050 250
Yurmatin 1350 300
Burizan 1650 300
Animikean 2200 400
Huronian 2450 150
Archaean
Randian 2800 350
Swazian 3500 700
Izuan 3800 300
Hadean 4000
Priscoan 4560 760
The above table is based in part on the material available at Jeff Poling's Geologic Ages of Earth History page. It uses Precambrian periods and dates from A Geologic Time Scale 1989 by Harland, W. Brian, Richard Armstrong, Allan Cox, Craig Lorraine, Alan Smith and David Smith
Summary of the three eras of Precambrian time The Priscoan (Hadean) Era Formation of the Solar System and the Earth. Formation of the rocky Earth out of collisions of planetoids. Formation of the Crust - Anorganic Chemical and physical Macrodynamics, Cosmic bombardment from comets and planetoids. Primal Ocean, Organic Substrate, Abiogenesis, Informed biomolecules, reducing atmosphere
The Hadean Era
The Archean Era Present crustal structure, Planetary Ocean, Formation of the continents and archaic regime of Continental drift. Gaia (Chemically and thermally self-stabilizing biosphere) - Bioenergetic Processes - Prokaryotes (Archaea, Eubacteria and Urkaria) - reducing atmosphere, Oxygen Crisis and the decline of the Archaea, colonial stromatolites
The Archean Era
The Proterozoic Era Endosymbiosis (Eukaryotes). Continental drift (present regime) begins. Proterozoic Ice Ages - Precambrian Pangeas. The first Multicelluar organisms
The Proterozoic Era
Precambrian Evolution of Life The Unicelluar Biosphere The evolution of life can be divided into two very unequal periods: the very long Precambrian (lasting over 3 billion years), when life for the most part remained at the microbial grade of organization, and the much shorter Phanerozoic, encompassing the Paleozoic, Mesozoic, and Cenozoic eras (about 540 million years in all), when much more complex, multicellular life, has flourished. The diagram on the left shows the comparative difference in duration of these eras.
Thus throughout most of the period the Earth has been in existence, there has been life, but life of a very primitive kind, analogous to modern bacteria. These single-celled microorganisms are distinguished from more advanced life in that their cells are not compartmentalized into distinct organs, but rather are of a simple and uniform nature. Such uniform cells are called Prokaryotes (the micro-organism illustrated at the left of the above figure), and they appeared long before the more complex cells, or Eukaryotes (shown at the right on the diagram). The accepted paradigm at present is that Eukaryotes are built up of Prokaryotes that came together in a symbiotic relation; the different organs of the cell descending from different prokaryotic organisms
It was only at the very end of the Precambrian that there was a sort of "quantum leap" in evolution from simple microbes to complex multicellular organisms.
Links
Links
The Divisions of Precambrian Time - an introduction to the Precambrian at the University of California Museum of Paleontology site
Wikipedia: Precambrian
Earth Floor- Geologic Time
Precambrian Era
Precambrian
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