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The Precambrian is sometimes referred to as an "." However, it actually has no rank. It is simply Precambrian time. The Precambrian is that stretch of geological time from the formation of the itself to the start of the period. This immensely long stretch of time - some four billion years or more - saw the formation of the Earth as a planetary body, including , , and , as well as the appearance of the and hence the transformation of the Earth from a dead planet to a living one.
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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 termed Precambrian - "before the Cambrian". During the twentieth century the term "" - age of hidden life" - was used to designate this period, whilst - "age of obvious (or revealed) life" - was used for those periods from which fossils of 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.
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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 ("age of first life") a preceding ("first life") and an even earlier (lifeless) era. Currently, Archeozoic and Azoic have been replaced by ("first", "primary") and the latter term referring to the hellish conditions of the very early Earth. More recently, the term , has been used to refer to the period where the geosphere was still forming and 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 and the 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.
| eon | era | period | end - began () |
|---|---|---|---|
| PR | NP | NP3 | 540 - 650 |
| NP2 | 650 - 850 | ||
| NP1 | 850 - 1000 | ||
| MP | MP3 | 1000 - 1200 | |
| MP2 | 1200 - 1400 | ||
| MP1 | 1400 - 1600 | ||
| PP | PP4 | 1600 - 1800 | |
| PP3 | 1800 - 2050 | ||
| PP2 | 2050 - 2300 | ||
| PP1 | 2300 - 2500 | ||
| AR | NA | 2500 - 2800 | |
| MA | 2800 - 3200 | ||
| PA | 3200 - 3600 | ||
| EA | 3600 -3800? | ||
| 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: , 2001, published by Micropress
| eon | era | period | when began (Mya) | duration (My) |
|---|---|---|---|---|
| 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 | ||
| Isuan | 3800 | 300 | ||
| Hadean | 4000 | 760 | ||
| Priscoan | 4560 |
The above table is based in part on the material available at Jeff Poling's 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
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The Priscoan (Hadean) Eon
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 Archean Eon
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 (, and stem-) - reducing atmosphere, oxygen crisis and the decline of the , colonial stromatolites.
The Proterozoic Eon
Endosymbiosis (eukaryotes). Continental drift (present regime) begins. Proterozoic ice ages - Precambrian Pangeas. The first multicelluar organisms. .
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The evolution of life can be divided into two very unequal periods: the very long Precambrian (lasting about 4 billion years), when life for the most part remained at the microbial grade of organization, and the much shorter , encompassing the , , and eras (about 540 million years in all), when much more complex, multicellular life, has flourished.
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 , and they appeared long before the more complex cells, or . 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 to complex multicellular organisms.
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