The Precambrian
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 Currently, Archeozoic and Azoic have been replaced by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
Neoproterozoic NP
Proterozoic PR
Mesoproterozoic MP
Paleoproterozoic PP
Archean AR
Hadean
period
end - began (Mya)
Ediacaran NP3
540 - 650
Cryogenian NP2
650 - 850
Tonian NP1
850 - 1000
Stenian MP3
1000 - 1200
Ectasian MP2
1200 - 1400
Calymmian MP1
1400 - 1600
Statherian PP4
1600 - 1800
Orosirian PP3
1800 - 2050
Rhyacian PP2
2050 - 2300
Siderian PP1
2300 - 2500
Neoarchean NA
2500 - 2800
Mesoarchean MA
2800 - 3200
Paleoarchean PA
3200 - 3600
Eoarchean 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: International Commission on
Stratigraphy, 2001, published by Micropress
era
period
began
duration
myrs
Ediacaran
610
40
Sturtian
800
190
Karatau
1050
250
Yurmatin
1350
300
Burizan
1650
300
2200
400
2450
150
Randian
2800
350
Swazian
3500
700
Izuan
3800
300
Sinian
(late Proterozoic)
Proterozoic
when
myrs ago
Riphean
(middle Proterozoic)
Animikean
Huronian
Archaean
4000
Hadean
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,
atmosphere
Informed biomolecules,
reducing
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 singlecelled 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.