Precambrian

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Aeonothem Arathem system Age
( mya )
later later later
P
r
o
t
e
r
o
z
o
i
k
u
m

Duration:

1959
Ma 		Neoproterozoic
Jungproterozoikum
Duration: 459 Ma 		Ediacarium 541
⬍
635
Cryogenium 635
⬍
720
Tonium 720
⬍
1000
Mesoproterozoic
Mittelproterozoikum
Duration: 600 Ma 		Stenium 1000
⬍
1200
Ectasium 1200
⬍
1400
Calymmium 1400
⬍
1600
Paleoproterozoic
Altproterozoikum
Duration: 900 Ma 		Statherium 1600
⬍
1800
Orosirium 1800
⬍
2050
Rhyacium 2050
⬍
2300
Siderium 2300
⬍
2500
A
r
c
h
a
i
k
u
m

Duration:

1500
Ma 		Neo-Archaic
Duration: 300 Ma 		2500
⬍
2800
Mesoarchean
Duration: 400 Ma 		2800
⬍
3200
Paleoarchean
Duration: 400 Ma 		3200
⬍
3600
Eoarchic
Duration: 400 Ma 		3600
⬍
4000

H
a
d
a
i
k
u
m

Duration:
600
Ma 		4000
▄
█
█
█
█
█
█
▀
4600

The Precambrian (from Latin prae = 'before' and Cambrian ) or early earth , the outdated term abiotic , is a period in the history of the earth . It covers the period from the formation of the earth about 4.56 billion years ago to the development of the animal world at the beginning of the Cambrian about 540 million years ago.

The Precambrian is as informal unit outside the rank system of Chronostratigraphy and includes the eons Hadean , Archaikum and Proterozoikum .

Origin of Life

The first living things developed in the Precambrian, but only a few fossils have survived. B. Bacteria such as the cyanobacteria . The Precambrian period, from which fossils are known, is sometimes referred to as the Cryptozoic and delimited from the Azoic , when there was probably no life on earth. In any case, very different chemical and climatic conditions prevailed in the early Precambrian than later.

Theories

So far there have only been various hypotheses or theories about the origin of life on earth (e.g. from Nobel Prize winners Manfred Eigen and Richard Kuhn ), since such origin processes have so far neither been observed in nature nor shown experimentally.

The theories mentioned are based on very general physico-chemical principles, e.g. B. self-organization according to the " principle of the smallest constraint " or adaptation to the conditions in "optimal cavities". It is assumed here that life could perhaps have formed in a “primordial soup” through chemical reactions (see Miller-Urey experiment ). The extreme environmental conditions in which life originated are similar to those around black and white smokers in the deep sea. Black smokers may have played a crucial role in the chemical evolution of life.

Other theories support the assumption that life was created or promoted by the impact of asteroids or comets , or that individual amino acids or even living cells with extraterrestrial rocks could have reached the earth ( panspermia ).

First living beings

The exact time of the development of life is not known. However, rocks around 3.8 billion years old on islands west of Greenland could be partly of organic origin. The oldest stromatolites , indirect geological evidence of the presence and activity of cyanobacteria (formerly known as blue-green algae) were found in Western Australia and are 3.46 billion years old. Another find in the same area indicates bacteria that are 100 million years older. At the end of the Precambrian, however, there are a number of well-preserved references to early species.

Towards the end of the Precambrian, multicellular organisms also emerged, some of which are counted among the forerunners of still existing groups of organisms such as sponges (Porifera) and cnidarians (Cnidaria), whose blueprints are in part hardly or not at all assigned to a group of living beings that still exists today can. Precambrian formations are rare on earth, and the ancient rocks were often reshaped several times by tectonic processes. The Ediacara fauna , a community of creatures of the late Precambrian, is named after the Ediacara Formation from Flinders Range in Australia , whose counterparts also occur in Canada and Namibia . This last section of the Precambrian is called Ediacarium , formerly also Vendium (see Vendobionts ).

Subdivision of the Precambrian

The Precambrian is divided into the following sub-units:

The boundaries of most of these units correspond to the mean values ​​of radiometrically determined age data of tectonic dormant phases, rounded to a full 50 or 100 million years and called Global Standard Stratigraphic Age (GSSA).

Reorganization of the Precambrian

In an effort to move away from the previous practice of fixing the boundaries of the subunits of the Precambrian to GSSAs and to replace them with the GSSP principle , which was already successfully applied in the Phanerozoic , a reorganization of the Precambrian was proposed. Although there are almost no fossils in Precambrian rocks that could be used to draw boundaries, the GSSPs of the Phanerozoic units are not exclusively defined by fossil markers, but also by purely geological, globally detectable event markers. Based on such purely geological markers, the unit boundaries are now to be determined in the Precambrian as well. The consequences of this reorganization are: a. the elimination of the Eoarchic, the division of the Hadaic into two eras and the subdivision of the eras of the Archean into periods.

According to this proposal, the Precambrian was structured as follows:

  • Aeon: Proterozoic (2,420-541 mya). The newly defined eon is 80 million years shorter, it lasts 1,879 million years.
  • Aeon: Archean (4030-2420 mya). Duration 1,610 million years. The redefined aeon is 110 million years longer, it begins 30 million years earlier and ends 80 million years later.
    • Era: Neo-Archean (2,780–2,420 mya). The newly defined era lasts 360 million years.
      • Period: Siderium (2,630–2,420 mya). So far attributed to the Paleoproterozoic. Huge strip ore deposits were formed over 210 million years .
      • Period: Methanium (2,780-2,630 mya). In this 150 million year period, methanotrophic eukaryotes occur more frequently.
    • Era: Mesoarchic (3,490-2,780 mya). The newly defined era lasts 710 million years.
    • Era: Paleo-Archean (4,030-3,490 mya). The newly defined era takes the place of the omitted Eoarchean, it lasts 540 million years.
  • Aeon: Hadean (4600-4030 million years BP ). The 570 million year aeon is 30 million years shorter.

Continents and climate

The fossils of the Ediacaran fauna are found on different continents today. These stratigraphic findings have proven that the land masses were then merged into a "supercontinent", which was given the name Rodinia . Traces of Precambrian glaciations in Namibia and Canada also speak for one or more global ice ages in the Precambrian ( Snowball Earth ). Only the transition to a warmer climate made the Cambrian Explosion , better known as Cambrian Radiation , of life on earth at the beginning of the Cambrian possible.

See also

literature

Individual evidence

  1. ^ A b M. J. Van Kranendonk, Wladyslaw Altermann, Brian L. Beard, Paul F. Hoffman, Clark M. Johnson, James F. Kasting, Victor A. Melezhik, Allen P. Nutman, Dominic Papineau, Franco Pirajno: A Chronostratigraphic Division of the Precambrian - Possibilities and Challenges. In: Felix M. Gradstein, James G. Ogg, Mark Schmitz, Gabi Ogg (eds.): The Geologic Time Scale 2012. Volume 1, Elsevier BV, 2012, pp. 299–392, doi : 10.1016 / B978-0- 444-59425-9.00016-0
  2. ^ Felix M. Gradstein, James G. Ogg: The Chronostratigraphic Scale. In: Felix M. Gradstein, James G. Ogg, Mark Schmitz, Gabi Ogg (eds.): The Geologic Time Scale 2012. Volume 1, Elsevier BV, 2012, pp. 31–42, doi : 10.1016 / B978-0- 444-59425-9.00002-0
  3. ^ Felix M. Gradstein, James G. Ogg, Frits J. Hilgen: On the Geologic Time Scale . In: Newsletters on Stratigraphy . tape 45 , no. 2 , 2012, p. 171-188 , doi : 10.1127 / 0078-0421 / 2012/0020 .
  4. ^ A b C. Goldblatt, KJ Zahnle, NH Sleep, EG Nisbet: The eons of Chaos and Hades . In: Solid Earth . tape 1 , no. 1 , 2010, p. 1-3 , doi : 10.5194 / se-1-1-2010 .