Precambrian

The Precambrian ( /priˈkæmbri.ən, -ˈkeɪm-/ pree-KAM-bree-ən, -⁠KAYM-;[2] or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon.

[4] This is because many Precambrian rocks have been heavily metamorphosed, obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata.

[10] Because the span of time falling under the Precambrian consists of three eons (the Hadean, the Archean, and the Proterozoic), it is sometimes described as a supereon,[11][12] but this is also an informal term, not defined by the ICS in its chronostratigraphic guide.

Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin.

By the middle of the following Cambrian Period, a very diverse fauna is recorded in the Burgess Shale, including some which may represent stem groups of modern taxa.

[30][31] While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas.

[35] During the Hadean Eon (4,567–4,031 Ma) abundant geothermal microenvironments were present that may have had the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form.

[36] It was shown that porous rock systems comprising heated air-water interfaces could allow ribozyme-catalyzed RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes.

Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and was lacking in free oxygen.

[40] At present, it is still believed that molecular oxygen was not 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 a chemically inert to an oxidizing atmosphere caused an ecological crisis, sometimes called the oxygen catastrophe.

It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than the current scheme based upon numerical ages.