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Precambrian time comprises that long period of geological history between the formation of the Earth, now estimated on the basis of meteorite analysis to have occurred 4.6 billion years ago, and the beginning of the Phanerozoic Eon, about 600 million years ago. The total duration thus is nearly 4 billion years, more than 80 percent of all geologic time.
The name stems from using the beginning of the Cambrian Period, the oldest major time unit of the Phanerozoic Eon, as the temporal reference. Prior to about 1965 the spelling pre-Cambrian prevailed, and this form still is preferred by some, notably those who favor the establishment of a still older time period within the Phanerozoic Eon. The term Cryptozoic, with essentially the same chronological definition as Precambrian but with connotations concerning life forms, was used for many years but is now obsolescent.
Unlike the Phanerozoic Eon, which has an internationally accepted hierarchy of time units that include 3 eras, 12 periods, and more than 40 epochs, the structurally complex and sparsely fossiliferous Precambrian is entirely lacking in standard subdivisions. Most of the schemes that have been advanced are rudimentary in form and provincial in derivation and application. A typical example of a regional scheme is that used for the United States by the U.S. Geological Survey. In it the Precambrian is divided into two eons--the older Archean and the younger Proterozoic--separated by a boundary set at 2.5 billion years ago. The Proterozoic in turn is divided into three subunits: Precambrian X (2.5 billion to 1.6 billion years ago), Precambrian Y (1.6 billion to 0.8 billion years ago), and Precambrian Z (0.8 billion to 0.6 billion years ago). The terms Proterozoic and Archean, with definitions comparable to those given above, appear in many regional time scales, including those of Canada, Russia, India, and Australia; so it now appears likely that this twofold subdivision will become the first element of an internationally accepted time scale for the Precambrian.
The lower limit of the Archean is essentially the age of the oldest known rocks, about 3.8 billion years ago. The unnamed earliest phase of Earth history, from the time of origin to 3.8 billion years ago, can only be inferred from evidence derived from meteorites and other solar bodies and from the inherited character of isotopic systems such as rubidium-strontium and lead-uranium.
Evidence of Life
Research has shown that biologic activity extends far back into Precambrian time. What appear to be fossilized algae or bacteria are found in South African rocks with an age in excess of 3 billion years. The existence of microbiota in these old rocks is further indicated by the presence of stromatolites, which are domal calcareous structures, in part of organic origin. Stromatolites become more common in carbonate rock of older Proterozoic age, such as the Transvaal Supergroup of South Africa; in strata of younger Proterozoic (Riphean) age in Russia they are sufficiently abundant and diverse to furnish a basis for time-stratigraphic classification.
Microfossils are particularly well preserved in certain dark cherts, notably in the Gunflint Formation of Ontario, with an age of about 2.0 billion years. Nucleated forms (eukaryotes) occur in younger Precambrian strata, as in the 850-million-year-old Bitter Springs chert of Australia. A few shelly fossils of tubular-shaped organisms appear in the rocks of the late Precambrian age (700-570 million years ago)--Cloudina, found in Namibia, and Sinotubulites, found in southern China. Both had shells composed of calcium carbonate, and they are considered to be precursors of the much more variegated shell fauna of the Cambrian age.
Much controversy has surrounded the Ediacaran fauna (first discovered in 1947 in the Ediacara Hills of South Australia), which also arose in the late Precambrian age. Paleontologists originally posited these animals as ancestors of certain jellyfish, worms, and soft corals. Further research led scientists to view the Ediacaran fauna as a distinct form of life that became extinct at the onset of the Cambrian age. However, findings in the early 1990s support the original view of a relationship to later animals. Characterized by flat, leaflike, sometimes quilted bodies, the water-dwelling Ediacaran fauna may have carried out metabolic functions directly through their outer skin (instead of developing complex inner organs as other multicellular fauna have done).
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