PRECAMBRIAN

Archean (Archeozoic): 4.6 to 2.5 billion years ago

Proterozoic: 2.5 billion years ago to 542 million years ago

CONTINENTS:
Existing evidence indicates that most of the modern continental landmasses formed early in earth's history. Although these continents have been joined during various periods, they have spent vast amounts of time apart as well. These continents were probably smaller and thinner earlier in earth's history and expanded as additional land masses fused to them. Ancestral North America, for example, probably began as the Canadian Shield consisting of central Canada and North Central U.S. Portions of the central U.S. were added in stages between 1 and 2 billion years ago. The eastern and western margins of North America, in addition to Mexico and Central America, were added in the last billion years. Ancestral North America included Greenland, part of Siberia, Norway, Scotland, and Ireland (Stokes, 1982).

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(One method of determining whether continents were separate or joined during a certain epoch is to examine rocks whose metallic elements indicate the direction of magnetic north at the time the rock was formed. Magnetic north has varied over time and, if two landmasses are joined, the record of polar wandering recorded in rocks that formed at different times should coincide. If two landmasses were not joined during a specific period of earth's history, the record of polar wandering on each would not coincide.)


By the end of the Precambrian (the Late Proterozoic Eon), there were five continents: Ancestral North America (also known as Laurentia), Ancestral Europe, Gondwana (a giant landmass composed of Africa, South America, Australia, Antarctica, and India), Ancestral Siberia, and Ancestral China. Ancestral North America included areas which today compose Ireland and Northwest Britain, Western Norway, and Northeastern Siberia but lacked the southeastern U.S., Mexico, and Central America. Much of the latter areas were attached to the African continent. The land which composes the modern Middle East was divided between Ancestral China and Gondwana.


During the Precambrian, there is evidence that these ancestral continents occasionally joined only to subsequently separate. About 1.1 billion years ago, a giant supercontinent consisting of North America, Europe, and Gondwana seems to have existed. North America seems to have been joined to Europe around 1.7 billion years ago and to Gondwana around 2.6 billion years ago. Many of earth's oldest rocks are known from North America (in areas such as Greenland, Labrador, and Minnesota). Somewhere during the interval of 2.7 to 2.2 billion years ago, a giant meteor struck the Hudson Bay area; the modern portion of the crater from this impact has a radius of 237 km (Seyfert, 1979).

At the end of the Precambrian, the Pacific edges of North America, South America, Australia, and Antarctica were formed. The western part of North America seems to have been joined to Antarctica at that time. The landmasses, which had been united in a supercontinent known as Rodina, began to separate (Dalziel, 1985).



CLIMATE

The primordial earth would have lacked a solid surface and an atmosphere. It would have been bombarded with meteorites and it is thought that a large collision very early in earth's history resulted in the formation of the moon. As comets bombarded the earth, the water they delivered formed oceans. The earliest atmosphere did not have much oxygen gas. Before photosynthesis, so much oxygen would have been absorbed by reduced geothermal outputs and by weathering rocks that very little would have been present in the atmosphere (Des Marais, 2000). By 2.1 billion years ago there was enough oxygen in the atmosphere to rust iron for the first time and by about 1.8 billion years ago oxygen was oxidizing significant amounts of iron. Other signs of oxygen also appear about this time such as sterane biomarkers from 1.7 billion years ago and oxidized sulfur deposits from about 1.6 billion years ago.

In the late Proterozoic, the earth experienced a number of great changes including severe ice ages. At its coldest, the temperature of the earth may have dropped to –50oC. Such frigid global temperatures would have frozen all landmasses and the oceans might have been covered with ice sheets. Life may have been restricted to hot springs and other areas of geothermal activity and to meltwater pools on the surface of the ice. These glaciations may have caused the extinctions of microscopic protist-like organisms referred to as acritarchs (Chuanming, 2001). Without evaporation from the oceans and the resultant rain, carbon dioxide from volcanic activity would have accumulated unchecked. Carbon dioxide levels would eventually greatly exceed modern concentrations (maybe by a thousand times) and this would lead to the warming of earth. This warming might have been quite extreme, perhaps to 50 degrees Celsius (Hoffman, 1998; Minnerop, 2000; Knoll, 1986).

 

LIFE ON LAND

The earth's surface has been devoid of life for almost all of earth's history. The Precambrian represents the majority of the history of life, yet not a single mammal, bird, reptile, amphibian, insect, spider, or plant existed during this time. Not only had these groups yet to evolve, the low levels of oxygen, the lack of an ozone layer to block harmful radiation from the sun, and ice ages would have inhibited terrestrial life during this time.

MARINE LIFE

Although the Archean represents about a third of the history of life on earth, the only form of life known during this time were the simplest cells similar to modern bacteria.

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In the Proterozoic, a larger, more complex cell type evolved. Ancestral organisms with this type of cell became multicellular and evolved into algae, fungi, and animals.

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The final part of the Proterozoic Eon is identified as the Ediacaran Period (630 to 542 million years ago). During this time, the simplest animals diversified to produce the ancestors of diverse lineages of higher animals. While most of this life was very small, some flat worms and wormlike forms could reach lengths of 2 to 4 meters. No fish or advanced invertebrates (higher arthropods and mollusks) are known.

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