The Proterozoic eon is a time in the Earth's history that occurred from 2.5 billion to 543 million years ago. Many important events in the geologic history of the Earth occurred during this eon. For example, stable continental masses first appear and began to significantly grow in size through the accretion of sedimentary deposits. Also occurring during this time is the fossilization of our planet's earliest organisms, mainly bacteria and archaeans. By about 1.8 billion years ago, eukaryotic organisms begin to appear in the fossil record as well. Finally, multicellular eukaryotic life-forms become common near the end of the Proterozoic.
During the middle of Proterozoic oxygen concentration in the atmosphere increases significantly. This global change reduced the abundance of many anerobic bacterial groups, but made possible for the domination of aerobic eukaryotic life-forms on our planet. These include multicellular algae, and toward the end of the Proterozoic, the first animals.
Geology of the Proterozoic
The Proterozoic is divided up into three eras: Paleoproterozoic (2.5 to 1.6 billion years ago), Mesoproterozoic (1.6 billion to 900 million years ago) and Neoproterozoic (900 to 543 million years ago) (Figure 1).
Near the beginning of the Proterozoic, stable continents first appeared and began to accrete, a long process taking about a billion years. Before the Proterozoic, the Earth's magma near the surface was hotter and less viscous, allowing the continents to move around more freely, leading to considerable crustal instability. Massive basaltic flood eruptions were common throughout the Proterozoic. A classic example of this type of volcanic flow occurred on the Canadian Shield of North America and is known as the Mackenzie dike swarm. This geologic event was the most extensive volcanic eruption of its kind on Earth. The source of the basaltic magma involved in this event is a mantle plume known as the Mackenzie hotspot.
In addition to dramatic eruptive events, the Proterozoic was a time of considerable rock weathering, erosion, and sedimentary deposition. For example, the Torridonian formations in western Scotland are extensive sedimentary deposits laid down during the Neoproterozoic (Figure 2). 
Analysis of Proterozoic geological formations that still exist today provides important clues in reconstructing the location, movement and development of ancient continents. For example, deductions regarding the development of the ancient continent of Laurentia can be determined from mapping of the exposures of Meso-Proterozoic outcrops of the Grenville orogen in Texas, the Adirondacks, and parts of Canada. At about 1.0 billion years ago, plate tectonic processes united the various continental masses into one supercontinent known as Rodinia. Rodinia broke-up about 800 million years ago.
Change in Atmospheric Chemistry
About 2.2 billion years ago the composition of the Earth's atmosphere changes radically. This change involves a significant increase in oxygen levels. Two pieces of evidence suggest this: 1) the presence of iron oxides in paleosols (fossil soils), and 2) the appearance of red colored sedimentary beds rich in metal oxides. Atmospheric oxygen levels in the Archaean (more than 2.5 billion years ago) had been less than 1% of present day levels. By about 1.8 billion years ago, oxygen levels were about 15% of current levels and rising steadily. This change in the Earth's atmospheric chemistry is commonly called the Great Oxygenation Event. This oxygen came from cyanobacteria (Figure 3) and algae which produced the oxygen as a byproduct of photosynthesis. The process of photosysthesis also removes carbon dioxide from the atmosphere by incorporating it into the production of carbon based organic molecules found in life.
Scientists refer to this drastic change in the composition of the Earth's atmosphere as a pollution crisis because of its negative environmental effect on the dominant anerobic life-forms alive at this time. Many species of bacteria and protists were killed off by the presence of higher concentrations of oxygen. As a result of this change, new types of organisms evolved with adaptations to biochemically render oxygen metabolically harmless. One of these biochemical methods, oxidative respiration found in most present day eukaryotic life-forms, had the advantage of producing large amounts of energy for the organism.
Climate of the Proterozoic
The climate of the Proterozoic can be characteriized by the appearance and retreat of our planet's first two great ice ages. Huronian Ice Age took place between 2.4 to 2.1 billion years before present. Geological evidence for this event comes from tillite deposits and glacial erosion artifacts found in North America and western Australia. Researchers believe the catalyst for this ice age was a sudden and drastic change in the chemical composition of the atmosphere. Before this event, the Earth’s atmosphere had significantly higher concentrations of carbon dioxide and methane, two strong greenhouse gases that add significant amounts of heat energy to the lower atmosphere. However, at about 2.4 billion years ago, the expansion of photosynthetic life caused the concentration of oxygen to rise significantly in the atmosphere. This Great Oxygenation Event changed the radiative characteristics of the atmosphere and decreased the amount of heat being generated by the Earth's natural greenhouse effect.
The next major ice age occurred between 850 to 600 million years ago. This time period, known as the Cryogenian, saw glaciation extend to the equatorial region. Scientists believe this glaciation event was the most severe in the history of the Earth. The trigger for this cataclysmic freeze, often called Snowball Earth, may have been the tectonic repositioning of the major continental land masses at the poles. The end of the Cryogenian coincides with the Cambrian Explosion of life and theappearance of many species of higher life-forms (Figure 4). The later Proterozoic climate was generally characterized by strongly seasonal variations in weather. Stratigraphic records indicate several episodes of abrupt climate change. Conditions were not only cold, but also arid and of high prevalent wind velocity.
Life and the Proterozoic
The first traces of life on Earth appeared nearly 3.5 billion years ago, in the early Archaean. However, clearly identifiable fossils remain rare until the late Archaean, when stromatolites, layered mounds produced by the growth of microbial mats, become common in the fossil record. Stromatolite diversity continued to increase through most of the Proterozoic. Until around one billion years ago, they flourished in shallow waters throughout the world. Their importance for understanding Proterozoic life is tremendous; stromatolites that have been silicified (forming a type of rock known as stromatolitic chert) often preserve excellent microfossils of the microbes (like cyanobacteria) that produced them (Figure 5).
In general, asexual prokaryotes and eukaryotic phytoplankton were prevalent life-forms that thrived in the early Proterozoic. Sexually reproducing arge celled phytoplankton appeared in large numbers in the later Proterozoic. Many of these organisms became extinct by the close of the Protoerozoic. Stromatolites began to decline in abundance and diversity starting about 700 million years ago. A popular theory for their decline (though certainly not the only possible explanation) is that herbivorous eukaryotes, perhaps including the first animals, evolved at about this time and began feeding on them. Stromatolites become rare in the fossil record after about 450 million years ago. Today, they are found only in restricted habitats with low levels of grazing, such as the shallow, saline waters of Shark Bay, Australia (Figure 6).
The oldest fossil that may represent a multicelluar organism is about 2.1 billion years old. Several types of fossil that appear to represent simple multicellular forms of life are found by the end of the Paleoproterozoic and become most common in the Neoproterozoic. These fossils, known as carbon films, are thin film coatings that are composed mainly of the chemical element carbon. These carbon films often resembling circles, ribbons, or leaves. Some resemble seaweeds and may represent eukaryotic algae. We know from independent evidence that red algae and green algae did first appear in the Proterozoic, probably over one billion years ago.
There are tantalizing hints from trace fossils and research in molecular biology that animals may have appeared as much as one billion years ago. However, the oldest relatively non-controversial, well-studied animal fossils appear in the last hundred million years of the Proterozoic, just before the Cambrian radiation of taxa. The time from 600-650 million years ago to 543 million years ago, known as the Vendian period, saw the origin and first diversification of soft-bodied organisms known collectively as the "Vendian fauna" or "Ediacara fauna" (after the Ediacara Hills of southern Australia, where the first abundant and diverse fossils of this kind were found).
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