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The Precambria1 - where the presence of...

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The Precambrian Archaea and Eubacteria are similar in size and shape. When we do recover fossilized bacteria those are the two features we will usually see: size and shape. How can we distinguish between the two groups: the use of molecular fossils that will point to either (but not both) groups. Such a chemical fossil has been found and its presence in the Ishua rocks of Greenland (3.8 billion years old) suggests that the archeans were present at that time. Many of the ancient phototrophs and heterotrophic bacteria lived in colonial associations known as stromatolites . Cyanobacteria occur on the outer surface, with other photosynthetic bacteria (anoxic, which do not produce oxygen from their photosynthesis process)) below them. Below these phototrophs are layers of heterotrophic bacteria. The layers in the stromatolites are alternating biogenic and sedimentologic in origin. Stromatolites become more common in the Proterozoic and decline during the Cambrian. Modern stromatolites are found in marine environments
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Unformatted text preview: where the presence of herbivorous :grazers" is limited. The Proterozoic Eon covers the time span from 2.5 billion to 544 million years ago. Simple, prokaryotic cells still dominated the world's environments until the evolution of simple eukaryotes approximately 1.5-1.2 billion years ago. With the appearance of eukaryotes comes the development of sexual reproduction, which greatly increased the variation that natural selection could operate on. A major enbvironmental change, initiated by living things, was the development of oxygenic photosynthesis. This led to increasing oxygen levels during later Proterozoic. Geologists refer to the "great iron crisis" when the rising levels of oxygen in the world's oceans caused the formation or iron oxide (Fe 2 O 3 ), often preserved as the banded iron formation (an important commercial source of iron)....
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The Precambria1 - where the presence of...

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