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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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