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Unformatted text preview: First signs of life Precambrian Oxygen levels were very low (there are pyrite grains in ancient sandstones you can erode and transport pyrite if there is oxygen in the atmosphere) The Early (secondary) atmosphere had little free oxygen, it let through UV radiation and was hostile to life. Meteorite bombardment was probably also a more important process then. The 3.8 B.Y . old Greenland rocks have carbon that is isotopically similar to that produced organically Stromatolites (fossil filamentous algae) are the first fossils found in sedimentary rocks. They date to 3.5 B.Y. old. These plants are Algae, very hardy and could have contributed to a buildup of oxygen. 3.5 Billion year old rock from South Africa reveals the remains of microscopic singlecelled marine organisms dividing, as does the 2 B.Y. old singlecelled marine organisms dividing, as does the 2 B....
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- Fall '10