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Early Precambrian (Archean) Life
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Lecture 15
3/20/98
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| | Today's discussion focuses on the early history of life and its impact on the planet |
| | Life's history may start with the oldest sedimentary rocks on the Earth. |
| | The oldest true fossils are from relatively complex organisms that lived 3.5 billion years ago |
| | These early organisms altered the earth's environment and set the stage for evolution of more complex organisms |
| | Isua Supergroup, Greenland |
| | sedimentary and volcanic rocks |
| | Warrawoona Series, Western Australia |
| | sedimentary rocks deposited in a nearshore evironment |
| | similar to modern cyanobacteria (blue-green algae) |
| | structures produced by sediment trapped in bactrial mats |
| | survive in limited (hyper-saline) environments |
| Atmospheric and Oceanic Changes |
| | Oxygen levels rise as a result of photosynthesis |
| | 2.5 to 1.8 Billion years ago |
| | Evidence for these changes |
| | Alternating beds of chert and iron oxides |
| | Commonly occur with stromatolites |
| | Form major iron-ore deposits around the world |
| | 2.5 to 1.8 billion years old |
| | Cannot occur in the presence of O2 |
| | Found as placer deposits in rocks older than 2.3 billion years |
| | Deposits from non-marine environments |
| | Contain a red tint from iron oxides in the sediment (rust) |
| | Oldest redbeds are around 2.3 Billion years old |
| | Results of increasing atmospheric O2 |
| | more complex life forms can evolve |
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