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					| Early Precambrian (Archean) Life |  
					| 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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