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Subject Item
dbpedia:Geological_history_of_oxygen
rdfs:comment
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2). Photosynthetic prokaryotic organisms that produced O2 as a waste product lived long before the first build-up of free oxygen in the atmosphere, perhaps as early as 3.5 billion years ago. The oxygen they produced would have been rapidly removed from the atmosphere by weathering of reducing minerals, most notably iron.
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