http://en.wikipedia.org/wiki/Geological_history_of_oxygen
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2).Oxygen was first produced by photosynthetic prokaryotic organisms that emitted O2 as a waste product. These organisms lived long before the first build-up of oxygen in the atmosphere,[3] perhaps as early as 3.5 billion years ago. The oxygen they produced would have almost instantly been removed from the atmosphere by weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event. This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current atmospheric rates, today's concentration of oxygen could be produced by photosynthesisers in 2,000 years.
The presence of O
2 provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore.
Under low oxygen levels, regular 'nitrogen crises' could render the ocean inhospitable to life. Significant concentrations of oxygen were just one of the prerequisites for the evolution of complex life.
Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a level was only reached immediately before metazoa first appeared in the fossil record.
The geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian - perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of photosynthesisers.
Before photosynthesis evolved, Earth's atmosphere had no free oxygen (O2).Oxygen was first produced by photosynthetic prokaryotic organisms that emitted O2 as a waste product. These organisms lived long before the first build-up of oxygen in the atmosphere,[3] perhaps as early as 3.5 billion years ago. The oxygen they produced would have almost instantly been removed from the atmosphere by weathering of reduced minerals, most notably iron. This 'mass rusting' led to the deposition of banded iron formations. Oxygen only began to persist in the atmosphere in small quantities about 50 million years before the start of the Great Oxygenation Event. This mass oxygenation of the atmosphere resulted in rapid buildup of free oxygen. At current atmospheric rates, today's concentration of oxygen could be produced by photosynthesisers in 2,000 years.
The presence of O
2 provided life with new opportunities. Aerobic metabolism is more efficient than anaerobic pathways, and the presence of oxygen undoubtedly created new possibilities for life to explore.
Under low oxygen levels, regular 'nitrogen crises' could render the ocean inhospitable to life. Significant concentrations of oxygen were just one of the prerequisites for the evolution of complex life.
Models based on uniformitarian principles (i.e. extrapolating present-day ocean dynamics into deep time) suggest that such a level was only reached immediately before metazoa first appeared in the fossil record.
The geochemical signatures found in ocean sediments reflect the atmosphere in a different way before the Cambrian - perhaps as a result of the fundamentally different mode of nutrient cycling in the absence of photosynthesisers.
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