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, 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. Of course, in the absence of plants, photosynthesis was slower in the Precambrian, and the levels of O2 attained were modest (less than 10% of today's) and probably fluctuated greatly; oxygen may even have disappeared from the atmosphere again around  These fluctuations in oxygen had little direct effect on life, with mass extinctions not observed until the appearance of complex life around the start of the Cambrian period, . 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.:214, 586
Since the start of the Cambrian period, atmospheric oxygen concentrations have fluctuated between 15% and a maximum of 35% of atmospheric volume towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of insects and amphibians at that time. Whilst human activities, such as the burning of fossil fuels, have an impact on relative carbon dioxide concentrations, their impact on the much larger concentration of oxygen is less significant.
Effects on life
The concentration of atmospheric oxygen is often cited as a possible contributor to large-scale evolutionary phenomena, such as the origin of the multicellular Ediacara biota, the Cambrian explosion, trends in animal body size, and other extinction and diversification events.
The large size of insects and amphibians in the Carboniferous period, where oxygen reached 35% of the atmosphere, has been attributed to the limiting role of diffusion in these organisms' metabolism. But Haldane's essay points out that it would only apply to insects. However, the biological basis for this correlation is not firm, and many lines of evidence show that oxygen concentration is not size-limiting in modern insects. Interestingly, there is no significant correlation between atmospheric oxygen and maximum body size elsewhere in the geological record. Ecological constraints can better explain the diminutive size of post-Carboniferous dragonflies - for instance, the appearance of flying competitors such as pterosaurs and birds and bats.
Rising oxygen concentrations have been cited as a driver for evolutionary diversification, although the physiological arguments behind such arguments are questionable, and a consistent pattern between oxygen levels and the rate of evolution is not clearly evident. The most celebrated link between oxygen and evolution occurs at the end of the last of the Snowball glaciations, where complex multicellular life is first found in the fossil record. Under low oxygen levels, regular 'nitrogen crises'[clarification needed] 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. Further, anoxic or otherwise chemically 'nasty' oceanic conditions that resemble those supposed to inhibit macroscopic life re-occur at intervals through the early Cambrian, and also in the late Cretaceous – with no apparent impact on lifeforms at these times. This might suggest that 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 planktivory.
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