The "close calls" of planetary catastrophe are repeatedly written large in the rock record of Earth. We have teetered on the brink many times. In this chapter we have recounted the obvious near misses, as manifested by the various mass extinctions. Yet a more subtle if no less sobering record can be found in the changes in atmospheric composition through time.
It is clear that the levels of carbon dioxide and oxygen have changed dramatically during the Phanerozoic (the last 530 million years), and these changes may themselves have been trivial compared to the longer but less readily sampled Precambrian interval of time. The ecological effects of these changes are very poorly understood. During the Paleozoic Era, CO2 values 20 times that of the present day are now confidently inferred to have been present in the lower Paleozoic, followed by a rapid decline in the Permo-Carboniferous time. The world then underwent a massive glaciation, as its greenhouse conditions gave way to far cooler climates.
Changes in the amount of oxygen in the atmosphere have also been profound, but they are far less well documented (or understood) than for carbon dioxide. Estimating ancient oxygen levels is fraught with uncertainty: One scientific group extracts oxygen from ancient amber, but others decry this method as yielding totally erroneous results. Without direct readings, we must estimate the ancient oxygen values in the atmosphere by studying the rate of organic carbon burial in sediment through time or by studying rates of rock weathering. Neither approach is very satisfactory. Yet if these methods have any validity, they tell us that the amount of oxygen has varied over the last 500 million years. For instance, there appear to have been far higher levels of oxygen in the atmosphere around 400 to 300 million years ago; levels as high as 35% by volume (compared to 21% today) are possible. This much oxygen would have made forest fires far more common and more devastating. There were also periods of depressed oxygen, and it is not too farfetched to envision scenarios where lowered oxygen levels had large-scale effects, perhaps even inhibiting the evolution or development of certain forms, such as those with very high metabolisms requiring abundant oxygen.
Thus even on a planet such as ours, the most important of all systems for life—the atmosphere—can be unstable over an interval 100 million years long. Planetary atmospheres can change enough to cause mass extinctions, and maintaining an atmosphere conductive to animal life for the staggering periods of time necessary for animal life to evolve and diversify may be the most difficult feat of all.
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