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The Oxygen Catastrophe, the first of several mass extinctions and die-offs. The Great Oxidization was the first of several times in the Earth’s history when Life has been threatened by dramatic changes. Asteroid impacts, tremendous volcanic action, continental shifts resulting in drastic changes in climate, and the emergence or transmission of new life forms, whether larger animals or microscopic germs, can upset old patterns of life and wipe out species. The boundary between the Permian age and the Triassic (about 250 millions years ago) was one of the great mass extinctions, and so was the more famous K-T boundary at the end of the Cretaceous, about 65 million years ago. Before these, there were tough times for life during the Cryogenian eras
(most recently ending about 630 million years ago). One of the earliest mass die-offs probably occurred
about 2.7 billion years ago when some microorganisms began to use photosynthesis, which gives off elemental Oxygen as a byproduct.
Oxygen shot up from 0.002 percent of the atmosphere to something closer to the level of elemental oxygen in our atmosphere
today (20 percent). Many older microorganisms had evolved in a low-oxygen atmosphere, and they were killed off by the new higher concentrations of oxygen.
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