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That time oxygen almost killed everything


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What if we told you that there was a time when oxygen almost wiped out all life on Earth? 3 billion years ago, when the world was a place you’d never recognize, too much of a good thing almost ruined everything for everybody.

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Autotrophs have always ruled the planet, first as chemoautotrophs in hydrothermal vents and later as photoautotrophs in shallow, sunlit seas. That is because at the beginning there was no other life to consume, making it impossible for heterotrophs.

Although the first heterotrophic microbes likely emerged soon after the autotrophs, complex animal life which then gave rise to complex plant life through endosymbiosis was only possible much later. Arguably the most important driver was the Great Oxidation Event caused by the emergence of oxygenic photosynthesis. After pioneering photosynthesis, cyanobacteria likely took an evolutionary hitch into the open ocean on the very animals they allowed to exist and still play an important role in the pelagic zone today. Oxygen was so critical because aerobic metabolism yields more energy and seems to be necessary for multicellular life. That is why ocean dead zones with little oxygen are such a cause for concern today.

The Great Oxidation Event was the first and definitely not the last planetary-scale intervention by plants. Oxygen produced by the arrival of more complex marine plants assisted the Cambrian Explosion. When they first appeared on land, plants enhanced rock weathering which resulted in the opposite effect: ocean deoxygenation and ultimately the End-Ordovician Mass Extinction. And in the Carboniferous, scale trees buried massive amounts of organic carbon, which caused planetary cooling and powered human industrialization through fossil fuels, ultimately leading to the Anthropocene Extinction.

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