The Proterozoic eon, oxygen, fossil fuels, and sex
The Proterozoic eon happened between 2.4 billion years ago and 550 million years ago. Throughout these 1.85 billion years, life, although solely unicellular, had ample time to generate vast amounts of oxygen and drastically change the Earth’s atmosphere. At the same time, their remains would accumulate and were destined to become the fossil fuel of today. Lasting four times more than the rest of history, it is no wonder why we haven’t run out of fossil fuels; it is very unlikely that we will run out before we drastically change the Earth’s atmosphere again—and this time not for the better. There was one other event that would have significant consequences on the future of the planet: the invention of sex. The earliest sexual reproduction (so necessary for genetic diversity) happened about 1.2 billion years ago.
Source: Carl Sagan, Cosmos (New York: Random House, 1980)
Image by Benoit Potin under CC BY-SA 3.0 license
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Full disclosure, I may occasionally borrow a sentence from Will Durant's Story of Civilization. I absolutely love that collection!