Once could argue that the most important event in the history of the Earth after its formation was the beginning of photosynthesis, about 2.7 billion years ago. This development by microscopic plants paved the way for oxygen to enrich the atmosphere, and allowed for oxygen-dependent animal life, including humans.
Even after oxygen began to accumulate in the atmosphere, it was many hundreds of millions of years, and many complex chemical reactions before the atmosphere was anything like it is today. And the growing concentration of oxygen was a serious crisis for the first life that appeared on earth—so much so that such life is relegated to obscure corners of the planet today.
Drawing by At09K9 via Wikipedia under Creative Commons license.
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