For the first 1 billion years of the Earth's evolution, the fraction of oxygen in its atmosphere was approximately:

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Quizplus · Accepted Answer

The first 1 billion years of the Earth's evolution is known as the Hadean Eon, during which the Earth's atmosphere was primarily composed of carbon dioxide, water vapor, nitrogen, and small amounts of other gases. Oxygen was absent from the atmosphere during this time because it was not being produced by photosynthesis. The appearance of oxygen in the atmosphere only occurred later, during the Great Oxygenation Event about 2.4 billion years ago.

For the First 1 Billion Years of the Earth's Evolution