At the beginning of the Proterozoic, Earth's atmosphere was mostly ____.

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It is believed that during the early Proterozoic (2.5 to 1.6 billion years ago), Earth's atmosphere was mostly composed of carbon dioxide, with possibly small amounts of nitrogen, water vapor, and methane. It was not until later in the Proterozoic, around 2 billion years ago, that photosynthetic bacteria began to produce oxygen and gradually transformed the atmosphere to the oxygen-rich environment we have today.

At the Beginning of the Proterozoic, Earth's Atmosphere Was Mostly