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There was not molecular oxygen in early Earth's atmosphere. It was not until photosynthetic blue-green algae began to release oxygen by taking in carbon dioxide, sunlight, and water to create glucose and oxygen. Today the atmosphere is about 21% oxygen due to the delicate dance of photosynthesis/cellular respiration from producers and consumers. The original atmosphere was composed of hydrogen and helium until volcanic eruptions released carbon dioxide, Nitrogen, water vapor, sulfates, ammonia, and Methane.
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