Answer:
The process of photosynthesis produced more oxygen that changed the Earth's early atmosphere.
Volcanic eruptions occurred almost constantly on the early Earth. Eruptions put water vapor, carbon dioxide and other gases into the air that helped create Earth's early atmosphere. Slowly, two processes changed Earth's atmosphere to one that is more oxygen-rich—like the one we have today.
or
Earth's original atmosphere was probably just hydrogen and helium, because these were the main gases in the dusty, gassy disk around the Sun from which the planets formed.
Explanation:
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