Several billion years ago, which gas was the least prevalent in the atmosphere

For the answer to the question above. The gas which was the least prevalent in the earth's atmosphere is oxygen or also known as O2. Because the earth that time is a void. There are no trees to provide Oxygen.

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oxygen

Explanation:
The first photosynthetic bacteria evolved on Earth around 2.4-3 billion years ago.

These bacteria used inorganic compounds such as water and carbon dioxide to produce their food and in return, they gave away glucose and oxygen.

This means that Oxygen was not prevalent until then.
This discovery is known as "The Great Oxygen Event".

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Answer( solar wind );