Question

The early atmosphere of the earth was a chemically ______ environment, whereas the present atmosphere is a chemically ______ environment. The event that changed the earth’s atmosphere from the first to the second type was the evolution of ______.

Answer 1

The early atmosphere of the earth was a chemically reducing environment, whereas the present atmosphere is a chemically oxidising environment. The event that changed the earth’s atmosphere from the first to the second type was the evolution of photosynthesis.

Explanation:

Initially, the Earth’s atmosphere was rich in hydrogen, methane, ammonia, and water vapor, but lacked free oxygen, creating a reducing environment. The evolution of photosynthetic organisms that released oxygen into the atmosphere gradually transformed it into an oxidizing environment. This oxygen buildup also led to the formation of the ozone layer, which protected life from harmful UV radiation and allowed life to move from water to land.