Question

What component of Earth's atmosphere exists entirely as a result of photosynthesis?

Answer 1

Oxygen is primarily produced by photosynthesis. In the beginning there was no free oxygen available in earth's atmosphere: It started to appear about 3.5 billion years ago due to evolution of photosynthetic bacteria.

Explanation:

Green plants and blue green algae use photosynthesis to produce Oxygen as a side product and Carbon Hydrates such as Glucose sugar for energy.

Most of the Oxygen in the atmosphere comes from photosynthesis

`6 CO_2 + 6 H_2O ===== 1 C_6H_12O_6 + 6 O_2`

However some Oxygen is produced by the decomposition of water in the atmosphere

`2 H_2O ===== 2 H_2 + 1 O_2`

The volcanos release volumes water that are decomposed into Oxygen and Hydrogen

Kenneth Towe concluded that the "early Earth very likely had an atmosphere that contained free oxygen (Nature 274 1978)