the earths atmosphere has changed a lot over the past 4 billion years. One big change was due to photosynthetic organisms. their photosynthesis increased the amount of what atmospheric gas

Answer 1

Here is a basic formula for photosynthesis:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Carbon dioxide (CO₂) and water (H₂O) are converted, through energy from the sun in the form of light, to a sugar molecule (C₆H₁₂O₆ ) and oxygen (O₂). The sugar molecule is used for energy or converted later into plant building materials. Some of the oxygen is used for cellular respiration, but some of it is released into the air. So the answer to your question is oxygen!