How would earth's atmosphere be different if organisms capable of photosynthesis has not evolved?

The Earth's atmosphere would have little or no oxygen. A by-product of photosynthesis is oxygen.

Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight into chemical energy. Oxygenic photosynthesis functions as a counterbalance to respiration; it takes in the carbon dioxide produced by all breathing organisms and reintroduces oxygen into the atmosphere.