Why was the evolution of photosynthesizing bacteria important to most modern organisms? these bacteria released free oxygen into earth's atmosphere for the first time; as their numbers grew, the amount of oxygen in earth's atmosphere grew?

Without oxygen, life cannot be sustained as it is an essential reactant in the respiration process. So the evolution of photosynthesizing bacteria increased the amount of oxygen in the atmosphere through converting water and carbon dioxide into oxygen with the help of sunlight. This provides a suitable environment for plants and vegetation to grow; plants also photosynthesise so they encouraged the increase of oxygen in the atmosphere even further, sustaining a suitable environment for the Animalia kingdom to evolve. Thanks to this, we now have a huge range of different modern species of animals and plants.