1) Photosynthesis is a life-sustaining process that occurs in plants, algae, bacteria and some animals through various mechanisms like oxygenic photosynthesis, symbiosis with photosynthetic organisms, and stealing genes from algae. 2) It involves the light and dark reactions - where light reactions use solar energy to produce ATP and NADPH, and the dark reactions use these to fix carbon and produce sugars like glucose. 3) Photosynthesis emerged over 2 billion years ago and was first developed by cyanobacteria, changing the atmosphere by producing oxygen and allowing the evolution of aerobic life. It remains a crucial process providing food and oxygen for most living things.