Astrobiology, which provides justifications for the current and future space missions, is the study of the origin, evolution, distribution and destiny of life in the universe (including our planet). I will review the current controversies concerning the evolution of major organisms (cyanobacteria, methanogens, methanotrophs, sulfate reducers, and eukarya) and their effects on the environment (atmosphere and ocean chemistry) on early Earth. I will also present new data (geochemical and biochemical) obtained from 3.8-1.8 billion-year-old rocks, which suggest the divergence of major organisms and the development of an oxygenated atmosphere took place more than 3.5 billion years ago.