photosynthesis is a source of oxygen & a source of something increases the amount of that something ->  photosynthesis increases the amount of oxygen in the atmosphere  photosynthesis increases the amount of oxygen in the atmosphere & the photosynthetic life performs photosynthesis ->  photosynthetic life increases the amount of oxygen in the atmosphere  early photosynthetic life is a kind of photosynthetic life & early photosynthetic life existed a long time ago ->  early photosynthetic life is a kind of photosynthetic life that existed a long time ago  photosynthetic life increases the amount of oxygen in the atmosphere & early photosynthetic life is a kind of photosynthetic life that existed a long time ago ->  early photosynthetic life increased the amount of oxygen in the atmosphere a long time ago  early photosynthetic life increased the amount of oxygen in the atmosphere a long time ago & earth had no / little oxygen a very long time ago & oxygen level describes an amount of oxygen -> early photosynthetic life increased the oxygen level in earth 's atmosphere