ABSTRACT
The Great Oxidation Event added oxygen to the atmosphere via photosynthesis and life,-removing hydrogen and carbon. Cyanobacteria, phytoplankton, and land plants all contributed to increasing atmospheric oxygen by photosynthesis. Rainforests and phytoplankton working together increase oxygen a great deal. Elevated oxygen also caused the diversification of Earth’s minerals, providing new niches for microbes. Oxygen is now 21% of the atmosphere due almost entirely to life. Without life’s building oxygen to high levels, higher life would not be possible. High oxygen levels generated by life formed the ozone layer that protects life from the sun’s ultraviolet radiation, controlled levels of the greenhouse gas methane and toxic hydrogen sulfide, and allowed favorable levels of fire for life. Oxygen is at near optimal levels for life, and has been so for much of the last 350 million years, because of life-generated negative feedback loops. There is also a negative feedback loop involving fire, keeping it at favorable levels for life. A favorable level of fire recycles nutrients, makes nutrients available to plants, and conserves water in forests. Life and Earth’s atmosphere coevolved.
