The level of oxygen in the atmosphere has gradually increased because of the steady increase in the earth's population of photosynthetic organisms--some of them still in their original bacterial form, the blue-green algae living in the waters of the earth, others now in the more complex and various forms of higher plants. From a level close to zero 3.5 billion years ago, oxygen now stands at 20 percent of the earth's atmosphere, all of it the product of life itself. There is another feature of that level of oxygen that seems to me equally remarkable: it stabilized at its present level around 400 million years ago, and it seems to be fixed there. It is a lucky thing for us, and for the life of the earth, that it did stabilize at that concentration. If it were to increase by more than two percentage points, most of the planet would ignite. If it were to decrease a few points, most of the life would suffocate. It is nicely balanced, against all hazards, at an absolutely optimal level.
Other gases in the atmosphere, including carbon dioxide, nitrogen, and methane, also seem to have been regulated and stabilized over long periods of time at optimal concentrations, despite the constant intervention of natural forces tending to push them up or down. At the moment, which is to say over the past century, the level of carbon dioxide has been rising slowly due to the heavy increase in the burning of fossil fuels, but the cliamtic consequences of this rise have not been observed.
Methane exists as a quantitatively minor constituent of the atmosphere, although it plays a crucial role in the intrests of living things. Most of what is there is the product of life itself, the tremendous populations of methanogenic baceria in the soil and water, in the intestinal tracts of ruminant vertebrates, and (a substantial source) in the hindgut of termites. How it is regulated so that the methane concnetrations are everywhere fixed and stable is not known, but it is known that if the level were to decrease appreciably the concentration of oxygen in the atmosphere would begin to rise to hazardous levels. There is probably a feedback loop, in which methane serves as a regulator of oxygen and vice versa.
The mean temperature at the surface of the earth has also remained remarkably stable over stretches of geological time, perhaps due in large part to the relatively stable concentrations of carbon dioxide in the atmosphere. From time to time fluctuations have occurred, with the cyclical development of ice ages as the result, but over all time, the temperature stays much the same. This also suggests a regulatory mechanism of some sort, since the radiant heat coming from the sun has increased by approximately 30 percent since life began.