Tropospheric oxygen at 21 percent is where human beings and most animals live. Interesting Earth is just right.

...Figures 1 and 2 (click here) show the progression of O₂ over earth's geologic history. While possibly not at the peak experienced 10's of millions of years ago, the level of atmospheric oxygen levels (~21%) are high enough to allow Earth's ecological systems (and humans) to flourish. Significant levels of O₂ are, cosmically, really quite rare! Oxygen will chemically react with almost anything, so having such a large amount of oxygen available in the atmosphere means that it must be continuously produced. The oxygen is produced by photosynthesis, but if that were to ever stop chemical reactions would make it all go away quite rapidly. If the chemical signature of molecular oxygen were found on another planet it would be a sure sign of life, since molecular oxygen wouldn't exist without life creating it....

Ya think chlorophyll would get some respect in the year 2021.

August 19, 2009 

By David Biello

It's hard to keep oxygen molecules around, (click here) despite the fact that it's the third-most abundant element in the universe, forged in the superhot, superdense core of stars. That's because oxygen wants to react; it can form compounds with nearly every other element on the periodic table. So how did Earth end up with an atmosphere made up of roughly 21 percent of the stuff?

The answer is tiny organisms known as cyanobacteria, or blue-green algae. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them down to this day.

For some untold eons prior to the evolution of these cyanobacteria, during the Archean eon, more primitive microbes lived the real old-fashioned way: anaerobically. These ancient organisms—and their "extremophile" descendants today—thrived in the absence of oxygen, relying on sulfate for their energy needs....