Most of the oxygen (O2) in the Earth's atmosphere was thought to have been generated though photosynthesis. Plants use energy from the Sun, taking in carbon dioxide and releasing O2 as a byproduct. Over time, this oxygen has built up in our atmosphere to its current ratio of 21%, with the rest nitrogen and other trace gases.
This ratio is very important to the search for life in the Universe. Over the next few decades, a fleet of spacecraft and experiments are being built that will be so sensitive, they'll be able to analyze the atmosphere of a distant Earth-sized world. Find oxygen or ozone in that planet's atmosphere - so goes the thinking - and you've found a world with life.
Chances are we'll find at least a modest handful of "living" extrasolar planets in the next few decades. A lot will change after that first discovery of a distant, life-harboring world. We'll find ourselves abruptly living on a planet we know is far from unique, and that promises some interesting existential consequences.
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