Our Earth has an atmosphere today only because of outgassing of the planetary interior, mainly through volcanic action after surface temperatures had fallen. The gases that were emitted were not those that were most common in the original material of the solar system, but those that could be trapped in chemical combination with minerals: water vapor, ammonia, hydrogen sulfide, carbon dioxide, and other small carbon and nitrogen molecules. The helium that was present initially was lost because it did not react chemically and could not be retained in a nonvolatile form.

Our present atmosphere, which essentially is 80% nitrogen and 20% oxygen, is quite different even from the original outgassed atmosphere. That primal atmosphere contained many components that would combine readily with oxygen, but did not contain free oxygen itself. Today's oxygen-rich atmosphere is the result of the slow action by one of the most remarkable phenomena to arise in the universe: Life. Out of this pool of carbon, oxygen, nitrogen, and hydrogen compounds, on the surface of a ball of silicate rock, there evolved the most complex and most subtle chemical systems that the universe has known: living organisms.

The story of how living organisms evolved and how they have transformed our planet is a fascinating one, but one that will have to wait until we have laid a chemical basis for understanding it. In the last chapters of this book we will return to this subject, as an attempt to tie everything together.