Air as Good as We Deserve
The evolution of advanced forms of animal life on Earth would not have been possible without molecular oxygen (O2), but too much of it is toxic to virtually all cells and organisms. Oxygen is the third-most abundant element in the universe after hydrogen and helium. It is formed at the heart of stars by the fusion of helium with carbon. More than 90% of the known universe is made up of hydrogen, whereas the other four major molecular building blocks of life, oxygen, carbon, nitrogen, and phosphorus, collectively account for less than a quarter of 1% of its composition. The abundance of hydrogen implies that the cooler spots in the universe are, in chemical terms, reducing environments. This means chemical energy is exchanged primarily by reactions that transfer electrons from hydrogen to suitable acceptors, such as carbon and nitrogen. These reductive processes are responsible for the production of many common, simple compounds in the universe, such as methane (CH4) and ammonia (NH3). This condition was certainly the case on Earth for billions of years, until photosynthesis appeared, which led to the generation of most of the O2 present in the atmosphere today (Gilbert, 1996).
The earliest life forms on Earth were unicellular organisms without a nucleus, (prokaryotes), which would have been destroyed by exposure to molecular oxy-