We have been discussing oxygen so continually that it seems superfluous to discuss it here in detail. Although we think of oxygen mainly as, an atmospheric gas, only one oxygen atom in 600,000 on our planet is found in the atmosphere. The remainder are locked in the silicates and other minerals in the crust and mantle. If one assumes that the primitive Earth lost its original atmosphere during a high-temperature phase of its early history, the secondary atmosphere obtained by outgassing of the planetary interior would have contained little or no free . This reduced atmosphere, in which life is believed to have evolved, would have consisted mainly of hydrogen and its compounds with the secondrow nonmetals - , and - with smaller amounts of . The best evidence that we have today suggests that our present oxygenrich atmosphere was a by-product of life itself, from the splitting of water during photosynthesis:

carbon source + (light energy) reduced organic molecules +

If we think of how much oxygen is in the atmosphere and its importance to us, the turnover of the atmosphere from reducing to oxidizing seems like an immense change. If we regard it only as a freeing of less than two parts per million of mineral oxygen, then the turnover seems less revolutionary. But such a two-parts-per-million change is the basis for all -breathing life.