The Earth's primary atmosphere, which was similar to that of the gas giant planets, was soon lost, and a secondary atmosphere was established by outgassing from the early Earth and from colliding debris. The composition of this atmosphere was probably similar to present-day volcanic emissions but with a tiny percentage of photochemically produced oxygen. In this atmosphere, early anaerobic bacterial life evolved, including photosynthesising cyanobacteria that produced oxygen. From then on, the evolution of the atmosphere was tied closely to the evolution of life, but with chemical constraints that meant progress was slow. In the last 500 million years, the changes have been dramatic, resulting in our current atmospheric composition, which is unique in the solar system. (Contains 4 tables and 11 figures.)