‘From slime to the sublime’ asks: How did Earth maintain an environment fit for life? However life started, once established, it persisted for over 3.5 billion years and evolved from microbial slime to the sophistication of human civilization. The most drastic changes of the Earth's atmospheric composition have been increases in oxygen, which were just as important for the evolution of life as variations in greenhouse gases. For most of Earth's history, oxygen levels were so low that oxygen-breathing animals were impossible. With the Great Oxidation, Earth's stratospheric ozone layer formed and the ancient atmosphere made the transition from hydrogen rich to oxygen rich.