Two-and-a-half billion years ago, the Earth's atmosphere was rich in hydrocarbons, similar to Saturn's moon, Titan. Before Earth's atmosphere ditched methane and began accumulating oxygen, though, our planet appears to have cycled back and forth every few million years between the two states years a hydrocarbon haze and clear skies. A sunlight-blocking haze most certainly affected the evolution of microbes that depend on light to photosynthesise and contributed to the delay before the final oxygenation of the atmosphere.