Actually, the sky was orange until about 2.5 billion years ago, but if you jumped back in time to see it, you’d double over in a coughing fit. Way back then, the air was a toxic fog of vicious vapors: carbon monoxide, carbon dioxide, nitrogen, cyanide, and methane. This last gas gave the sky an orange tint and the land a strange glow. But then something happened that would change the sky—and the planet—forever. Blue-green microbes called cyanobacteria formed in the ocean that were capable of a special trick that transformed the planet: photosynthesis. Later used by plants, this natural process converts sunlight and carbon dioxide into energy, creating oxygen as a by-product. Fed by nutrients in the sea and powered by the sun, cyanobacteria exploded across the ocean, pumping more and more oxygen into Earth’s atmosphere. Slowly, over the next two billion years, oxygen in the atmosphere rose to its present levels, and the sky took on the blue hue on view today.