Comet Enthusiast

Two and a half billion years ago the atmosphere contained almost no oxygen, Arizona State University scientists say, and life as we know it was not possible in an environment devoid of oxygen. The presence of oxygen in Earth's atmosphere came from ancient microbes inhabiting "oxygen oases" in the oceans billions of years ago, when microbes known as cyanobacteria appeared that could capture sunlight to create energy through photosynthesis, and in the process altered Earth's atmosphere by the production of oxygen, a waste product to cyanobacteria but essential for later life.