Thank you, cyanobacteria!
We humans exist because of tiny organisms—cyanobacteria, or blue-green bacteria—that evolved about 2.7 billion years ago. They flourished in Earth’s warm oceans of that time. They released oxygen gases from carbon dioxide, water, and sunlight—the process called photosynthesis. As cyanobacteria created more free oxygen, the amount of oxygen in the atmosphere reached 1% of today’s level of oxygen. The bacteria continued to release free oxygen and build the oxygen atmosphere. By 2 billion years, the methane haze cleared from Earth’s atmosphere, and the skies turned blue. About 430 million years ago, life moved onto land (and the cyanobacteria joined these new life forms as symbiotic partners). This new life used oxygen directly from the atmosphere. And at 400 million years the oxygen content in the atmosphere reached today’s level. The oxygen atmosphere at 21% and the continued decrease of carbon dioxide in the atmosphere allowed the earth’s atmosphere to cool. By 300 million years ago, this change in temperature generated great masses of dead plants which got buried and removed carbon from the atmosphere. This led to the formation of the oil, coal and natural gas reserves that we have been extracting from the ground since the 18th century. Thank you, cyanobacteria, for giving us the atmosphere we breathe!