Cyanobacteria were allegedly some of the first microbes that existed on earth. Evidence for their existence can be found in stromatolites, layered masses of They inhabited the seas and survived through photosynthesis, using sunlight, water, and carbon dioxide to produce oxygen and glucose. Initially this oxygen reacted with iron in the water and formed the deposits that are today known as iron banded formations. After the rust was used, however, oxygen had nowhere to go but up. It started filling the atmosphere over time. And so began the oxygen revolution.
Oxygen was just as much of a toxin as it was a bringer of new life – it just depended upon who you were. Microbes that could not tolerate oxygen, called obligate anerobes, either died off or retreated to anaerobic habitats. These unusual locations, such as deep sea thermal vents, are being explored today and mined for new types of microbial life. However, the abundance of oxygen also set the stage for the the diversification of life on earth. Microbes that could survive the increased amounts of oxygen often made use of it to become more efficient. Some of them were swallowed up by other living things and came to live inside them as mitochondria, producing oxygen for their hosts. Eventually multicellular oxygen breathers emerged, and over time a vast array of oxygen breathing life populated both land and water. Rarely, however, did these new life forms think about cyanobacteria - the microbes that made their existence possible.
