Gloeotrichia sp. (filamentous colonies of Cyanobacteria). 40x magnification. Darkfield microscopy. 

Cyanobacteria are a phylum of bacteria. As a bacterial prokaryote, it does not have a nucleus or membrane bound organelles in its cells. It does however have thylakoid membranes where the chemical reactions of photosynthesis take place which make Cyanobacteria the most important organism on this planet. They are the only photosynthetic bacteria able to produce oxygen. Until about 2-2.5 billion years ago, the Earth was a hot and scary place with an atmosphere of carbon dioxide, methane, and ammonia. Once Cyanobacteria evolved and were able to use photosynthesis to take in some of that CO2 and release oxygen (O2), the atmosphere changed from a reducing to an oxidizing environment and this made it possible for organisms to evolve that could take in oxygen and use it to make energy.

If all of that wasn’t cool enough, here’s another reason why we wouldn’t exist without Cyanobacteria. Eukaryotic cells are those that contain a nucleus and membrane bound organelles like chloroplasts and mitochondria. We are eukaryotes and so are plants, fungi, protists, and other animals. In the early and mid 1900s, a few scientists (including Lynn Margulis) started to notice that chloroplasts and mitochondria look like bacteria and the theory of endosymbiosis/symbiogenesis was developed. Basically, a primitive (think billions of years ago) microbe ate a primitive Cyanobacterial cell that was just photosynthesizing and minding its own business and after many generations of this behavior and genetic changes in the two cells, they were able to live and divide as one complex cell with its own chloroplasts inside converting light and CO2 to oxygen and chemical energy. Something similar also happened to produce mitochondria. Once microbes were able to evolve to this level of complexity (as eukaryotes), all other forms of eukaryotic life (including humans) were able to evolve.