What Early Earth Organism Was Responsible For Major increases in Oxygen Levels?

A long, long time ago, roughly 2 billion years ago according to most scientists, there was no oxygen in Earth’s atmosphere. Nearly all life on Earth was anaerobic bacteria, or bacteria that break down their food and obtain energy without oxygen. The oxygen-lacking atmosphere of early Earth was perfect for these organisms, and the oceans were full of them.

Then cyanobacteria, also known as blue-green algae, entered the scene and everything changed. Up until now, energy entered the food chain via chemosynthesis (obtaining energy from the breakdown of non-living chemicals) or a primitive form of photosynthesis fueled by non-visible light that produced sulfur as a waste product. Cyanobacteria were different; they could harvest energy from visible sunlight and produce free oxygen.

As humans who need to breathe, we think of oxygen as something good and life giving. For the anaerobic bacteria living billions of years ago, oxygen was essentially poison.

The emergence and success of cyanobacteria triggered a mass extinction event, which scientists call The Great Oxygenation Event. Many species of anaerobic bacteria went entirely extinct, while others were relegated to sparse, oxygen-free environments like the depths of the ocean.

Cyanobacteria, as you may know, live on to this day. They can be found in nearly every aquatic habitat on Earth, from freshwater to saltwater. Some cyanobacteria were engulfed by the cells of early plants and formed a symbiotic relationship. These cyanobacteria are now known as chloroplasts, and they are what allow plants to photosynthesize.

Without cyanobacteria, and the extinction of hundreds of anaerobic bacteria species, the current, oxygen-rich atmosphere that allows us to live would never have existed.