Cyanobacteria are single-celled organisms that led to the Great Oxygenation Event and paved the way for complex life. There was no free oxygen in the atmosphere 3.5 billion years ago and anaerobic organisms thrived until the phenomenon of photosynthesis developed in cyanobacteria and began polluting the atmosphere with oxygen. Photosynthesis gave cyanobacteria an advantage and their populations began to multiply for the next several hundred million years until the atmosphere was full of oxygen.

Why watch this video?

• Have you ever wondered when and where the oxygen in our atmosphere came from?
• Would you like to know what the first mass extinction on Earth was?
• Do you have trouble understanding the evolution of life from a single celled organism to the complex life we see today?

Key terms

Cyanobacteria: Single-celled organisms that were the first to develop photosynthesis

Anaerobic: Organisms that require a lack of oxygen in order to survive

Photosynthesis: a process in which plants produce their own food by converting water and carbon dioxide into sugar and oxygen.

Loose ends

1. Although cyanobacteria introduced free oxygen into the atmosphere there was still oxygen present 3.5 billion years ago. However, the oxygen at this time was trapped in molecules such as water and carbon dioxide, and the atmosphere largely contained nitrogen. It is helpful to think of cyanobacteria as a converter and not a creator for they simply took the oxygen that was in the water and carbon dioxide and converted it into free oxygen.
2. The introduction of oxygen into the atmosphere seems to have been beneficial as it resulted in the life we see today, so how could it have caused a mass extinction? Well, the organisms at the time were anaerobic, meaning they required a lack of oxygen to function. The introduction of oxygen into the atmosphere was actually toxic to these anaerobic organisms.
3. The introduction of oxygen into the atmosphere also impacted the climate and led to one of the first ice ages. Methane in the atmosphere at the time acted as a potent greenhouse gas and kept the Earth warm. Oxygen reacted with this methane and created a thinner atmosphere, preventing the trapping of heat as methane had previously done.

Self-Test Questions