Cyanobacteria are the oldest known fossils, more than 3.5 billion years old, and they are still one of the largest existing group of bacteria on the planet Earth. These tiny living species are very special because they created the oxygen in our atmosphere. Before the existence of these bacteria, a small amount of oxygen was present as a compound in rocks but there was no oxygen in the atmosphere.
1Earth without oxygen in the atmosphere
Our Earth was entirely a different place when the first living species were evolved.
Life was evolved as unicellular organisms in the oceans.
2The great oxygenation event
Cyanobacteria developed a unique ability to create their own food using sunlight, aka photosynthesis. Oxygen was produced as a waste product of the photosynthesis. At first, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The great oxygenation event was the point in time when these oxygen sinks became saturated, at which point oxygen, produced by the cyanobacteria, was free to escape into the atmosphere. Still, there is no other natural source of free O2 on earth but photosynthesis. Cyanobacteria evolved exponentially and they wiped out almost all other existing living microbes.
3The first extinction event(Oxygen Holocaust)
The increased production of oxygen was toxic for other existing microbes. It was the first known mass extinction event. Most of the existing microbes were wiped out. There were many other extinction events in the history of the evolution of species. Today, 99.9 percent of all species that have existed on earth are extinct.
4The First Ice Age (Huronian Glaciation)
According to the recent research, the excess production of oxygen reacted with methane in the existing atmosphere and produced carbon dioxide and water, which does not retain heat as well as methane does. There was a drastic global temperature fall because of the reaction which ultimately turned into the first ice age.
These unicellular organisms slowly evolved into multicellular organisms and rest is the history explained by Charles Darwin.