The most important change in Earth's environment since life began is the buildup of free oxygen in the air. Before life became established on Earth, and even for some one or two billion years afterwards, the air contained no free oxygen. Today, the atmosphere is 21 percent oxygen.

Oxygen has always existed in Earth's interior, atmosphere and oceans. 4.5 billion years ago, the heavier oxygen gases would have reacted with the atmospheric gases (methane) or minerals on the Earth's surface. If oxygen was to produced even faster 4.5 billion years ago, faster than it was to be removed, oxygen could have existed a lot longer. (1) Oxygen producted two important gases, a) breathable oxygen and b) ozone (also known as O3). By doing so, this allowed life emerging onto land and to evolve into a much wider diversity that exists on Earth today. And cyanobacteria played an important prior the rise of oxygen.

What exactly is Cyanobacteria?

external image cyanobacteria3.jpg Cyanobacteria also known as blue-green algae (they are actually photosynthetic prokaryotes) brought about a massive increase in the concentration of atmospheric oxygen. When the oxygen began to accumulate, it constituted a wholesale pollution of the environment, a "holocaust" for all living things on Earth. cyanobacteria are believed to perform plant-like photosynthesis. Because of that, therefore it helped changed the life forms on Earth by producing an oxidizing atmosphere. These microbes live mainly in seawater.

Cyanobacteria were capable to use the Sun's energy and to convert carbon dioxide (CO2) and water into food and the waste product? Oxygen. These blue-green algae lived in shallow seas and they were protected from full exposure to the harmful radiation of the Sun. Around 3.5 billion years ago, they became the first microbes to produce oxygen by photosynthesis. They produce organic carbon and they release oxygen gas (O2) and then the oxygen gas enters the seawater and then it escapes into the atmosphere from the seawater. Although the Earth took a somewhat hundreds of millions of years to have first gained significant amounts of oxygen. (2)


  • Cyanobacteria are found in habitat that's protected from the Sun's harmful UV (oceans, fresh water and rocks)
  • Cyanobacteria are mostly found in fresh water (or damp soil)


  • they produce oxygen as a by-product
  • they are known to be the first organisms to create oxygen in the atmosphere

Some bacteria however were able to endure the oxygen atmosphere. Oxygen-based metabolism came into being. The environment changed and life randomly followed.

What else is producing oxygen?

phy.gifThere is another life form, known as the phytoplankton, which also produced oxygen through photosynthesis. These planktons may not be as old as cyanobacteria. However, they did play an important in producing half of the earth's oxygen. But then phytonplanktons didn't really exist 4.5 billion years ago. They are the main producers of oxygen today and half of the world's oxygen today is produced via phytoplankton photosynthesis. (3)

The green plants die and they will either fall to the ground or sink to the ocean floor. A very small amount of their organic carbon is buried and the carbon remains there for millions of years while abosorbing substances like oil, coal and shale.

Evolutionary changes

(that caused our atmosphere to change, or were caused by the change in atmosphere )

external image evolution1.jpg Oxygen is what made living organsisms more complex. This is because, in order to build a complex living organsism. With all the signalling between these cells, and to form a symbiosis between these organsisms, they will need energy. Without oxygen or mitochondria, these complex organisms cannot get enough of energy and will not develop. Most organsisms containing two or three different cell types appeared after the oxygen became available on the surface environment (this happened around 2,300 million years ago). This is also a time when cells became able to extract energy from energy and also mitochondria. (4)

Life forms appearing after oxygen was produced, followed by the evolution of organelles. Plastids that are found in plants evolved around 1,500 million years ago, and during the next 500 million years, organisms contained up to 50 different cell types evolved. These included algae (they produce their own oxygen, also a reason why the amount of oxygen is increasing throughout these years since the cyanobacteria ever started producing oxygen), early animals and fungi and they could use the extra oxygen to provide energy for development. (4)


Other sources:

Related Links:
Eukaryotic Cells