At three and half billion years ago, the first stromatolites were seen. This is assumed to be of biological origin, with cyanobacteria, as in present day stromatolites. Stromatolites are commonly thought to have been formed by the trapping, binding, and cementation of sedimentary grains by micro-organisms, especially cyanobacteria this is why evidence has shown cyanobacteria was present at this time though could have been around before this time.

Stromatolites were more abundant on the Earth in Precambrian times. The Precambrian times spans from the formation of Earth around 4500 million years ago to the evolution of abundant macroscopic hard-shelled fossils, which marked the beginning of the Cambrian, the first period of the first era of the Phanerozoic eon, some 542 million years ago. The Precambrian atmosphere was rich in carbon dioxide, but prior to 2.4 billion years ago, it lacked the oxygen that sustains the complex multicellular life that has evolved since about 600 million years ago. Stromatolites in the geological record declined sharply in both diversity and number during the latest Precambrian and are present, but not common during the Paleozoic. They are uncommon in modern day marine environments because of burrowing or grazing animals. Modern stromatolites are mostly found in hypersaline lakes and marine lagoons where extreme conditions exclude animal grazing. An example of such a location is Hamelin Pool Marine Nature Reserve, Shark Bay in Western Australia where excellent examples are found.

Stomatolites in Shark Bay, W.A.

Cyanobacteria or “blue bacteria” are aquatic and photosynthetic which means that they can live in the water and can also manufacture their own food. Because they are bacteria, they have the same qualities and properties as any other bacteria. Cyanobacteria are quite small and unicellular, though they often grow in colonies large enough to see.

Cyanobacteria play an important role on earth. Many Proterozsoic oil deposits are attributed to the activity of cyanobacteria. They are also important providers of nitrogen fertilizer in the cultivation of rice and beans and have also been tremendously important in shaping the course of evolution and ecological change throughout earth's history. Numerous cyanobacteria during the Archaean and Proterozoic Eras generated the oxygen atmosphere that we depend on today. Before that time, the atmosphere had a very different chemistry, unsuitable for life, as we know it today. They played a very important role in the atmospheric change of chemicals to the present day atmosphere. Similarly, plants today play that same important role as cyanobacteria where they have the same bi-product of oxygen. Cyanobacteria are still around today and are one of the largest and most important groups of bacteria on earth.

Microscope images of Cyanobacteria.

There is also great contribution to the cyanobacteria as the origin of plants. The chloroplast with which plants make food for themselves is actually a cyanobacterium living within the plant's cells.

Around 3.5 to 2.8 billion years ago, from above photosynthesis occurred. Blue-green algae was one of the first plants which releases oxygen molecules into the atmosphere and steadily works to strengthen the ozone layer and change the Earth's chemically reducing atmosphere into a chemically oxidizing one. Blue- green algae and other plants not discovered yet changed the earth’s chemical atmosphere on a much larger scale then the cyanobacteria due to his size. Though, even plants at this time were not numerous and big enough to stabilize and reach to the present day level in its oxygen levels which took another billion years to do so.

This was a very important process in the earth’s history as it created an ozone layer (discussed later), which protected humans today and ensure our survival in today’s conditions and the dangerous sun’s rays. This process is not only important to us, humans but to other organism’s properties as well such as having mitochondria, respiration, etc.