Great oxidation event (GOE) was a time in Earth’s history when oxygen gas (O2) first began to build up in the atmosphere. Before about 2.4 billion years ago, Earth’s atmosphere and oceans held almost no oxygen. Scientists believe that after 2.4 billion years ago, O2 began to build up in the atmosphere. This gradual build-up of oxygen appears to have continued for the next 2 billion years. The rise in oxygen leveled off at roughly 10 percent of the amount found in the atmosphere today. It then stabilized at or near its present level. However, scientists do not have a precise way to measure exactly how much O2 was present during Earth’s past. Most evidence for the GOE comes from rocks that formed during the period. Most scientists think that cyanobacteria were the first organisms capable of producing O2.
The strongest evidence for the build up of O2 comes from the study of rock formations. Rock that formed before 2.4 billion years ago has a different chemical composition than rock formed after this time. By knowing the chemistry involved in forming each type of rock, scientists can estimate when the O2 appeared as well as the upper and lower limits for how much there was in the environment.
Cyanobacteria release O2 as a by-product of their photosynthesis. Photosynthesis is the process of using the energy from sunlight to make carbon compounds. Some scientists think that cyanobacteria produced O2 for several hundred million years before the start of the GOE. The first O2 produced was most likely used by other organisms. The cyanobacteria themselves may have needed some of the O2 formed during the day to breath at night. Oxygen was only able to build up in the atmosphere once it was being produced faster than organisms and reactions with rocks could remove it.
It is thought that the level of atmospheric oxygen reached about 10 to 50 percent of today’s level by 580 to 550 million years ago. This time period coincides with the appearance of the first animal fossils. Almost all organisms use food for energy more efficiently when oxygen is available. Scientists think that an atmosphere with O2 at or near to its present level is essential for the development and maintenance of complex multicellular life.
