Final answer:
The atmosphere was oxygenated by cyanobacteria, which evolved from early phototropic organisms and transformed Earth's anoxic environment into one suitable for aerobic life. This process marked the Great Oxygenation Event, a critical development in the history of Earth and its biological systems.
Step-by-step explanation:
The first organisms that oxygenated the atmosphere were cyanobacteria. During the initial two billion years of Earth's existence, the atmosphere was anoxic, which means there was no molecular oxygen present. This period was dominated by anaerobic organisms that did not require oxygen for survival.
Phototrophic organisms, which are autotrophic organisms capable of converting solar energy into chemical energy, appeared around one billion years after Earth's formation. These phototrophs were the precursors to cyanobacteria. Cyanobacteria, sometimes referred to as blue-green algae, evolved from these simple phototrophs and were responsible for the significant process of oxygenating our planet's atmosphere. This was a profound transformation, as the oxygen produced by cyanobacteria as a by-product of photosynthesis led not only to increased levels of atmospheric oxygen but also enabled the development of ozone (O3) which helps shield Earth from harmful ultraviolet radiation.
Ultimately, the increase in oxygen concentration allowed for the evolution of other life forms and the development of more complex ecosystems. This oxygenation event, known as the Great Oxygenation Event, marked a turning point in Earth's history and the history of life on our planet.