Final answer:
Evidence indicates that during the first two billion years of Earth's existence, the atmosphere was anoxic. Cyanobacteria evolved from simple phototrophs and began the oxygenation of the atmosphere, enabling the evolution of other life forms.
Step-by-step explanation:
Evidence indicates that during the first two billion years of Earth's existence, the atmosphere was anoxic, meaning that there was no molecular oxygen. Therefore, only those organisms that can grow without oxygen—anaerobic organisms-were able to live. Autotrophic organisms that convert solar energy into chemical energy are called phototrophs, and they appeared within one billion years of the formation of Earth.
Then, cyanobacteria, also known as blue-green algae, evolved from these simple phototrophs one billion years later. Cyanobacteria began the oxygenation of the atmosphere. Increased atmospheric oxygen allowed the development of more efficient O₂-utilizing catabolic pathways. It also opened up the land to increased colonization, because some O₂ is converted into O₃ (ozone) and ozone effectively absorbs the ultraviolet light that would otherwise cause lethal mutations in DNA. Ultimately, the increase in O₂ concentrations allowed the evolution of other life forms.