Final answer:
Cyanobacteria were the first organisms that oxygenated the atmosphere, transforming it from an anoxic state and enabling the evolution of aerobic life forms.
Step-by-step explanation:
The First Organisms to Oxygenate the Atmosphere
The first organisms that oxygenated the atmosphere were cyanoabacteria. During Earth's early history, the atmosphere was anoxic, which means it contained no molecular oxygen. This implies that only anaerobic organisms could survive. However, about one billion years after Earth's formation, phototrophic organisms appeared, which were capable of converting solar energy into chemical energy. From these phototrophs evolved cyanobacteria, and it is these cyanobacteria that are credited with beginning the oxygenation of the atmosphere through photosynthesis, ultimately increasing atmospheric oxygen and allowing the evolution of other life forms.
The first organisms that oxygenated the atmosphere were cyanobacteria. For the first two billion years of Earth's existence, the atmosphere had no molecular oxygen, and thus, the first organisms were anaerobic. Cyanobacteria, which produce oxygen as a by-product of photosynthesis, appeared within one billion years of Earth's formation. From these organisms evolved phototrophic organisms.