Final answer:
The first cells were anaerobic, not aerobic. Cyanobacteria evolved and produced oxygen, leading to the oxygenation of the atmosphere.
Step-by-step explanation:
The first cells were actually anaerobic, meaning they did not require oxygen to survive. For the first two billion years of Earth's existence, the atmosphere had no molecular oxygen, so the first organisms had to be anaerobic. Cyanobacteria, which evolved within one billion years of Earth's formation, produced oxygen as a by-product of photosynthesis and eventually led to the oxygenation of the atmosphere.