Oxygen content on Earth increased significantly over time. Initially, the atmosphere was largely devoid of oxygen and composed primarily of methane, ammonia, and water vapor. Photosynthetic bacteria and cyanobacteria were instrumental in producing oxygen as a byproduct of photosynthesis. Oxygen levels gradually accumulated, facilitating the development of aerobic organisms and leading to a dramatic transformation of the planet's ecosystem. These processes paved the way for oxygen-dependent life forms.