Final answer:
The Earth formed around 4.5 billion years ago, with microbes appearing shortly after. Cyanobacteria evolved at least one billion years later and initiated oxygenic photosynthesis, leading to the oxygenation of the atmosphere and enabling the development of aerobic life forms.
Step-by-step explanation:
The Earth is estimated to have formed approximately 4.5 billion years ago. Initially, Earth's atmosphere contained no molecular oxygen and was therefore anoxic. The first life forms, which were anaerobic, appeared quite early in Earth's history, after about 3.8 to 4.0 billion years. The microbes that first inhabited Earth were phototrophic, meaning they could convert solar energy into chemical energy, within the first billion years. With time, these early phototrophs evolved into cyanobacteria, a significant group of organisms which began the process of oxygenating the atmosphere through oxygenic photosynthesis, contributing to the dramatic environmental transformation on Earth.
Cyanobacteria are thought to have appeared at least one billion years after Earth's formation, marking a pivotal moment in Earth's history as they produced oxygen as a by-product of photosynthesis. This process not only changed the composition of the atmosphere but also enabled the emergence of aerobic life forms. This transformation due to oxygenic photosynthesis likely evolved before 3.7 billion years ago, leading to an increase in atmospheric oxygen and the eventual colonization of land by more diverse forms of life.