Final answer:
The activity of photosynthetic bacteria increased Earth's molecular oxygen and oxygen levels due to cyanobacteria producing oxygen via photosynthesis during the Great Oxygenation Event.
Step-by-step explanation:
Due to the activity of photosynthetic bacteria, the levels of molecular oxygen (O2) in the atmosphere went up, but so did the Earth's oxygen levels. This process, known as the Great Oxygenation Event, was primarily driven by cyanobacteria, which produce oxygen as a byproduct of photosynthesis. Over time, the increase in atmospheric O2 led to the development of more efficient aerobic metabolic pathways and the evolution of more complex life forms, thanks to the creation of an ozone layer that protected organisms from UV radiation.