Final answer:
Oxygen levels began to decline from a peak of 35% after starting to accumulate around 2.4 billion years ago due to cyanobacteria's photosynthesis. This period marked the formation of the ozone layer, protecting life and enabling its land colonization.
Step-by-step explanation:
Oxygen levels began to decline from their peak of 35% down to today's levels after they first started to significantly accumulate in the Earth's atmosphere. This increase in atmospheric oxygen occurred around 2.4 billion years ago, during an event known as the Great Oxygenation Event caused primarily by the activity of cyanobacteria.
Before this period, Earth's atmosphere was anoxic, making it impossible for aerobic organisms to exist. The rise of cyanobacteria, and subsequently oxygenic photosynthesis, led to the accumulation of free oxygen. This resulted not just in the proliferation of aerobic life forms but also in the formation of the ozone layer, which protected life from the Sun's ultraviolet rays and allowed it to thrive on land.
While oxygen levels have fluctuated since then, it is during this critical time frame that our atmosphere transformed into an oxygen-rich environment, paving the way for more complex life forms to evolve.