Final answer:
Oxygen levels began to decline from their prehistoric highs, which might have reached up to 35% during the Carboniferous period, starting during the Oligocene around 30 million years ago. The drop to current levels was gradual and influenced by numerous environmental and biological factors.
Step-by-step explanation:
The question pertains to the historical levels of oxygen in the Earth's atmosphere and when these levels began to decline from 35% to today's levels. Oxygen levels started to decline during the Oligocene, about 30 million years ago, moving from elevated percentages to lower levels that are more similar to what we experience today. Throughout Earth's history, atmospheric oxygen content has fluctuated due to a number of factors, including biological processes, geological activity, and changes in Earth's orbit and axis tilt.
Initially, the free oxygen produced by photosynthesis began accumulating in our atmosphere around 2.4 billion years ago, which contributed to the formation of Earth's protective ozone layer. Although the provided information does not specify when oxygen levels were exactly 35%, we know from geological and fossil records that oxygen levels have been higher in the past, potentially reaching up to 35% during the Carboniferous period, around 300-359 million years ago. The reduction in oxygen levels to those of today has been a gradual process influenced by complex interactions within the Earth's biosphere, geosphere, and atmosphere.