Did Rising Oxygen Levels Fuel Mammal Evolution?
Then there was a spike to about 23 percent during the Eocene period between 55 and 38 million years ago. Over the past 10 million years, the concentration of oxygen declined slightly to today’s 21 percent. The rapid decline in oxygen levels at the end of the Permian and the beginning of the Triassic was a major factor in contributing to the extinction of many land animals, mostly reptiles.
