This is neato science.

According to this article, the great extinction of 270 million years ago resulted from a decline in oxygen levels to approximately 50% of current levels. Sea level oxygen content back then would have been like being at 14,000 feet today. Some animals - specifically, dinosaurs - developed lungs that would compensate for this diminished oxygen. The descendants of those animals still have the same adaptations which allows them to survive at altitudes with diminished oxygen.

We call those descendants "birds."

How bizarre. Oxygen levels dropped by 50% because of changes in the enironment? Don't tell the liberals or that's going to be the next big scare.