I do believe that different conditions have existed at different times throughout the earth's history. And some of that is confirmed by geological processes. BUT again, things have been different at different times. We just don't know EXACTLY when. With all sorts of flying things now, I don't know about the flying stuff.
Maybe moog is refreing to things like the dragonflies from the Cambrian that had 2 foot wingspans. It was estimated that the atmosphere may have been as high as 40% oxygen back then. That's the concentration necessary for such large insects to respire and survive.