There is a chap I listen to who has gone into great depth on this matter. He is a chronologist and through studying ancient scripts and historical texts has come to the conclusion that a few times in our recent past (last 6k years) we have had an atmosphere with increased oxygen where humans, animals and plants grew much larger and healed quicker. He calls it a Vapour Canopy. There is even an experiment done (in Texas I think) where they simulated conditions in a controlled environment and found insects growing much larger. Fascinating stuff.

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