Watched an interesting documentary about pre-historic animals and during a certain era they were discussing on why almost all of the supporting life forms were gigantic and huge. It was mainly due to the higher oxygen levels in the atmosphere which were at around 33 percent oxygen and the rest nitrogen. Today the oxygen levels are around 20 percent. With higher oxygen levels in the atmosphere, evidently animals blood vessels were smaller since not as much oxygen needed to flow through since the air was richer allowing more muscle to grow, hence why you would get huge bugs and mammals etc. Can you mimic this today in a room where the air is controlled to a certain oxygen level? I'm sure it can be done by scientists.