According to Graham and Dudley, escalating Paleozoic oxygen levels may have helped speed oxygen transport in the longer tracheae of bigger insects. The environment itself could have opened the respiratory door for Paleozoic insects, allowing giant species to evolve. Insects do not breath the same way that we do. For a given tube diameter and temperature, gas molecules diffuse over distance at a rate proportional to the source concentration. In other words, air that contains more oxygen allows the minimum amount needed for metabolism to reach farther into the insect's tracheae.
Some insects canincrease oxygen delivery by a mechanical pumping action of their bodies.
Humans and other vertebrates are less likely to be affected by atmospheric oxygen concentration, since oxygen is delivered by blood that is pumped through the tissues. Harrison's personal research interest is focused on insect respiratory physiology. The ASU scientist always wondered whether respiration could have been the limiting factor in insect gigantism. Shortly before Graham and Dudley's hypothesis was published, Harrison learned about the Paleozoic oxygen pulse from Dudley himself. The two were chatting at a national biology conference.