I think it arises from consideration of the evolution of the lung. Basically, if you look at the lung fish, they are a tidal bidirectional breather when they are using their lungs, so, I believe, are amphibians (which are also capable of respiration through skin absorption). Please Register or Log in to view the hidden image! Source Lungs (lung fish) evolved in the early Devonian: Please Register or Log in to view the hidden image! Tiktaalik didn't evolve until nearly 40 million years after lungs evolved.