DAVID: Look at the supposed uses. to me that is Darwin-speak, all suppositions, no proof stated.

dhw: I’m sorry, but it seems to me that enhancing stability in the water, filtering carbon dioxide out of the bloodstream, oxygenating the blood in hypoxic environments, all represent a far more rational supposition than “God anticipating land creatures”. Do you think your God gave them primitive lungs for no reason at all except to reduce the work he needed to do when they stepped onto land?

God anticipated land creatures, and preplanned for their development. Swim bladders and lungs may have diverged from the same beginnings, but have vastly different functions. No swim bladders on land, but lungs are.