To Omnirizon:
Yes, making gills work in air is actually relatively simple. As for lungs underwater - well, they have enough area to emulate gills - the main problem (besides lung surface is adapted in different way than gills' ) is that breathing muscles are too weak to regularly move water back and forth... but this can be fixed in a number of ways.
As for carbon sink - swamps can do this work quite well... and actually they did it in some eras. I don't want to go into calculations now, but there are possibilities to have human-like creatures on a planet with less active water surface than Earth has - even without making universe with slightly different physical laws... and fantasy worlds are such by definition. And as for Lucas - iirc he actually placed some marks in the film which say that SW universe is also slightly different from our own...