The lungs originate from the intestinal tract. Some fish would gulp air. This got better and better... and hence became lungs. (FYI, the gills turned into vario

The lungs originate from the intestinal tract. Some fish would gulp air. This got better and better... and hence became lungs. (FYI, the gills turned into various features of the neck and lower face including ear bones and the jaw, with some of this happening long before functional gills were lost)