Early Fish Had Lungs
Our ability to breathe above the water originated long before we began to live on dry land. The earliest boney fish, which were the ancestors of anything from cod to humans, had already developed primitive lungs. One of them was the 33- cm-long Guiyu, discover in 2009. Guiyu is the oldest known bony fish, and unlike the earliest vertebrates, it had a spine consisting of robust bones instead of cartilage. Moreover, the skull had been furnished with an important addition: a lower jaw, allowing the fish more possibilities of consuming food.
The Guiyu fossil also demonstrates that the foundation stones of our arms and legs were being laid. Whereas other fish moved their fins by means of muscles close to the spine, Guiyu's muscles were probably located in its fins. This new evolutionary trait made the fins much more flexible.
Although Guiyu’s lungs have not been preserved, scientists are quite sure that they must have existed. The ancestor of bony fish must have had lungs, because all modern bony fish have lungs of some kind. In many fish, but not all, the primitive lungs have been converted into swim bladders, which help the animals control their upward momentum in the water. Like the modern bowfin, Guiyu used its lungs as a supplement to its gills to get extra oxygen for an active life in the water.