What Organ in Fish Later Evolved to Become Lungs?

The transition of vertebrate life from the ocean to the land has always been a fascinating example of evolution. Imagining an early fish evolving simple legs and dragging itself onto dry sand for the first time is certainly a dramatic image.

Of course, to live on land these early fish had to be able to breathe air. When living in water, most fish use organs called “gills” to get oxygen. Gills have a high surface area and are able to perform gas exchange to pull oxygen from the surrounding water. It seems intuitive that lungs would have evolved from gills to allow fish to breathe air; however, this is not the case.

It turns out that another fish organ, the swim bladder, much more closely resembles a primitive lung. Swim bladders help many fish regulate their buoyancy, but some fish can also use them to breathe. For example, fish that are found in very stagnant water with low levels of dissolved oxygen may need to gulp air from the surface and “breathe” through their swim bladder rather than through their gills.

Darwin believed that lungs must have evolved from swim bladders, but more recent scientific evidence suggests that swim bladders and modern lungs both evolved from an earlier, more primitive organ that served the purpose of both a lung and a swim bladder. This early organ may have originally come from gill tissue or a sac of tissue in the digestive system used for regulating gas.

One of the incredible feats of evolution is its ability to adapt the same organ or piece of tissue to serve completely different purposes in different species. Who would have thought that our air-breathing lungs could have the same origin as the buoyancy-regulating swim bladder of a fish!