Polypterus bichir |
Humans, like all terrestrial tetrapods, have a couple of major differences with the bony fish we are all believed to be descended from via an intermediate species such as Tiktaalik roseae, which crawled out onto land about 370 million years ago; the major differences being our jointed limbs and our lungs, all thought to be adaptations to walking on land and breathing air.
However, researchers at the University of Copenhagen believe they have found evidence to show that these structures could have been present in our ancestral fish some 50 million years before that. In fact, lungs may be the forerunner of the swim-bladder found in bony fish, rather than the swim-bladder being the forerunner of lungs.
The conventional view is that bottom-feeding fish, in low-oxygen environments, used their swim-bladder as a fresh-air supply by gulping air from the surface. The swim-bladder is connected to the gut. This, over time, evolved into the lungs and enabled this bottom-feeders to use the limbs they were evolving to crawl around in the silt, to come out onto land.
This research challenges that view by suggesting that some early bony fish may have had lungs that later evolved into swim-bladders in fish that needed to vary their buoyancy while some retained them as functional lungs which server the purpose of a fresh air store for foraging in an anoxic environment.