The transition of vertebrates from the salty ocean to life on land required the ability for these animals to change how they dealt with salt. While salt is essential for many cellular functions, salt balance must be tightly regulated to prevent illness or even death. Animals that live in a salty environment have evolved the ability to get rid of excess salt from their bodies whereas land-dwelling animals evolved the ability to retain salt in an environment with limited availability.
One way that animals prevent loss of sodium is by recovering salt from the urine as it is being formed in the kidneys. This is accomplished with the help of special sodium channels (ENaCs) in the kidneys that move sodium from the urine back to the bloodstream. Where salt goes, water follows. That means that as the animals recover salt from the urine, they are also preventing water loss and dehydration. You can imagine that if this system fails, animals would lose both salt and water and likely succumb to dehydration. These ENaC transporters are also found in the lungs of tetrapods to help regulate the fluids that line the inner surface of the lungs. Failure of these ENaCs results in excess fluid in the lungs. In contrast, if these transporters are working too much, they can lead to high blood pressure by retaining too much salt, and hence water, in the body as well as dehydration of the lungs much like what happens with cystic fibrosis.
Drs. Lukas Wichmann and Mike Althaus recently published a review in the American Journal of Physiology – Regulatory, Integrative and Comparative Physiology exploring the evolutionary history of these important channels. Because these channels are found in the gills of nonteleost freshwater fish, the scientists speculate that ancestral vertebrates may have evolved the ability to take in sodium from the surrounding water, which contains less salt than what is found inside the animals.
Oddly, ENaCs are also found in some modern marine vertebrates including hagfish and coelacanth. Researchers speculate some modern fish may have had ancestors that transitioned to freshwater habitats before resettling in the oceans during times of mass extinctions. In fact, some of the oldest fossils of shark ancestors were actually discovered in the sediment of freshwater, even though modern sharks live in saltwater. As for hagfish, their plasma is nearly as salty as seawater, although they too have a form of a sodium transporter. Researchers think the hagfish sodium transporters might be used by the animals to regulate acid base balance since they do not seem to control salt balance.
The transition to land also required additional adaptations to allow the animals to breathe air. In fact, some teleost fish and African lungfish are obligate air breathers and ENaCs have been found in their gills, but not their lungs. Although, it is found in the lungs of terrestrial vertebrates. Gas exchange in the lungs of terrestrial vertebrates requires water, kind of like the gills of fish. A thin layer of water is important in our lungs as oxygen and carbon dioxide are dissolved in the water before being transported across the lung tissue, thus allowing gas exchange to occur between the atmosphere and the bloodstream. It is pretty neat that we still have a form of “water-breathing” in common with fish!
L Wichmann, M Althaus. Evolution of epithelial sodium channels: Current concepts and hypotheses. American Journal of Physiology – Regulatory, Integrative and Comparative Physiology. 319(4): R387-R400, 2020. DOI: 10.1152/ajpregu.00144.2020