Previously, we explored the evolution of the Sarcopterygians or Lobe-Finned Fish, which possessed specialized fins for movement in shallow freshwater habitats and primitive lungs for breathing in oxygen-poor habitats. Lobe-finned fish are most notable for being the ancestors of Tetrapods, or four-limbed vertebrate animals (mammals, reptiles, birds, and amphibians). Being mammals, we humans are tetrapods as well. Lobe-finned fish would eventually evolve the first limbs, lose their gills, and evolve into terrestrial creatures. Prior to their conquest of the land, their unique adaptations for oxygen-poor shallow waters allowed them to dominate many freshwater ecosystems. 

But since then, the tables have turned. Today, Earth’s waters are full of and dominated by members of the sister taxon to lobe-finned fish, known as Ray-Finned Fish, which comprise over 50% of all living vertebrate species. On the other hand, lobe-finned fish are now rare and restricted to a couple of disparate niches, with their living representatives only consisting of six species of Lungfish and two species of Coelacanth. 

These two types of fish are surviving lobe-finned fish and Tetrapods are directly descended from lobe-finned fish by evolution. Therefore, the Lungfish and the Coelacanth, along with lobe-finned fish as a whole, are more closely related to Tetrapods than they are to the much more common ray-finned fish.

The Coelacanth is rather famous for being what is known as a “Lazarus Taxon.” For many years, this fish was known only from fossils and believed to have died out 66 million years ago along with the dinosaurs until a live specimen was found living off the coast of Africa hundreds of feet below the surface in 1938. There are two living species, the West Indian Ocean Coelacanth and the Indonesian Coelacanth, but they were once more diverse.

The modern Coelacanth can weigh around 200 pounds and grow over 6 feet long. Based on analysis of annual growth marks on their scales, it is estimated that they can live up to 100 years and reach maturity at around 55 years of age. Shy, elusive, and nocturnal, these ambush hunters live in underwater rock formations deep in the ocean and feed mainly on smaller fish and various cephalopods. The Coelacanth is popularly and somewhat erroneously known as a “living fossil,” as scientists once thought that it evolved into roughly its current form 400 million years ago. However, in 2013, a research group analyzed the genome sequence of the Coelacanth and found that, although these fish are indeed ancient, they have continued to evolve and change since the Devonian period in which they appeared. Furthermore, the two living species are not very ancient themselves, appearing only around 20,000 years ago. So while Coelacanths have a fossil record dating back to roughly 410 million years ago and did indeed originate before land living vertebrates did, the living West Indian Ocean and Indonesian Coelacanths are not primitive relics from a lost ancient world, but more of a late-surviving expression of a large and fairly diverse group of fish that has been around for hundreds of millions of years. 

Although the two living species still only inhabit deep water marine ecosystems, the Coelacanth was once thought to be very closely related to Tetrapods, meaning they could provide us with some clues as to how they evolved. Prior to being rediscovered and living specimens being extensively studied, scientists once believed that the Coelacanth was capable of using its fins to crawl on dry land to move from pool to pool when water was scarce, just as they once believed other lobe-finned fish were capable of. But when it was found living in deep waters off the coast of Tanzania, it was discovered that, although its fins rotate in a different manner and direction from those of ray-finned fish, it uses its fins exclusively for swimming. This discovery strongly backed up the idea that most of the prehistoric lobe-finned fish that were once believed to be able to use their fins to traverse out of the water were not and used their fins solely for pushing their way through waters filled with roots and vegetation. Nonetheless, the Coelacanth was still formerly thought to be the closest living fish relative to Tetrapods, including humans. However, through an analysis of the Coelacanth’s genome sequence, scientists concluded that the Lungfish is more closely related to Tetrapods than the Coelacanth is. 

Like Coelacanths, Lungfish have Devonian origins, appearing in the fossil record nearly 400 million years ago. But unlike Coelacanths, Lungfish possess both gills and lungs, all living species possess lobed fins, and all species are capable of traversing on land. These are all features they share with the most immediate fish ancestors of Tetrapods. That being said, the most recent fish ancestors to Tetrapods are extinct, making Lungfish their closest extant relatives rather than their direct semi-terrestrial ancestors. Therefore, the answer as to whether or not Lungfish serve as living model organisms that elucidate Tetrapod ancestry isn’t particularly well understood.