How aquatic vertebrates evolved into land vertebrates has been difficult for evolutionary biologists to study, in part because the shift from water to land appears to have occurred rapidly and has yielded a scarce fossil record. Prior to the advent of DNA sequencing, the primary guideposts in tracing the emergence of tetrapods have been morphological considerations, which have highlighted the coelacanth and the lungfish as species of interest.
     Coelacanths and lungfish are distinct from other fish in that they are lobe-finned species. Lobe-finned species, like ray-finned fishes such as tuna and trout, possess not cartilage but a bony skeleton, a key prerequisite for survival on land. Lobe-finned fish species are distinguished from ray-finned species by fins that are joined to a single bone and which thus have the potential to evolve into limbs. Coelacanths and lungfish are two of the only lobe-finned species that are not extinct, and since they have evolved minimally since the time of the appearance of tetrapods, they are sometimes referred to as "living fossils." In fact, the first live coelacanth was discovered more than 100 years after the species had been discovered in fossilized form.
     Whether the coelacanth in particular is rightly called a living fossil and whether it is the closest living relative of the original tetrapods are two questions that have been illuminated more recently by genetic analysis. The coelacanth's genome has recently been sequenced, and this analysis has led to the conclusion that the lungfish is the closer relative of tetrapods. Moreover, the coelacanth DNA has shown evolution over time--although at a rate much slower than that of most animals. Possibly, the fish's morphology and its environment deep in the Indian Ocean have created favorable conditions allowing a more slowly evolving species to have survived for the last 400 million years.