The Vivacious Relic of the Seas

For a long time, scientists only knew Coelacanth from fossils, assuming that they went extinct over 60 million years ago. However, in 1938, a live Coelacanth was captured near South Africa, as described in Coelacanth, one of Sir David Attenborough’s first documentaries. Scientists were astounded by this discovery because it was a novel clue as to how amphibians, reptiles, and mammals evolved. Their long, fleshy, lobe-like fins were an intermediate between fish fins and amphibian limbs. These fins might have been the origin of rudimentary legs for amphibians, allowing them to leave the water and start living on land. The documentary also mentions that Coelacanth started to develop air-breathing lungs, which are features of land animals. Thus, the coelacanth, rediscovered in 1938 after its alleged extinction over 60 million years ago [1], is a living representation of the evolution of reptiles, amphibians and land organisms, as well as of the transition from fish to tetrapods. It is an important organism to study both physiologically and genetically.

While fascinating, Coelacanth, released in 1952, presents some outdated scientific knowledge. Recent findings provide insight into the evolutionary implications of the coelacanth. Analysis of novel data has elucidated the relationship between the Coelacanth and other organisms, including early land animals. Sequencing of 44 genes elucidated the divergence of coelacanth and tetrapod lineages [2], and another analysis sequencing 3,500 base pairs of DNA suggested that coelacanths are closely related to land vertebrates [3]. These genotypic relationships translate to phenotypic similarities between the coelacanth and early land animals. For example, the primitive pattern of lobed fins, as described based on recently discovered Coelacanth fossils, is the precursor of digit-bearing limbs [4]. Because of this, the recently sequenced coelacanth genome is seen as a blueprint for understanding tetrapod evolution because within it are contained genes underlying the fin-to-limb transition [1].

Before the discovery of the Coelacanth, many biologists turned to other primitive species, like lungfish, as a model of tetrapod evolution. Today, the Coelacanth and its genome serve as a template to study this important evolutionary step.

by Ali Murad Büyüm, Andrew Cho, Wendy Lee & Amanda Zhang  

BBC Earth- Scientific Sensation of the Century! Coelacanth, starting at approximately 2:06

References

1. Amemiya C, et. al. 2013. The African Coelacanth genome provides insights into tetrapod evolution. Nature 496: 311–316.
2. Takezaki N, Figueroa F, Zaleska-Rutczynska Z,Takahata N & J Klein. 2004. The Phylogenetic relationship of tetrapod, coelacanth, and lungfish revealed by the sequences of forty-four nuclear genes. Molecular Biology and Evolution  21: 1512–1524.
3. Zardoya R & A Meyer. 1996. Evolutionary relationships of the coelacanth, lungfishes, and tetrapods based on the 28S ribosomal RNA gene. Proceedings of the National Academy of Sciences  93: 5449–5454.
4. Friedman M, Coates MI & P Anderson. 2007. First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs. Evolution & Development 9: 29–337.