The fish that reappeared
On December 22, 1938, Marjorie Courtenay-Latimer, a museum curator in East London, South Africa, received a call about an unusual fish. Caught in a shark net by Captain Hendrik Goosen near the mouth of the Chalumna River, the five-foot-long, mauve-blue fish was unlike anything she had ever seen. It was covered in hard, armor-like scales, had four fleshy, limb-like fins, and a strange three-lobed tail. After she sent a sketch to ichthyologist J.L.B. Smith, he confirmed her suspicion that it was a coelacanth, a fish known only from fossils and believed to have gone extinct with the dinosaurs around 66 million years ago.
The discovery was a zoological sensation. The fish was named Latimeria chalumnae in honor of Courtenay-Latimer and the river where it was found. Coelacanths today look remarkably similar to their ancestors from over 300 million years ago, called "living fossils." For decades, scientists wondered if this external stability was reflected in their genetic makeup. The answer had to wait for the development of genome sequencing.
A static blueprint
The full genome of the African coelacanth was published in 2013, providing a molecular explanation for its slow evolution. The analysis showed that the coelacanth's protein-coding genes are evolving significantly slower than those of other fish and land vertebrates. Its genome, which is 2.86 billion base pairs long, helps scientists understand understanding the transition of vertebrates from water to land.
The sequencing confirmed that lungfish are the closest living relatives of tetrapods (four-limbed vertebrates), not coelacanths. The coelacanth genome is an important reference point. It contains ancient versions of genes that land animals later adapted for life on shore. For example, it possesses enhancers for genes like bmp7 and gli3, which help with limb formation and are conserved between coelacanths and tetrapods but not in other fish. Scientists also identified changes in genes related to immunity and the detection of airborne odors, modifications that were necessary for the move onto land. While the fish's body plan appears static, its genome recently acquired 62 new genes from parasitic DNA elements called transposons about 10 million years ago, showing that even "living fossils" are not entirely frozen in time.