There is a fascination with living organisms that closely resemble fossils dated at millions of years old. The classic definition of a living fossil is an extant organism that closely resembles fossil specimens.
In 1938 Miss Marjorie Courtenay-Latimer of South Africa recognized that among recently caught fish, one specimen was unfamiliar. It turned out to be “the only living member of an ancient group of lobe-finned fishes that was known previously only from fossils and believed to have been extinct since the Late Cretaceous period approximately 70 million years ago (Myr ago)”1 Much later, in 1997, a second species of this fish was discovered in the seas around Indonesia. The two species were named Latimeria chalumnae and L. menadoensis respectively.
Although the two populations of coelacanth resemble each other, they were given different species names because of some genetic differences between the two groups. The similarity in appearance however also extended to the fossil specimens. As an article in Nature declared: “Fascination with these fish is partly due to their prehistoric appearance – remarkably, their morphology is similar to that of fossils that date back at least 300 Myr, leading to the supposition that among vertebrates [animals with backbones], this lineage is markedly slow to evolve.”1
The resemblance of the living coelacanth fish to such apparently ancient fossils, naturally raised the question whether “the genome of the coelacanth is as slowly evolving as its outward appearance suggests.”1 Indeed, recent genetic work on the coelacanth had suggested that this fish does not show a lot of genetic variation. But a serendipitous discovery changed this view.
A graduate student at University of Toronto was assigned an evolution-based project: to find out what organism(s) display ancestral versions of the human gene CGGBP1 which is involved in gene regulation. Thus, Isaac Yellan searched genetic databases of different animals for sequences which resemble this gene. Very unexpectedly he found that the coelacanth has many variations of this gene, 62 of them to be exact.2 This is far more than any other vertebrate, and the coelacanth is not even included in the category of reptiles, birds and mammals where an organism might be expected to display such a gene.
The speculation which the authors offer is that the CGG Binding Protein is similar to a specific family of jumping genes (transposons) which might several times have invaded the germ line of the coelacanths so that all offspring would express these genes. Even if this were possible, why does the coelacanth exhibit astonishingly high versions of a gene when it was not expected to have even one? However, it is not even certain that the CGGBP genes came from jumping genes. What we do know is that this living fossil harbours far more genetic diversity than anyone ever expected. Whether the rate of change has been fast or slow, it is evident that the coelacanth has only been around for thousands not millions of years.
- Chris T. Amemiya et al. Nature 496 #7445 pp. 311-316.
- Isaac Yellan, Ally W. H. Yang, and Timothy R. Hughes. 2021. Molecular Biology and Evolution. 1-15.
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