Let’s go far back in time to talk about a fish that was once considered a living fossil.
In December 1938, local angler Captain Hendrick Goosen hauled in his fish-catch off the east coast of South Africa and saw a fish he had never seen before. This unusual looking creature was brought to the attention of Museum curator Marjorie Courtenay-Latimer, who had been interested in seeing unusual specimen brought in by fishermen. Marjorie took the fish back to her museum and tried, unsuccessfully, to identify it in her books. She asked an ichthyologist friend to help her identify it but he was away. She did not have facilities at her museum to preserve the fish, so she sent it to a taxidermist. When her ichthyologist friend J.L.B Smith returned, he immediately recognized it as a coelacanth, a fish that was thought to be extinct for 66 million years. Smith called it Latimeria chalumnae after Marjorie and the Chalumna River, where it was found. It would remain the only specimen of the fish for 14 years, when another one was brought in.
So, the evolutionary line of the coelacanth disappeared from the fossil record for over 60 million years, only to reappear much later as a living species. Since 1938, West Indian Ocean coelacanth have been found off the coasts of east and south Africa. And then in 1999, a second species was described in North Sulawesi, Indonesia – known as the Indonesian coelacanth or Latimeria menadoensis. These are the only two living species known today. Related to lungfish, and tetrapods such as amphibians and reptiles, coelacanths were initially considered transitional species between fish and tetrapods. Therefore, it was thought that they might be the most recent shared ancestor between terrestrial and marine vertebrates. In 2013, its genome was sequenced to discover that this was not the case. Instead, the lungfish is the most recent shared ancestor between marine and terrestrial vertebrates. The coelacanths had already diverged from lungfish around 390 million years ago; long before the latter ventured on to land.
Coelacanths are large, plump, lobe-finned fish that can grow to more than two meters (6 feet 6 inches), weigh around 90 kilograms (200 pounds) and are estimated to live for 60 years or more. They have leg-like lobed fins (which is probably why they were thought to be a transitional species) and are more closely related to humans and our closest fish relatives: the lungfish, than any other type of fish.
Having emerged around 400 million years ago, they were considered a living fossil because scientists thought they had not changed since the cretaceous period, when flightless dinosaurs still roamed the Earth. Now, new genetic evidence shows that the coelacanth is not a living fossil because it has undergone evolutionary changes. In fact, it has gone through widespread changes, many times – all by hijacking genes from other species!
The research was published in science magazine Molecular Biology and Evolution in February 2021. Lead author and University of Toronto molecular geneticist Isaac Yellen was looking through genetic databases for the ancestral version of a particular human gene involved in gene regulation. He found that one species of coelacanth (L. chalumnae), the kind found in the Indian Ocean in 1938, had many versions of this gene CGGBP1 and all the variations did not share a common ancestor. This means that some time in their evolutionary history, the coelacanth hijacked 62 of these genes from other unrelated species. This probably happened around 23 million years ago, when the two coelacanth species separated from each other. The Indonesian coelacanth does not have the same variety of these genes.
This type of transfer is known as horizontal gene transfer and the genes that can do this are called transposons. If they can move into the right place on the genome, they can be copied like all the other genes. In such a case, they can also be retained if they are useful to the host, and can lose their ability to jump, thus becoming part of the host species. But they can also jump into the wrong place and become parasitic, causing diseases and other problems.
In the case of the coelacanth, the genes found a position on the genome that was useful for the fish and were therefore retained in their new host genome – and this seems to have happened many times. According to the researchers, it is unusual to find so many transposons. Like the human version, in the coelacanth too, they seem to be involved in gene regulation, although their exact function is still not clear. More investigation into their genome can help us understand our own evolution.
How did we know that the coelacanth even existed if no one had seen it? Fossil records were found from 360 million years ago and 80 species have been described – all but two are extinct. Here is another cool thing. Right after this study was published in February 2021, fossilised remains of a coelacanth – a bony lung – were identified by scientists from the University of Portsmouth, from a private collection. This one was the largest fossil of its kind (based on the size of the lung that was found) and was from phosphate deposits in Morocco. It was discovered next to a pterodactyl fossil, indicating that it lived in the cretaceous era— about 66 million years ago. Usually, coelacanths grow up to two meters in length but this one was probably five meters.
While we may have found coelacanths alive and well in our oceans, they are still rare and are also endangered, so further studies are going to be limited. What we do now know is that the idea of ‘living fossil’, a species that has not changed genetically over millions of years may need to be revisited. Although coelacanth genes have evolved more slowly than other fish, reptiles, and mammals, they have nonetheless evolved.
I heard about the coelacanths when I was in school and the idea of a species unchanged for millions of years was extremely exciting for me. Even though the notion of a living fossil must be put to rest, the coelacanth is still exciting because of its leg-like fins, its ability to remain undetected for so many years, and now because it stole genes from other species. Evolution is cool like that.
All images Creative Commons and Public Domain/ Wikimedia Title music: Hovering Thoughts by Spence (YouTube Music Archive) Featured Music: 3 Water Spring (Filmora Wondershare Free Music Archive)
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