Sharks have finally been sequenced, a remarkable feat in light of their giant genomes. Specifically, a team of Japanese scientists painstakingly analyzed two species of tiny sharks that they could conveniently grow in aquariums.
Among other things, the brownbanded bamboo shark and the cloudy catshark have equivalents of the human genes that regulate reproduction, sleep, growth – and metabolism, which includes the propensity to obesity, the RIKEN Center team reports in Nature Ecology and Evolution.
If sharks and humans have the same genetic elements (albeit somewhat differentially evolved), these genetic elements apparently existed in the animal ancestor common to sharks and mammals that lived more than 450 million years ago.
Ergo, the genes for obesity, sleep regulation, reproduction, etc., would have existed in the last common ancestor before the split-off of sharks and bony fish.
Sharks are cartilaginous fish. They don’t have bones. Bony fish have bones.
Sharks evolved into other sharks. The first bony fish would go on to evolve into other bony fish – which would evolve into every vertebrate we know, from dinosaurs to lizards to humans.
We cannot know who exactly the last ancestor of sharks and men was. It must have been a gnathostome (a jawed fish that diverged into the cartilaginous fish and the vertebrates), but which gnathostome will forever remain obscure.
A 2012 paper in Nature described a fish that lived in the Paleozoic era 300 million years ago, Acanthodes bronni, which was ancestral to sharks and fish. Not surprisingly, the University of Chicago team found that Acanthodes looked like a shark. Meaning the earliest bony fish looked like sharks, not vice versa.
Now, why would shark genomes be so big – bigger than a human’s?
First of all, the tomato also has more genes than you do. So it goes. Humans have about the same number of genes as the nematode worm. A 2015 paper by University of Haifa statisticians and geneticists theorized why You and Nematode could have about 20,000 genes each, yet be so vastly disparate in complexity: Interaction of genetic networks, whose effect is so weak as to be almost undetectable by usual statistical means, but which culminates in Us not Worm.
Or shark. The research team, led by Shigehiro Kuraku at Japan’s RIKEN Center for Biosystems Dynamics Research, found that the shark’s giant genome is due to massive insertions of repetitive elements along the DNA.
In this era of CRISPR-expedited genetic engineering in the garage, the layman may not realize just how enigmatic genetics remains as a science. We may sequence DNA, but the rule – not the exception – is that we don’t know what most of it actually does. We know that our DNA has repetitive elements, too. And we know that sometimes this can affect expression of characteristics. But what the repetitive elements in shark DNA achieve remains to be elucidated.
One possibility why shark genomes kept repetitive elements is that sharks seem to have very slow genetic evolution rates, says the team. Their glacial genetic change rate indicates that they kept many ancestral gene repertoires.
In that, sharks aren’t only living fossils in the sense that they look much the same now and hundreds of millions of years ago. They can be thought of as “living fossils” in a genomic sense too, writes the team.
It’s ratfish to you
To be meticulous, the team points out that one “shark” genome had been sequenced before their work, namely that of the elephant shark – which, the team points out, is not a shark. It’s a ratfish.
Wondrously, like sharks, the elephant shark (aka ghost shark) has also been around for hundreds of millions of years. It also has a glacially slow rate of genetic evolution – the slowest among living animals, according to a 2015 paper. That in turn means that the so-called elephant shark gene set is even closer than real sharks to the primordial ancestor of us all.
Yet if there is one trait science would prefer from the shark rather than the ratfish, it’s longevity. The elephant shark-née-ratfish lives maybe for 15 years. Sharks can live for centuries. Greenland sharks in particular – large fish with slow metabolisms that slowly thrive in icy waters – can definitely live for up to 400 years, while analysis of eye tissue indicates a possible age of over 500 years (though that has yet to be proved to the satisfaction of science).
While about analyzing the genomes of captive bamboo sharks and cloudy catsharks, the Japanese team also spruced up the science of sequencing the massive whale shark, which, unlike its pint-sized relatives, dives as much as 2 kilometers (1.2 miles) deep. The whale shark turned out to have eyes able to see in the depths, while the smaller sharks did not.
If there’s one thing that doesn’t seem to have developed in that ancient ancestral gnathostome, it’s the nose. People have better senses of smell than they necessarily realize: We seem to be more olfactorily advanced than the Neanderthal, for instance. All the sharks sequenced so far proved to have relatively few olfactory receptor genes, implying that with all due respect to the smell of blood in the water, they depend on other systems – such as sensing electromagnetic fields – for navigation.