We may need to rethink what ‘sharks’ are, thanks to a new study from Yale University.
Evolutionary biologists compared the DNA of different sharks to one another, and also to similar but non-shark species such as rays, skates and chimaeras.
They found that sharks are not necessarily each other’s closest relations, throwing into question the entire concept of sharks as a biological category.
“Our research casts doubt on the idea that sharks all share a common ancestor, exclusive of rays and skates,” first author Chase Brownstein, a PhD candidate at Yale, told BBC Science Focus.
“This can be read two ways: either sharks are not a natural group, insofar as they don’t share exclusive common ancestry, or rays and skates are simply another type of shark.”
In total, the scientists analysed genetic data from 48 different species, looking at their entire genomes.
“Genomes are the sum total of the genetic information stored in our cells,” last author Thomas Near, professor of ecology and evolutionary biology at Yale, told BBC Science Focus.
“By comparing whole genomes as opposed to only a handful of genes, we can get a better understanding of how organisms are related to one another.”
Brownstein explained that they investigated “how different ways of sampling DNA sequences from across the genome might affect our understanding of how sharks, rays and other cartilaginous fishes are related.”
The evidence they found was conflicting. When they looked at some segments of DNA, called exons, Brownstein and Near found evidence that all sharks might be part of the same family.
But other sections of DNA suggested the opposite – that certain species of shark (such as frilled and cow sharks) might be more closely related to rays than other shark species.
These weird sharks might, therefore, just be distant cousins to the rest of the shark world, having split off from the genetic tree far earlier in their evolutionary history.
But, if sharks are a distinct group, the biologists could use their genetic data to estimate when they first emerged, evolutionarily.
“We estimated that this group appeared about 300 million years ago,” said Brownstein. “This about coincides with – or is slightly more recent than – the time we last shared a common ancestor with living amphibians!”
The scientists explained that sharks, skates and rays form one of the most ancient branches of vertebrates – that is, animals that have a hard, segmented backbone.
“They are the first type of jawed vertebrate to diverge from the tree, meaning that they provide critical information on the first animals with jaws and bony skeletons,” continued Brownstein. “Our hope is that this new research gets attention on this understudied part of the vertebrate tree.”
The Yale study is currently awaiting publication.
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