Researchers have found 214 RNA viruses that have never been seen before, lurking inside animals usually ignored when it comes to viral infections - frogs, fish and reptiles.
Not only does the finding have implications for studying human virology in the future, it also shows that viruses have an evolutionary history dating back millions of years - and they evolved concurrently with vertebrates.
"This study reveals some groups of virus have been in existence for the entire evolutionary history of the vertebrates - it transforms our understanding of virus evolution," says study co-author, epidemiologist Eddie Holmes from the University of Sydney, Australia.
"For the first time we can definitely show that RNA viruses are many millions of years old, and have been in existence since the first vertebrates existed."
Holmes and his team looked at the RNA of over 186 species, including reptiles such as snakes, turtles and lizards; amphibians such as frogs, salamanders and caecilians; and fish such as jawless, cartilaginous, ray-finned, lancelets and lungfish.
"We extracted total RNA from the gut, liver, and lung or gill tissue of these animals, which was then organised into 126 libraries for high-throughput RNA sequencing," the researchers wrote in their paper.
"In total, we generated 806 billion bases of sequence reads that were assembled and screened for RNA viruses."
They focussed on viruses specific to vertebrates, and the 214 viruses they found fell into every family or immediate sister-groups of RNA viruses associated with vertebrate infection.
Twenty-four percent of those viruses were recovered from different tissues within the same individual.
"Fish, in particular, carry an amazing diversity of viruses, and virtually every type of virus family detected in mammals is now found in fish," Holmes said. "We even found relatives of both Ebola and influenza viruses in fish."
But that doesn't mean we have to freak out about getting something like Ebola from a fish. Humans and fish are very different from one another - and further analyses of the viruses revealed that they had been evolving alongside the animals they infect, arriving on the scene at the same time as vertebrates.
The team concluded that modern viruses emerged from the ocean with our fishy ancestors, and have been with us the entire time.
But this also means that modern fish influenza, for example, has evolved to live optimally inside fish, not mammals - and hopefully we are too genetically different for cross-species transmission to be possible (although it can't be completely ruled out).
The study does show that there are many more RNA viruses out there than we ever expected, and this could help identify RNA viruses that may afflict humans in the future.
"This study emphasises just how big the universe of viruses - the virosphere - really is. Viruses are everywhere," Holmes said.
"It is clear that there are still many millions more viruses still to be discovered."
The research has been published in the journal Nature.