Tracing the ancient origin of retroviruses — the family of viruses that includes HIV — is a big undertaking, partly because of the absence of fossils. But a new study conducted by researchers at Oxford University suggests that retroviruses are nearly half a billion years old, significantly older than previously thought.

Until now, scientists thought that retroviruses traced back roughly 100 million years, about as old as terrestrial placental mammals. But at half a billion years old, retroviruses probably developed in marine vertebrates.

According to the study, retroviruses made the transition from the sea to land along with the evolution of terrestrial vertebrates. "Their widespread distribution is a result of ancient origins, not simply the tendency of retroviruses to cross species boundaries," Aris Katzourakis, associate professor at Oxford University's Department of Zoology and author of the study, told Seeker.

"We tend to think of retroviruses over recent timescales, for example, HIV-1 crossed from chimpanzees to humans about 100 years ago, leading to the AIDS pandemic," Katzourakis said. "Our work provides a context in which to understand the association of retroviruses with their hosts," he said.

The researchers used genome sequences from retroviruses that resemble the 'foamy' viruses, a group of viruses that are widespread in mammals. They unearthed genomic fossils for so-called endogenous retroviruses in highly diverse hosts, such as ray-finned fish and amphibians that had no previous link to retroviruses.

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Contrary to the thinking that retroviruses evolved rapidly, the researchers also discovered that the rate of retrovirus evolution slowed the farther back in time they looked.

With this new context in mind, researchers can look back at retroviruses through a deeper lens. "We really need to think about the interactions of retroviruses and host immunity as the product of an ancient arms race stretching back to the early origins of vertebrates," Katzaourakis said.

"As we understand the nature of the interaction between viruses and host immunity, we will be better placed to intervene in this delicately balanced arms race in order to develop novel treatments and interventions," Katzourakis noted.

The findings were reported in the journal " Nature Communications."

Caption: HIV-infected T-cells shown under high magnification.

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