HIV belongs to a family of viruses, retroviruses, that could be as much as half a billion years old. Scientists have now discovered that retroviruses could have emerged in ancient marine vertebrates, and could have been present in these hosts through the evolutionary transition from sea to land.

This would push the date of retroviruses' emergence back by several hundred million years, to the early Palaeozoic era, some 450 million years ago. "Previously the oldest robust age estimate for the origins of retroviruses was about as old as mammals. We have pushed this back to the origins of oldest vertebrates," study author Dr Aris Katzourakis, from the University of Oxford, told IBTimes UK.

It is usually very difficult to study the deep evolutionary history of viruses, because of the rapid speed at which their genome changes. "Viral genomes can mutate at speed over a thousand times greater than mammals' genomes. Information about the evolutionary history of the virus is thus lost very quickly," Katzourakis added.

Another problem is the patchiness of the geological fossil record.

Over a very long period of time, viral genomes have been inserted into hosts, so a record of ancient viruses does exists, but it remains relatively patchy, especially concerning events beyond a certain time frame.

However, this research, published in Nature Communications, is based on the discovery of a rich fossil record and an innovative method to extrapolate the ancient history of viruses in modern genomes.

Fish and amphibian hosts

The researchers looked at the genome sequences from endogenous retroviruses that resemble the "foamy" viruses – a group of viruses that tend to diverge alongside their hosts. This involved analysing 36 lineages of amphibian and fish foamy-like endogenous retroviruses.

The scientists showed that the first major retroviral lineage may have emerged in these aquatic jawed vertebrate hosts more than 450 million years ago, much earlier that previously expected.

This finding provides them with new clues to retrace the history of retroviruses, and could prove useful to find treatments against HIV.

"These findings give us context to understand the arms race that is taking place between the retroviruses such as HIV and their hosts. We discovered that the origins of retroviruses coincide with the beginnings of adaptive immunity in vertebrates, and this can help us better understand the interactions between viral and host immunity, and how hosts' respond to retroviruses," Katzourakis said.

Future studies may want to investigate specific host innovations in response to the virus, in the hope that this will lead to new therapeutic options. "For instance, if we find that a particular species developed a gene to fight retroviruses, we may identify an analogous gene in humans or a way to translate this in treatment", the researcher concluded.