Retroviruses, the family of viruses that includes HIV, are almost half a billion years old, according to new research by British scientists.
This means they are several hundred million years older than previously thought.
The new research suggests the viruses have ancient marine origins, and they stayed with their animal hosts through the evolutionary transition from sea to land.
The study could lead scientists to discover of tricks immune systems have developed to beat them in the past to develop more effective treatments in the future.
Retroviruses are a family of viruses including the HIV virus, responsible for the AIDS pandemic.
They can cause cancers and problems with the immune system in a range of animals.
‘Very little has been known about the ancient origin of retroviruses, partly because of the absence of geological fossil records,’ said study author Dr Aris Katzourakis, from Oxford University’s Department of Zoology.
‘Retroviruses are broadly distributed among vertebrates and can also transmit between hosts, leading to novel diseases such as HIV, and they have been shown to be capable of leaping between distantly related hosts such as birds and mammals.
‘But until now, it was thought that retroviruses were relative newcomers – possibly as recent as 100 million years in age.
The new research shows retroviruses are at least 450 million years old, if not older.
They must have originated together with, if not before, their vertebrate hosts in the early Paleozoic era.
‘They would have been present in our vertebrate ancestors prior to the colonisation of land and have accompanied their hosts throughout this transition from sea to land, all the way up until the present day.’
The ‘retro’ part of the name retrovirus comes from the fact they are made of RNA.
The viruses can convert this RNA into DNA and insert into their host genome, the opposite direction to the normal flow of information in a cell.
This property means they can occasionally be inherited, as endogenous retroviruses.
Retroviruses are a family of viruses including the HIV virus, responsible for the AIDS pandemic. Scanning electron micrograph of HIV-1, pictured
These endogenous retroviruses form a virtual ‘genomic fossil record’ that can be used to look back into their evolutionary history.
The new research, published in Nature Communications, used genome sequences from endogenous retroviruses that resemble the ‘foamy’ viruses – a group of viruses that tend to diverge alongside their hosts.
Foamy viruses are widespread in mammals, but in the new study the researchers unearthed genomic fossils for foamy-like retroviruses in highly diverse hosts.
These include ray-finned fish and amphibians in which retroviruses had not previously been found.
During this study, the researchers overcame one of the key limitations in studying the deep evolutionary history of viruses: their rapid evolution.
This trait facilitates the reconstruction of viruses’ recent history but obscures their more distant past.
However, a new model used in this research – in combination with the genomic fossil records of the foamy-like viruses – allowed the scientists to account for an apparent slowdown in the rate of evolution the further back they went.
‘These findings show that this medically important group of viruses is at least up to half a billion years in age – far older than previously thought,’ Dr Katzourakis added.
‘They date back to the origins of vertebrates, and this gives us the context in which we should consider their present-day activity and interactions with their hosts.
‘For example, we need to consider the adaptations that vertebrates have developed to combat viruses, and the corresponding viral countermeasures, as the product of a continuous arms race that stretches back hundreds of millions of years.
‘Our inferred date of the origins of retroviruses coincides with the origins of adaptive immunity, and thus it is likely that retroviruses have played an important role in the emergence of this key tool in vertebrate antiviral defence.
‘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.
‘And as we build a clearer picture of the origins of the diverse groups of viruses that infect us today, we should come closer to unravelling the mystery of their ultimate origins.’