An activist from a non-governmental organisation (NGO) lights candles during an AIDS awareness campaign on the eve of World AIDS Day in Agartala, capital of India's northeastern state of Tripura, Nov. 30, 2011. REUTERS/Jayanta Dey

When our vertebrate ancestors first moved to land in the late Devonian period over 400 million years ago, they may have been carrying an unwanted guest in their cells. A study published Tuesday in the journal Nature Communications suggests that retroviruses — the family of viruses that includes HIV — may have been with their animal hosts when they made their transition from land to sea, making these parasites hundreds of millions years older than previously believed.

Prior to this study, scientists estimated that these viruses, which are known to possess the capability to jump between distantly related hosts such as birds and mammals, were only about 100 million years old.

“Very little has been known about the ancient origin of retroviruses, partly because of the absence of geological fossil records,” study co-author Aris Katzourakis from the University of Oxford said in a statement. “Our new research shows that retroviruses are at least 450 million years old, if not older, and that they must have originated together with, if not before, their vertebrate hosts in the early Paleozoic era. Furthermore, 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.”

In order to fill the gap in our understanding of the origin and evolutionary history of retroviruses, the researchers studied “foamy” viruses — named after their ability to give infected cells in a tissue culture a distinctive foam-like appearance. These viruses can infect a wide variety of animals, making them ideal for investigating viral evolution across a wide lineage of hosts.

“Retroviruses ... can occasionally leave genomic fossils within their host genome, known as endogenous retroviruses (ERVs). ERVs are relics of past infections, resulting from viral genomic integrations that occur in host germ-line cells, and are in turn passed down from parents to offspring as part of the host genome,” the researchers wrote in the study. “Similar to other retroviruses, FVs occasionally leave viral genomic fossils in their host genomes. ... Additional ERV data from multiple vertebrate classes could help resolve the ancient origins of FVs, and also shed light on the early history of retroviruses as a whole.”

Using a combination of traditional techniques — which involve creating an evolutionary tree by using mutation rates — and a tailor-made mathematical model, the researchers were able to trace back the evolution of 36 fish and amphibian lineages of FV-like ERVs (FLERVs), some of which have the largest known retroviral genomes. This allowed them to work out that retroviruses first emerged between 460 and 550 million years ago, if not earlier.

Getting a clearer picture of the phylogenetic history of this medically important group of viruses is crucial for scientists seeking to intervene in the “arms race” between viruses and their hosts. The researchers said that doing so could allow them to develop newer and better treatments for viral infections.

“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,” Katzourakis said. “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.”