Are they an older, simpler form of life — or are they parasites that arose only once cells had evolved?
It is thought this microorganism is host to a fragment of DNA that can build a capsule around itself, thus answering that elusive question of how viruses first arose.
According to Ricardo Cavichioli and his team from the University of New South Wales, they found a microorganism in the lakes of the Rauer Islands off the coast of Antarctica that might shed some light on the question.
The organism, which they named Halorubrum lacusprofundi R1S1, is an Archaean: a kind of single-celled organism that looks like a bacterium, but actually belongs to a separate domain of life.
Recognizing viruses often play an important role in Antarctic ecosystems, they searched for viruses inside the organism's cells and found something unexpected: a plasmid.
Plasmids are small fragments of DNA, often circular, that reside in living cells.
They are not part of the cell's main genome, and can replicate themselves independently. Often, a plasmid will carry a gene that is somehow useful to the cell: for instance, antibiotic resistance genes. The plasmid — which the team calls "pR1SE" — is unusual as it allows the genes it carries to make protective bubbles and seek out new hosts.
In other words, pR1SE looks and acts a lot like a virus. "There really are no major distinctions left between plasmids and viruses," says Cavicchioli.
He suggests that viruses could have evolved from plasmids like pR1SE, by acquiring genes from their host that allowed them to make a hard capsid shell rather than a soft vesicle. It also lines up with existing evidence on the origin of viruses.
Previously there have been three leading ideas: either viruses originated before cells, or some cells evolved simpler forms and became viruses, or genes "escaped" from cells and became viruses.
This third escape hypothesis has gathered support in recent years: in March 2017, a study suggested that many capsid proteins can be traced back to proteins found in cells.
The evidence implies that such escapes began early in the history of life, says Patrick Forterre of the Pasteur Institute in Paris.
"Traditionally the escape hypothesis has been associated with the idea that viruses are recent. Now the escape hypothesis should be viewed in a broader context."
The first viruses may have escaped from some of the first cells on Earth.