The two-toed sloth is a walking hotel. The animal is so inactive that its fur acts as an ecosystem in its own right, hosting a wide variety of algae and insects. But the sloth has another surprise passenger hitching a ride inside its body, one that has stayed with it for up to 55 million years – a virus.
In the Cretaceous period, the genes of the sloth’s ancestor were infiltrated by a “foamy virus“, one of a family that still infects humans, chimps and other mammals today. They are examples of retroviruses, which reproduce by converting an RNA genome into a DNA version and inserting that into the genome of whatever animal they’re infecting. If these hitchhikers become permanent tenants, as so often happens, they become known as endogenous retroviruses or ERVs.
ERVs act as a sort of viral fossil record, telling us about the ancient viruses that infected ancestral animals. In the sloth’s case, its ERVs tell us that foamy viruses must have been doing the rounds among ancient mammals over 100 million years ago, back when the dinosaurs still ruled the planet.
Despite the passing of a geological age, their descendants still circulate today and are astonishingly unchanged. The modern viruses look very similar to the one that inserted its genetic material into the sloth’s ancestors. That’s especially amazing because retroviruses – take HIV as an example – have a reputation for mutating at incredibly high rates.