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.
Aris Koutzarakis at the University of Oxford screened every available mammal genome for ERVs based on foamy viruses, and he found hundreds of them in the DNA of the two-toed sloth. These viral fossils have changed substantially since they first invaded the sloth's ancestor, and most have lost the ability to produce proteins of any kind.
Sloths belong to Xenartha, one of the oldest lineages of mammals and Koutzarakis searched for the sloth ERVs in other members of the group. He found some in three-toed sloths but not anteaters or armadillos. That suggests that the viruses first infiltrated the mammalian dynasty between 21 and 55 million years ago, before the two families of sloths split apart, but after they branched off from the anteater or armadillos lines. Calculations based on the xenarthans' rate of genetic evolution arrived at similar estimates: 39 to 45 million years ago.
But the foamy viruses were infecting mammals long before this point. By comparing the sloth ERVs with their modern relatives, Koutzarakis worked out that the evolutionary history of these viruses perfectly matches that of the mammals. They have co-evolved with us for around 105 million years.
Reference: Science 10.1126/science.1174149
Image: Sloth by Leyo