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.
The natural world is rife with leftovers. Over the course of evolution, body parts that no longer benefit their owners eventually waste, away leaving behind shrivelled and useless anatomical remnants. The human tailbone is one such example. Others include the sightless eyes of cavefish that live in total darkness, the tiny spurs on boas and pythons that hint at the legs of their ancestors, and the withered wings of the Galapagos cormorant, an animal that dispensed with flight on an island bereft of land predators.
Animal genomes contain similar remains. Just like organs, genes also waste away if they stop being useful. They accumulate crippling mutations that kill their ability to make proteins and turn them into functionless “pseudogenes“. They have no useful role other than to tell inquisitive geneticists about their histories.
So organs can degenerate and genes can decay. The two processes should clearly run in parallel, but there are few documented examples of this. Robert Meredith from the University of California Riverside, has uncovered just such an example, a beautiful case study where the decay of a gene called enamelin clearly parallels the loss of a body part – tooth enamel.
Enamel is an extremely tough material that coats the outside of our teeth. Many proteins are essential for making it, including the aptly named enamelin, which is produced by a gene of the same name. Meredith’s team sequenced the enamelin gene (ENAM) in 20 species of mammals that either have teeth without enamel caps (like aardvarks, sloths and armadillos) or that lack teeth altogether (like anteaters, pangolins and several whales).
Today, every single one of these species has a broken version of enamelin. Mutations have crept into these genes, which stop the production of the protein before it’s fully formed. The result is a busted gene that produces a runty, useless protein. Other mammals don’t suffer from this problem; Meredith found that ENAM is fully functional in 29 other toothed mammals, including cats, cows and dolphins.
This is exactly what you’d expect, but the clear link between the lack of enamel and a broken enamel-producing gene is exciting nonetheless. It’s a tale that, in Meredith’s own words, provides “manifest evidence for the predictive power of Darwin’s theory”.