DNA analysis indicates these viral remnants are ancient hepadnaviruses, a family of DNA viruses that includes the hepatitis B virus (HBV), which infects roughly one-third of the world’s human population, causing a variety of acute and chronic liver diseases.
These ancient viruses are endogenous viruses: they insert themselves or pieces of themselves into the genome of an organism, and are then passed from one generation to the next. Up until now, nearly all endogenous viruses found have been retroviruses. This is not surprising since insertion of the retrovirus into the host’s genome is an essential part of its life cycle. But when deactivated fragments of these viral freeloaders reside in a host’s genome for millions of years, they are referred to as “fossil viruses” and the scientists who study them are palaeovirologists.
As genome sequencing becomes easier and cheaper, palaeovirologists are now uncovering remnants of other types of virus such as hepadnaviruses in a variety of organisms ranging from fungi to plants – and even including humans.