An ancient retrovirus has been found in human DNA – and it might still be active

Striking evidence has emerged that an ancient virus previously known only from fossil evidence has persistently infected some humans at very low levels for hundreds of thousands or even millions of years. This ancient retrovirus is a kind of living fossil, and the discovery of an intact copy of it within the human genome poses questions as to how it has survived, and suggests others from the distant evolutionary past may lie dormant in the DNA of many species.

A retrovirus replicates by inserting its genome into that of an infected cell. Occasionally, retroviruses infect germ line cells – those found in eggs and sperm – and if these cells survive and go on to create a new organism, that new organism will contain the retrovirus as an inherent part of its genome. In this way the genomes of many mammals, birds and other vertebrates have accumulated many DNA sequences derived from retroviruses, known as endogenous retroviruses (ERVs). About 8% of the human genome is comprised of ERVs, for example.

The vast majority of these sequences are genomic fossils in an advanced state of decay, and incapable of producing any sort of infectious particles. Intriguingly, however, some ERVs have been co-opted to perform physiological functions within the host organism, for example to provide immunity. These domesticated virus sequences, though functional, have effectively become part of the host’s genome. They cannot produce infectious virus particles either, having generally lost the genetic equipment required to do so.