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 percent of the human genome is comprised of ERVs, for example.
