Paleovirology is the study of viruses that existed in the past but are now extinct. In general, viruses cannot leave behind physical fossils,[1] therefore indirect evidence is used to reconstruct the past. For example, viruses can cause evolution of their hosts, and the signatures of that evolution can be found and interpreted in the present day.[2] Also, some viral genetic fragments which were integrated intogermline cells of an ancient organism have been passed down to our time asviral fossils,[2] orendogenous viral elements (EVEs).[3] EVEs that originate from the integration ofretroviruses are known asendogenous retroviruses, or ERVs,[4] and most viral fossils are ERVs. They may preserve genetic code from millions of years ago, hence the "fossil" terminology, although no one has detected a virus in mineralfossils.[2] The most surprising viral fossils originate from non-retroviralDNA andRNA viruses.
Although there is no formal classification system for EVEs, they are categorised according to the taxonomy of their viral origin. Indeed, all known viral genome types and replication strategies, as defined by theBaltimore classification, have been found in the genomic fossil record.[5][6] Acronyms have been designated to describe different types of viral fossil.
NIRV: Viral fossils originating from non-retroviral RNA viruses have been termedNon-retroviral Integrated RNA Viruses or NIRVs.[7][8] Unlike other types of viral fossils, NIRV formation requires borrowing the integration machinery that is coded by the host genome or by a co-infecting retrovirus.[9]
