Paleovirology
Paleovirology is the study of viruses that existed in the past but are now extinct. In general, viruses cannot leave behind physical fossils 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. Also, viral genetic material that was integrated into another organism has been passed down to our time as a viral fossil. Viral fossil is an informal term for regions of genomes that originate from ancient germline integration of viral genetic material. The scientific term for such regions is endogenous viral element or EVE. EVEs that originate from the integration of retroviruses are known as endogenous retroviruses, or ERVs, and most viral fossils are ERVs. They may be traced to millions of years back, hence the terminology, although strictly speaking, no one has detected a virus in fossils. The most surprising viral fossils originate from non-retroviral DNA and RNA 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 the Baltimore classification, have been found in the genomic fossil record. Acronyms have been designated to describe different types of viral fossil.
NIRV: Non-retroviral Integrated RNA Virus
HERV: Human Endogenous Retrovirus
Other viral fossils originate from DNA viruses such as hepadnaviruses (a group that includes hepatitis B).
Viral fossils originating from non-retroviral RNA viruses have been termed Non-retroviral Integrated RNA Viruses or NIRVs. 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.
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