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Viruses are found wherever there is life and have probably existed since living cells first evolved

The origin of viruses is unclear because they do not form fossils, so molecular techniques have been used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose

There are three main hypotheses that try to explain the origins of viruses:

1. Regressive hypothesis:

  • Viruses may have once been small cells that parasitised larger cells
  • Over time, genes not required by their parasitism were lost
  • The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells
  • This support the theory
  • This is also called the degeneracy hypothesis, or reduction hypothesis

2. Cellular origin hypothesis:

  • Some viruses may have evolved from bits of DNA or RNA that “escaped” from the genes of a larger organism
  • The escaped DNA could have come from plasmids or transposons
  • Once called “jumping genes”, transposons are examples of mobile genetic elements and could be the origin of some viruses
  • They were discovered in maize by Barbara McClintock in 1950
  • This is sometimes called the vagrancy hypothesis, or the escape hypothesis

3. Coevolution (virus-first) hypothesis:

  • Proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on Earth and would have been dependent on cellular life for billions of years
  • Virusoids of plants and the hepatitis delta virus of humans have an RNA genome but depend on other viruses for their replication (thus referred to as satellites)
  • These satellites may represent evolutionary intermediates of viroids and viruses