Where do viruses originate

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If the proteins on the surface of the new virus have significantly changed from currently circulating influenza virus strains, then no one will have immunity and the new virus can easily spread. This shift in the influenza virus is called antigenic shift. This is what we think happened with the H1N1 influenza epidemic, with the shift occurring in pigs and then jumping to humans to start the outbreak.

There is also genetic evidence that this mechanism can occur in coronaviruses, although its role in the emergence of SARS-CoV-2 remains to be determined.

New viruses can also emerge through genetic mutations within the virus genome, which are more common among viruses that, instead of DNA, store their genetic information in the similar molecule RNA.

This because these viruses with the exception of coronaviruses lack a way to check for mistakes when they replicate. Most of the mutations produced during replication will be damaging to the virus but some will enable it infect a new host more effectively.

Recent analysis of the genome suggest that the virus had been circulating in a very similar form to today for approximately 40 years. The closest relative of the virus that we can identify is one found in bats. However, this virus and SARS-CoV-2 probably shared a common ancestor approximately years ago, and so this bat virus is not the cause of the outbreak. Although these viruses share a common ancestor, 40 years of evolution since then has separated them.

Closely related viruses have been found in pangolins , for example. The second model is called the regressive hypothesis, sometimes also called the degeneracy hypothesis or reduction hypothesis.

This one suggests that viruses were once small cells that parasitized larger cells, and that over time the genes not required by their parasitism were lost. The discovery of giant viruses that had similar genetic material to parasitic bacteria supported this idea.

The third model is escape hypothesis , or vagrancy hypothesis , and states that viruses evolved from bits of RNA or DNA that escaped from genes of larger organisms. For example, bacteriophages viruses that infect bacteria came from bits of bacterial genetic materials, or eukaryotic viruses are from bits of genetic material from eukaryotes like us.

However, in this model, it would be expected that viral proteins would then share more qualities with their hosts, but this is largely not the case. Some recent discoveries of giant viruses have even further complicated the question about the origin of viruses. These discoveries also challenge many of the classical definitions of what makes a virus, such as the size requirement, gene behavior, and how they replicate.

Giant viruses were first described in Mimiviruses are different from viruses in that they have way more genes than other viruses, including genes with the ability to replicate and repair DNA. The pandoravirus, discovered in , is even larger than the mimivirus and has approximately genes, with 93 percent of their genes not known from any other microbe. The pithovirus was discovered in from a Siberian dirt sample that had been frozen for 30, years.

However, the pithovirus possesses some replication machinery of its own. While it contains fewer genes than the pandoravirus, two-thirds of its proteins are unlike those of other viruses. Tupanvirus was discovered in Brazil. It holds an almost nearly complete set of genes necessary for protein production. The discoveries of these giant viruses and others not listed here have made some researchers suggest they lie somewhere between bacterium and viruses, and might even deserve their own branch on the Tree of Life.