ORIGINS OF VIRUSES Viruses are found in all ecosystems and probably have existed since the origins of cellular life. They don’t form fossils so this cannot be proven. They are passed vertically (which means through the offspring of organisms) by inserting their DNA into organisms that then divide, allowing the virus to be in the offspring of an organism. This is how virologists have determined that viruses have existed for millions of years. There are three main hypotheses of how viruses originated, including these: •
Viruses may have once been smaller cells that were parasites of larger cell but lost all of the genes except those involved in parasitism. There are examples of “middle ground” bacteria, such as chlamydia and rickettsia, which are living things that can only reproduce inside cells (but, unlike viruses, these are cellular). Viruses may have degenerated from organisms like this. This is referred to as the “reduction hypothesis” or “degeneracy hypothesis”.
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Viruses may have evolved from escaped bits of nucleic acids that “escaped” from nucleic acids in larger organisms. This can happen when plasmids (which are pieces of DNA that escape from cells) or transposons (which are pieces of DNA that move to a different part of the same cell) get out of the cell. This is called the “escape hypothesis” or the “vagrancy hypothesis”.
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They may have evolved from pieces of DNA and RNA at the beginning of time and may have been dependent on cellular life for billions of years on earth. This is called the “virus first hypothesis” and means that viruses have always been around. People who propose this idea point to viroids, which are molecules of RNA that have no protein coat, commonly infecting plants. These are called subviral particles. There are also “satellite viruses” that are also in-between viruses. They have a protein coat that isn’t technically their own but belong another virus that they need to infect the cell before they can be released. These defective viruses include the hepatitis D virus that needs a coinfection with hepatitis B before it can become infective.
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