Virus Evolution

Zika viruses, illustration
The Zika virus. KATERYNA KON/SCIENCE PHOTO LIBRARY / Getty Images

All living things must exhibit the same set of characteristics in order for them to be classified as living (or once living for those that have died off at some point in time). These characteristics include maintaining homeostasis (a stable internal environment even when the external environment changes), ability to produce offspring, have an operating metabolism (meaning chemical processes are happening within the organism), exhibiting heredity (the passing down of traits from one generation to the next), growth and development, responsiveness to the environment the individual is in, and it must be made up of one or more cells.

Are Viruses Alive?

Viruses are an interesting topic virologists and biologists study due to their relationship to living things. In fact, viruses are not considered to be living things because they do not exhibit all of the characteristics of life that are referenced above. This is why when you catch a virus there is no real “cure” for it and only the symptoms can be treated until the immune system hopefully works it out. However, it is no secret that viruses can cause some serious damage to living things. They do this by essentially becoming parasites to healthy host cells. If viruses are not alive, though, can they evolve? If we take the meaning of “evolve” to mean change over time, then yes, viruses do indeed evolve. So where did they come from? That question has yet to be answered.

Possible Origins

There are three evolutionary based hypotheses for how viruses came into being that are debated among scientists.

Others dismiss all three and are still looking for answers elsewhere. The first hypothesis is called the “escape hypothesis.” It was asserted that viruses are actually pieces of RNA or DNA that broke out, or “escaped” from various cells and then began invading other cells. This hypothesis is generally dismissed because it does not explain intricate viral structures such as capsules that surround the virus or mechanisms that can inject the viral DNA into host cells.

The “reduction hypothesis” is another popular idea about the origin of viruses. This hypothesis claims that viruses were once cells themselves that became parasites of larger cells. While this explained much of why host cells are needed for viruses to thrive and reproduce, it is often criticized for the lack of evidence including why small parasites do not resemble viruses in any way. The final hypothesis about the origin of viruses has come to be known as the “virus first hypothesis.” This says viruses actually predated cells or at least were created at the same time as the first cells. However, since viruses need host cells in order to survive, this hypothesis does not hold up.

How We Know They Existed Long Ago

Since viruses are so small, there are no viruses within the fossil record. However, since many types of viruses integrate their viral DNA into the genetic material of the host cell, traces of viruses can be seen when DNA of ancient fossils is mapped out. Viruses adapt and evolve very quickly since they can produce several generations of offspring in a relatively short amount of time. The copying of the viral DNA is prone to many mutations in every generation since the host cells checking mechanisms are not equipped to handle “proofreading” the viral DNA.

These mutations can cause the viruses to quickly change over a short period of time driving viral evolution to be done at very high speeds.

What Came First?

Some paleovirologists believe that RNA viruses, those that only carry RNA as a genetic material and not DNA may have been the first viruses to evolve. The simplicity of the RNA design along with these types of viruses’ abilities to mutate at an extreme rate make them excellent candidates for the first viruses. Others believe, however, that the DNA viruses came into being first. Most of this is based off of the hypothesis that viruses were once parasitic cells or genetic material that escaped their host to become parasitic.

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Scoville, Heather. "Virus Evolution." ThoughtCo, Dec. 3, 2017, thoughtco.com/virus-evolution-overview-1224539. Scoville, Heather. (2017, December 3). Virus Evolution. Retrieved from https://www.thoughtco.com/virus-evolution-overview-1224539 Scoville, Heather. "Virus Evolution." ThoughtCo. https://www.thoughtco.com/virus-evolution-overview-1224539 (accessed January 17, 2018).