Viruses gym web 
 Illustration: Nicole Elmer

The origin of viruses is a hotly debated topic. It’s unclear how they first evolved. However, there are many ideas floating around out there. There are three classical hypotheses but many new ideas and discoveries challenging them.

The first one is the virus first hypothesis, and states that since viruses are so much simpler than a cell, they must have evolved first, and that ancestors of modern viruses could have provided raw material for the development of cellular life. The key data that supports this is apparent when you look at virus genes, compare them and their genetic sequence with cellular life data available in genetic databases. This will reveal a mismatch that suggests viruses aren’t a simpler version of cellular life, but are different fundamentally and might have predated cellular life altogether. This model also suggests there was an ancient virosphere from which all viruses evolved. However, some scientists dismiss this hypothesis because of one key feature.  According to the classical definition of viruses, they need a host’s cell to replicate. So, how could viruses have survived before the existence of cellular life?

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. But what it can’t explain is why the tiniest of cellular parasites don’t resemble viruses at all.

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. This model also doesn’t explain the unique structure viruses have that is not seen in cells.

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 2003. The first specimen was Acanthamoeba polyphaga mimivirus (APMV), isolated from an amoeba in cooling tower in England. The name “mimivirus” stands for MImicking MIcrobe virus because of the way amoebae mistake it for their typical meal of bacteria. 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 2013, is even larger than the mimivirus and has approximately 2500 genes, with 93 percent of their genes not known from any other microbe.

amb web 
 Illustration: Nicole Elmer 

The pithovirus was discovered in 2013 from a Siberian dirt sample that had been frozen for 30,000 years. It’s larger than the pandoravirus, as well as some bacteria, and behaves differently than viruses when it comes to reproduction. According to the classical definition of viruses, they must have a host’s cell to reproduce and cannot do it on their own. 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. This would create a yet undescribed fourth domain of life aside from Bacteria, Archaea, and Eukaryotes. And in case you’re worried if these big viruses can infect us human being, rest easy. You only need to worry if you happen to be an amoeba.

In our next posting about viruses, we'll look at how they might be the most successful of earth's inhabitants.

SOURCES

Arnold, Carrie. “Could Viruses Be the Origin of Life on Earth?” July 17, 2014. National Geographic. (accessed online: https://www.nationalgeographic.com/news/2014/7/140716-giant-viruses-science-life-evolution-origins/)

Brown, Paige. “Once upon a time: The possible story of viruses.” August 1, 2012. Scitable: A Collaborative Learning Space for Science. (accessed online: https://www.nature.com/scitable/blog/student-voices/once_upon_a_time_the/)

Dockrill, Peter. “These Viruses Found in Brazil are So Huge They’re Challenging What We Think a ‘Virus’ Is”. February 28, 2018. Science Alert. (accessed online: https://www.sciencealert.com/giants-viruses-discovered-brazil-among-largest-most-complex-ever-found-tupanvirus-mimivirus)

Koonin, Eugene and Dolja, Valerian. “A virocentric perspective on the evolution of life.” Current Opinion in Virology. Published online July 2, 2013. (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4326007/)

McCrae, Mike. June 13, 2018. “These Abnormally Huge Viruses Create Their Own Genes, Defying Standard Biology.” Science Alert (accessed online: https://www.sciencealert.com/genome-analysis-new-species-pandoravirus-intergenic-generation)

Turner, Paul. “Virus Ecology and Evolution: from Virus Adaptation to Phage Therapy. Part 1: Introduction to Virus Ecology and Evolution.” iBiology.  (accessed online: https://www.ibiology.org/microbiology/virus-ecology-evolution-virus-adaptation-phage-therapy/)