Every family of a certain size seems to have “that” side of which they don’t speak, you know, “those” people. The familial rift could have been caused by a disagreement over a spousal choice, a series of unforgivable holiday faux pas, or even cheering for the wrong football team. Now it seems biologists may have inadvertently given the expansive family of life itself a taboo relative – viruses. Viruses may just be life’s equivalent of Aunt Betty, whom your mom didn’t tell you about until you were 18.
Since their discovery in the late nineteenth century, it has been scientific dogma that viruses are NOT alive. They were too small and their genomes were too simplistic (the most damning evidence was that no known virus possessed genes for translating RNA, i.e. viruses have to hijack a host cell’s ribosomes to reproduce). The discovery of giant viruses in 2003 forced scientists to rethink their position.
The first giant virus to be discovered, the Mimivirus, was found in 1992 during a search for the microscopic miscreant behind a pneumonia outbreak in Bradford, England. Researchers observed the Mimivirus in amoebae (tiny little protist creatures) and mistakenly believed it to be a type of bacterium. Further analysis, published in 2003, told them they had stumbled upon an unprecedented new kind of virus – one that is shockingly similar in size and complexity to bacteria. A growing number of microbiologists began pushing to reexamine whether viruses should be classified as a form of life.
A recent study published by open access publisher BioMed Central heightens the taxonomic drama and claims to have re-imagined the tree of life and found a long lost branch – the viruses. Genomic sequencing is the standard method for tracing evolutionary history among organisms; however, this method is prone to inaccuracies due to genes’ pesky proclivities for insertion, deletion, and horizontal swapping. Typically evolutionary history is traced by identifying common genes among organisms – by tracking relative gene prevalence researchers can identify how closely related different species are. For organisms with smaller genomes, like viruses, errors can lead to relatively large oversights.
The researchers, to gene-proof their taxonomic mapping, instead decided to examine protein structures within organisms in place of genes. This paper, published by the same researchers, provides rationale for using this method. They traced the commonality of structural components within the cells of 981 different organisms (70 Archaea, 652 Bacteria, and 259 Eukarya) and regrew a new tree of life based on their data. Their tree had the standard three super kingdoms, Eukarya, Bacteria, and Archae, but it also had a new super kingdom of Viruses.
Not only are viruses on this alternate tree of life, but the researchers claim viruses may have predated LUCA, life’s Last Universal Common Ancestor (the grandmother of all life on the planet). I spoke with Arshan Nasir, an author on the paper, and he explained this would indicate there are two main forms of life that we know of – one cellular (ourselves and everyone else) and one not cellular (viruses). He told me to imagine life’s emergence and propagation on our planet as a cross-country car race: It starts with 20 cars (different forms of life) and throughout the race many cars experience breakdowns, detours, and getting lost. The three cars that make to the end represent the three superkingdoms. Perhaps viruses were a fourth form of life that, in order to finish the race, needed to hitch onto a more able car. In order to be towed along, the fourth car would have to jettison all unnecessary equipment in order to be as light and unnoticeable as possible – hence the simplified form we see viruses in today.
This claim is not without controversy – biologists are still suspicious of the new taxonomic method and more robust studies will be needed to satiate their skepticism. More research needs to be completed before we can welcome viruses into our family of life, but this study is a tantalizing hint that our concept of the evolutionary history of life is far from complete.