Over 200 years of research, and we're still not sure if viruses are alive

By: Maude Hamilton, Université de Sherbrooke

Our world was turned upside down on March 11, 2020 when the World Health Organization declared the COVID-19 pandemic. Our microscopic invader is a virus, a parasite that infects organisms to survive. It is estimated that altogether ten nonillion (a number with 31 zeros!) viruses are present among us on earth. This astronomical number represents more stars than there are in the known universe! Fortunately, not all those entities can cause problems to humans, like the novel coronavirus is causing right now. Even though we have learned so much about viruses since their first discovery at the end of the 19th century, scientists still do not agree on a fundamental question: are viruses really alive?

All living organisms are described and classified in biology by the tree of life. This tree shows a connection, a common ancestry, at some point between all organisms and currently contains three main branches: archaea, bacteria, and eukaryotes. All those organisms are considered living things as they possess the following: they have the capacity to grow, reproduce, they have functional activity like metabolic processes, they can respond to stimuli and adapt to their environment. Not all scientists, however, agree on a precise definition of life, leaving the status of viruses up for debate. For the moment, viruses do not have a place in this tree.

In one way, yes, viruses can grow, reproduce, and evolve - but they cannot do it on their own. This is a big part of the argument against the inclusion of viruses as living things. They are dependent on other living organisms to survive as they are primarily constituted of genetic material, either DNA or RNA, wrapped in a coat of proteins. This way, they cannot carry their own metabolic processes, instead using the host’s cellular machinery to their own advantage to replicate and produce new viruses. Even mutation of their genetic code is dependent on host cells, which means that their evolution is also not independent. Variants of the novel coronavirus is an example of evolution in viruses. Furthermore, viruses cannot survive for a prolonged period of time outside of cells. In summary, as viruses do not possess the capacity to replicate and, thereby, even to evolve independently of cells, could they be considered alive?

To complicate this question further, the origin and evolution of viruses remain a mystery. One theory supporting the classification of viruses as living things is that they could have originated from cells. That would mean that viruses have a common ancestry with a cellular form, and this would give them a place in the tree of life by adding a fourth branch. The discovery of giant viruses that share common genes with the three known branches of life – archaea, bacteria, and eukaryotes – could point in this direction. Nevertheless, the possible addition to the tree of life remains controversial. Another hypothesis of the origin of viruses includes the possibility that they have coevolved beside cellular organisms.

A tricky thing about this dilemma is that depending on our conceived definition of life and its robustness, it could let in the realm of living some things that people would probably never consider as living organisms. For example, if the definition of life includes self-replication and evolution, independent of how it is achieved (in this case for viruses, using cells), computer viruses could be considered alive. For some scientists, allowing viruses in the realm of living would also mean that DNA molecules are considered alive, which could be puzzling. Overall, it is funny in an ironic way to think that a parasite that has caused us countless losses over the last year could not even be a living organism.

Edited by participants of the 2021 Science Writing Internship program and B.G. Borowiec. Header photo from Wikimedia Commons.