If a virus evolved from living organisms it would make sense to consider them to be alive. But are they alive or are they more like zombies?
Imagine yourself walking on a park as some of the people around you behave like the zombies on the set of The Walking Dead. You would certainly recognise the difference between a person, an animal, a tree and one of those zombies, wouldn’t you?
Unfortunately, many scientists haven’t been able to classify viruses, because, unlike people, animals or trees, they do not belong to any of those groups. In theory, they would be like those brainless zombies who walk around with no clear direction.
Not being able to recognise to what group a virus belongs makes it harder for scientists to study them and find solutions to the health challenges they may present.
Where do they belong?
Scientists ask this question every day when they try to find out the relationship between different living things.
The answers are not simple or trivial. Biological relationships not only serve to elaborate a catalog of life, but also to understand how it has evolved to give rise to its many forms.
Viruses are a perfect example. They pose a problem to biologists because they have no cells, so they are not part of any of the three main groups, or “domains”, of living things to which all other organisms belong: bacteria, archaea and eukaryotes.
Some scientists argue that viruses cannot be considered living things and it is better to view them as independent genetic material that cannot replicate on its own and that needs to sequester a host cell.
Others believe that viruses evolved from cellular organisms and, therefore, can be considered a fourth domain of living beings.
This last theory was backed by the discovery, a decade ago, of giant viruses that are more similar to cellular organisms.
But a new study, published in the journal Science, on the genomes of these giant viruses, calls into question that idea. So will scientists have to start again to look for the origins of viruses?
Viruses are tiny, minimal beings that escape the nuances of cell life.
They are usually composed of genetic material – DNA or RNA- , often surrounded by a protein shell called capsid, which sometimes has additional layers taken from a host cell.
Viruses can only replicate within a host cell by sequestering their metabolism, and a different version of these cellular squats infects each domain of living beings.
This tremendous dependence on the host cells places them within the limits of the definition of life, so that some consider them living beings and some not.
It is not surprising that most zombie stories involve a virus.
Perhaps it would be easier to consider living dead viruses. The big question is: where do they come from?
There are competing theories that try to explain the evolution of viruses. One of them presents the virus as descendants of an ancient lineage of cellular organisms that lived inside other cells and whose structure was simplified with the time.
This would make them the only survivors of a fourth domain of living beings, long disappeared, that left behind its cellular structure.
If viruses evolved from living organisms, it would now make sense to consider them to be alive.
Another theory proposes that viruses emerged as independent genetic entities, vagabonds of the genome that escaped their cellular confinement. They could be related to the transposons, able to be copied or cut from the genome and then sticking to other areas of DNA.
In this case, viruses would be the result of molecular accidents that became evolutionarily stable. Which would mean that they have never been whole living organisms, just as a computer virus is not a complete computer.
Both proposals have weaknesses. The first fails to explain the simplicity of viruses. There is no other known organism with that degree of simplification.
On the other hand, the second theory does not explain why viruses are much more complex than other mobile genetic elements, none of which has a shell comparable to the capsid.
In 2004, scientists discovered a species of giant virus that seemed to tip the scales in favor of the cellular origin of viruses. There are good reasons to call them giants.
Some are 10 times larger, both by size and genome length, than the influenza virus and have no less than 2,500 genes, compared to the meager 11 genes contained in influenza.
This additional genetic material contains instructions for making proteins, some of which other viruses are largely lacking, but are found in other forms of life.
The molecular system is not complete and giant viruses also have to invade cells to make more giant viruses. But some researchers point out that these genes could be remnants of a cellular past, which would support the existence of a fourth domain of living things.
On the other hand, the bouncy nature of viruses makes them prone to take genes from other organisms. This has led others to argue that all these additional genes for giant viruses are a consequence of evolutionary theft.
What is the nature this virus material?
Now, a new study has confirmed the “borrowed” nature of all these virus genes.
The research uses the most advanced methods, called New Generation Sequencing to map DNA taken from a wastewater treatment plant in Austria.
In recent years, NGS-based studies have revealed a myriad of new life forms and, in this particular case, NGS has revealed a completely new lineage of giant viruses.
Among all the giant viruses, Klosneuvirus possess the largest set of genes involved in the manufacture of proteins.
By comparing the genomes of the various giant viruses and carefully reconstructing their evolution, the researchers convincingly point out that the protein-making machinery of these giant viruses is a relatively recent genetic addition and not the remains of a larger ancestral genome.
In the study it is argued that the host cells that these viruses tried to kidnap might have developed a defense strategy based on hiding the proteins from the invaders.
As a result, the viruses adapted by incorporating some of these genes into their genome.
The researchers conclude that the giant viruses analyzed in this study have evolved at different times from smaller viruses, ruling out the idea that they evolved from cellular organisms.
However, the new tests do not end completely with viruses. In the tree of life new knots are discovered every day, and a new finding could still provide a link between cell and acellular life.