30 November - December 6 2020
How old are viruses?
Can we give a date to the first appearance of viruses?
The recent COVID-19 pandemic is a instructive example of the kinds of pressure and danger that viruses bring to our health and our economy. This episode has sparked great public interest in virology.
Viruses are non-living biological entities, since they don’t actually reproduce, which is a necessary characteristic in the eyes of scientists for them to be considered as living. But paradoxically they are infectious since they parasitize a host cell in by-passing its defences and by manipulating it so it is forced to produce new viral particles (a virion factory) instead of dividing itself. The virions are then excreted by the host cell and go on to infect new cells in their turn. In a similar way to human gametes or seeds from plants, the job of the virions is to spread genetic information.
The question of the origin of viruses is much debated by scientists. They are often defined and classified according to physical or genetic criteria, but also according to the biological properties of their hosts. In order to study their origins, the standard guide is phylogenetics, which studies parental links between species by comparing their genomes. According to the authors, this approach is not adapted for the study of the origin of organisms, in particular concerning viruses, because it contains many variables. For example, the principal studies have been in viral genes which have homologues in host cells, which immediately suggests that the viruses have inherited these genes from the cells. The authors’ recommendation is to consider the way in which the viruses spread their genetic material (parasitism), and they prefer to concentrate on the viral genes that don’t have cellular equivalents.
Certain theories suggest that the nucleic acids (ADN, ARN) appeared independently of cells. Then cells appeared, consisting of a membrane enveloping the nucleic acids, so forming living entities that reproduce autonomously. A first ancestral cell may have appeared and then evolved to become the Last Universal Common Ancestor (LUCA, around 3,5 billion years ago), the common ancestor of all modern cells (archea, bacteria and eukarya)
Viruses could not therefore have appeared before cells evolved, since they use them to multiply. In all likelihood they would have appeared before LUCA since we find in modern viruses, which infect the three domains of living organisms, the eukarya (animal and vegetable cells), bacteria and the archea, proteins which have similar structures, suggesting a common origin.
According to the authors, these ancestral cells were already capable of parasitizing other cells through the production of vesicles, small organelles that we find today in communication between cells. The vesicles seems to have become gradually capable of repeating the parasitic cycle autonomously, eventually becoming virions. The virion factories act like a “pseudo nucleus”, and certain authors think that they may have evolved into the cell nucleus of modern day eukaryote cells. The ancestral cells probably had an RNA genome as well as the first viruses.
Despite these various possible scenarios, evolution towards cells and DNA viruses remains a mystery. Where scientists do agree, however, is on the important role of retroviruses in evolution: they are capable of transforming their RNA genome into DNA in order to penetrate the host-genome, as with HIV. This is confirmed by the fact that the human genome contains around 8% of ancestral retrovirus genes. While viruses are generally considered to be harmful, we do largely underestimate the benefits that they bring to the living world.