Ancient Evolution of Viruses
Even before the outbreak of COVID-19, scientists had been seeking to understand the peculiar nature of viruses. Viruses, as an agent of disease separate from bacteria, were hypothesized in 1892 Martinus Beijerinck by studying infections in tobacco plants in Holland, though he imagined it as a liquid rather than a particle. The structure of the virus was eventually confirmed in 1941 through photographs on electron microscope. But it was only in the 21st century that research began to fully explain the relationship between the virus and the cell it uses as a host.
By definition, viruses are fully dependent on host cells to reproduce and not considered alive since they cannot replicate on their own. Under the escape gene hypothesis, it was theorized that viruses were bits of cellular RNA that had become parasitic. This would suggest that viruses are more recently evolved compared to the rest of life on Earth.
It was the discovery of giant viruses, starting with the mimicking microbe virus in 2003 followed by the even larger pithovirus and pandoravirus in 2013 that began to fuel another theory. Giant viruses have large DNA-based genomes on par with parasitic bacteria and could actually be the true ancestors of modern viruses. Additionally, when researchers compared protein folds between viruses and bacteria, they found 442 shared folds and only 66 folds unique to viruses. The 442 shared folds can be traced back 3.6 billion years to the time of the last universal common ancestor (or LUCA). Based on analysis of fold superfamilies (FSF), it was theorized that viruses could make up their own separate branch after splitting from cellular organisms. Though viruses are not considered alive, the similarity of protein folds indicates they at least share a common ancestor with all life on earth. Further evidence that virsues coexisted with the last universal cellular ancestor (or LUCELLA) is seen by the fact that viruses infect organisms across all three branches of life.
While viruses are known primarily for the diseases they cause, ancient viruses may have contributed to our complexity. Recent estimates attribute 45% of the human genome to coming from retroviruses. HIV is an example of a retrovirus — meaning it inserts its own code into our DNA. Retroviruses that infected our cellular ancestors could have eventually lost their pathogenicity and become integrated parts of our functional genome. For instance, this mechanism potentially supplied the syncytin proteins necessary for the development of the placenta in mammals. The placenta protects the fetus from the mother’s immune system which enabled these species to carry their young until birth rather than laying eggs.
Evolutionary pressures caused the genomes of cells to increase in size while viruses instead became streamlined. The simplicity of modern virus genomes led us to believe that they owed even their origins to the cells they infected. However, scientists are seeing proof that viruses are much older and that current life, as we know it, may actually owe its exisitence to these viruses from the past.
