At the end of the 19th century, Dmitry Ivanovsky was the first to discover viruses. However, it was not until the early 20th century that scientists identified viruses of various types. With the invention of the electron microscope, scientists could observe the structure of viruses and study their evolution in greater detail.
1. The First Person to Discover Viruses
Dmitry Ivanovsky was the first to discover viruses. He was born in the village of Nizy, Gdov Uyezd, and studied at Saint Petersburg University under Andrei Famintsyn. In 1887, Ivanovsky was sent to Ukraine and Bessarabia to investigate a disease affecting tobacco plants, which was causing significant damage to plantations at the time. Three years later, he was again dispatched to examine a similar outbreak, this time in Crimea. Ivanovsky found that the disease in both cases was caused by an extremely small infectious agent that could pass through the Chamberland filter, something bacteria could not do. Ivanovsky described his findings in a paper (1892) and a thesis (1902). He later worked in Warsaw and Rostov-on-Don.
Six years later, in 1898, Dutch biologist Martinus Beijerinck conducted similar experiments and declared the discovery of a new infectious organism, naming it "virus." Neither Ivanovsky nor Beijerinck understood that viruses were particles called virions; Ivanovsky believed they were toxins produced by bacteria. It wasn't until the invention of the electron microscope in the 1950s that scientists discovered the tobacco mosaic virus was a tiny hollow rod formed by a single helical RNA strand surrounded by a protein coat.
2. The Origin of Viruses
Viruses have existed since ancient times. Molecular studies have revealed connections between virus systems that infect one of the three domains of life, suggesting that viral proteins existed before the divergence of life. This implies that some viruses emerged early in life’s evolutionary history and may have arisen multiple times. Some theories suggest that new viral groups have continuously emerged at all stages of evolution, often through genetic recombination.
There are three classical hypotheses regarding the origin and development of viruses:
- The Virus-First Hypothesis: Viruses evolved from complex molecules of proteins and nucleic acids before cells appeared on Earth. According to this hypothesis, viruses contributed to the rise of cellular life. This is supported by the fact that viral genomes encode proteins without cellular homologs. However, some scientists reject this hypothesis because viruses require a host cell to replicate, which contradicts their definition.
- The Regressive Hypothesis: Viruses were once small cells parasitic on larger cells. The discovery of giant viruses with genetic material resembling parasitic bacteria supports this hypothesis. However, this fails to explain why even the smallest parasitic cells bear little resemblance to viruses.
- Escape hypothesis (ambiguous hypothesis): Some viruses evolved from DNA or RNA molecules that ‘escaped’ from the genetic system. This does not explain why the structures for viruses are unique and not found anywhere in cells. It also does not explain the complex outer protein coats of viruses and other structures of viral molecules.
However, virologists are in the process of reassessing all these hypotheses.
- Coevolution Hypothesis (Bubble Theory): In the early stages, a community of replicators (genetic information fragments capable of self-replication) existed near food sources. This food source also created lipid-like molecules that self-assembled into vesicles to contain the replicators. Near the replicators, the food source thrived, but further away, only undiluted resources were inside the vesicles. Therefore, evolutionary pressure could push the replicators along two developmental paths: merging with a vesicle to create cells; and infiltrating a vesicle, using its resources, multiplying, and leaving for another vesicle, giving rise to viruses.
- Hypothesis on the Origin of Genetics: Based on analyses of the evolution of replication modules and the structure of viruses, a genetic scenario for the origin of viruses was proposed in 2019. According to this hypothesis, the replication modules of viruses originated from a primitive gene pool, although their long subsequent evolution involved many shifts by replication genes from cellular hosts. Conversely, the genes encoding the main protein structures were developed from host proteins with diverse functions throughout the evolution of the viral world. This scenario differs from the three traditional scenarios but incorporates features of both the first hypothesis and the escape hypothesis.
One of the challenges in studying the origin and evolution of viruses is their high mutation rate, especially in the case of RNA retroviruses like HIV/AIDS. However, a recent study based on the comparison of viral protein folding structures is providing some new evidence. Fold Super Family (FSF) are proteins that show similar folding structures independent of amino acid sequences and have found evidence of viral phylogeny. Thus, viruses have been found to potentially be divided into 4 FSFs. The viral protein system still contains traces of ancient evolutionary history, which is currently being studied. Research on FSF proteins shows the existence of ancient cell lineages common to both cells and viruses before the emergence of the ‘last universal cellular ancestor’ that created modern cells. Evolutionary pressure to reduce genome and particle size may have eventually reduced vero cells to modern viruses, while other coexisting cell lineages eventually evolved into modern cells. Furthermore, the long genetic distance between RNA and DNA FSFs suggests that the RNA world hypothesis may have new experimental evidence, with a long intermediate stage in the evolution of cellular life.
A final exclusion of a hypothesis about the origin of viruses is very difficult to achieve on Earth because viruses and cells interact everywhere. Therefore, from a biological perspective, it has been proposed that on celestial bodies like Mars, not only cells but also traces of previous virions or viroids should be actively sought. If only traces of virions but no cells are found on another celestial body, this would be a strong indication of the first hypothesis about viruses
3. History of virology
Virology is the science that studies viruses and the infections they cause, beginning in the late 19th century. Although Louis Pasteur and Edward Jenner developed the first vaccines to protect against viral infections, they did not know that viruses existed. The first evidence of viruses came from experiments with filters small enough to retain bacteria. In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased tobacco plant remained infectious to healthy tobacco plants even after filtration. Martinus Beijerinck named the infectious agent “virus,” and this discovery is considered the beginning of virology.
Subsequent discoveries, along with the research of Frederick Twort and Félix d’Herelle on bacteriophages, further catalyzed the field. By the early 20th century, scientists had discovered many types of viruses. In 1926, Thomas Milton Rivers defined viruses as parasites. Wendell Meredith Stanley demonstrated that viruses were molecules, not liquids. The invention of the electron microscope in 1931 allowed scientists to see the structure of viruses clearly.
4. Evolution of Viruses
The evolution of viruses is a field within evolutionary biology and virology, focusing on the study of viral evolution. Viruses have short generation times, and many types of viruses, especially RNA viruses, have relatively high mutation rates (on the order of one or more point mutations per genome per replication cycle). This high mutation rate, combined with natural selection, allows viruses to quickly adapt to changes in their host environments. Additionally, most viruses reproduce very quickly, so any mutated genes can be passed on to many generations rapidly. Although the ability to mutate and evolve can vary depending on the type of virus (double-stranded DNA, double-stranded RNA, single-stranded DNA, etc.), in general, viruses have a high mutation capacity.
The evolution of viruses is a crucial aspect of the epidemiology of viral diseases such as influenza (influenza virus), AIDS (HIV), and hepatitis (e.g., HCV). The rapid mutation of viruses also poses challenges in developing vaccines and antiviral drugs, as drug-resistant mutations often appear within weeks or months after treatment begins. One of the main theoretical models applied to viral evolution is the quasispecies model. A quasis species is defined as a group of closely related viral strains competing in an environment.
References: en.wikipedia.org, lindahall.org, britannica.com.
To arrange an appointment, please call HOTLINE or make your reservation directly HERE. You may also download the MyVinmec app to schedule appointments faster and manage your reservations more conveniently.
