Send Us Email support@drharsh.co.in The evolutionary history of viruses represents a fascinating, albeit murky, topic for virologists and cell biologists. Because of the great diversity among viruses, biologists have struggled with how to classify these entities and how to relate them to the conventional tree of life. They may represent genetic elements that gained the ability to move between cells. They may represent previously free-living organisms that became parasites in further evolution. They may be the precursors of life as we know it.
We know that viruses are quite diverse. Unlike all other biological entities, some viruses, like poliovirus, have RNA genomes and some, like herpesvirus, have DNA genomes. Further, some viruses (like influenza virus) have single-stranded genomes, while others (like smallpox) have double-stranded genomes. Their structures and replication strategies are equally diverse. Viruses, do, however, share a few features: First, they generally are quite small, with a diameter of fewer than 200 nanometers (nm). Second, they can replicate only within a host cell. Third, no known virus contains ribosomes, a necessary component of a cell’s protein-making machinery.
Biologists generally agree that all living organisms exhibit several key properties: They can grow, reproduce, maintain internal homeostasis, respond to stimuli, and carry out various metabolic processes.
We probably all realize that viruses reproduce in some way. We can become infected with a small number of virus particles. For example, by inhaling particles expelled when another person coughs, for instance — and then become sick several days later as the viruses replicate within our bodies. Likewise, we probably all realize that viruses evolve. Hence, we need to get a flu vaccine every year primarily because the influenza virus changes, or evolves, from one year to the next.
Viruses do not, however, carry out metabolic processes. Most notably, viruses differ from living organisms in that they cannot generate ATP. Viruses also do not possess the necessary machinery for translation, as mentioned above. They do not possess ribosomes and cannot independently form proteins from molecules of messenger RNA. Because of these limitations, viruses can replicate only within a living host cell. Therefore, viruses are obligate intracellular parasites. According to a stringent definition of life, they are nonliving. Not everyone, though, necessarily agrees with this conclusion. Perhaps viruses represent a different type of organism on the tree of life — the capsid-encoding organisms, or CEOs.
Contemplating the origins of life fascinates both scientists and the general public. Understanding the evolutionary history of viruses may shed some light on this interesting topic. To date, no clear explanation for the origin(s) of viruses exists. Viruses may have arisen from genetic elements that gained the ability to move between cells. They may be descendants of previously free-living organisms that adapted a parasitic replication strategy. Perhaps viruses existed before cell origin and led to the evolution of, cellular life. Continuing studies may provide us with clearer answers or future studies may reveal that the answer is even murkier than it now appears.
Cells are considered the foundation of life, but viruses with all their genetic diversities may share that role. Our planet’s earliest viruses and cells likely evolved in an intertwined and often symbiotic relationship of predator and prey. Evidence even suggests that viruses may have started as cells but lost their autonomy as they evolved to thrive as parasites on other cells. This dependent relationship began a long history of co-evolution. Viruses living in cells cause their hosts to adapt, and those changes then cause viruses to adapt in a never-ending cycle of one-upmanship.
As far back as 150 million years ago, viruses infected mammals and left genes that led to a dramatic evolutionary advance; the placenta, which allows nutrients and oxygen to reach the fetus and waste and carbon dioxide to pass out. Humans and other mammals with the placenta can move around with their unborn young, making them less vulnerable to predators. In humans, two genes originating from virus-syncytin-1 and syncytin-2 help to form the placental membrane that attaches to the uterus. This membrane also may aid in preventing the mother’s immune system from attacking the fetus as a foreign object.
The ancestral virus probably resided in a bat, possibly a horseshoe bat, belonging to a genus of small, insectivores creatures with the horseshoe-shaped nose, which commonly carry coronaviruses. If recombination did occur, adding some crucial new elements from a different coronavirus, this could have happened in a bat or possibly in another animal. Pangolin has been suggested; other species could also be candidates. Scientists are exploring these possibilities and others by sequencing and comparing genomes of the viruses found in various potential hosts. All we know for now is that SARS-Cov-2 as it exists today in humans is a subtle capable of further evolution.
So virus gives and the virus takes away. Maybe the reason they are difficult to place on the tree of life is that life’s history, after all, isn’t quite shaped like a tree. The arboreal analogy is just our traditional way of illustrating evolution, made canonical by Charles Darvin. But Darvin, great as he was, knew nothing about horizontal gene transfer. He knew nothing about genes. He knew nothing about viruses. Everything is very complicated as we realize now. Even viruses, which seem to be so simple at first glance, are very complicated and seeing them in all their complexity gives us a clearer vision of the tangled connectedness of the natural world. Reflecting upon our viral contents takes away some of our sublime detachment, then we leave it to all to say whether those are benefits or harms.