Would we be oviparous without viruses?
The word virus, which in Latin means toxin, poison, always resonates in the head as a synonym for disease and destruction. Perhaps this has not always been the case and these organisms, on the contrary, millions of years ago have actively participated in important evolutionary processes.
And they keep doing it, so much so that they could produce significant changes in animal speciation.
Although viruses reproduce very quickly and almost always with violent results, they are so rudimentary that many scientists do not even consider them living beings. A virus is nothing more than strands of genetic material, mostly DNA, wrapped in a bundle of proteins — a parasite, unable to function on its own. To survive, you must find a cell to infect. Only then can the virus make use of its talent, which is nothing more than taking control of the cellular machinery and using it to make thousands of copies of itself.
Nothing has posed such a persistent threat to humanity as viral diseases: measles and smallpox have caused epidemics for thousands of years; smallpox could have killed about half a billion people in the 20th century alone. However, these viruses, although highly infectious, have their impact limited by something as important as that if they destroy their victim (an entire population for example), they also disappear. Therefore, not even the smallpox virus has the evolutionary power to influence humans as a species, alter genetic makeup. This would require the germ cells to be infected.
We have some virus
Only retroviruses, which in exchange for DNA are composed of RNA have this possibility. When they infect a cell, they use an enzyme that changes RNA into DNA and integrates it into the cell’s DNA forever: when the cell divides, the virus goes with it. If the virus infects germ cells, ova, or sperm, which is rare, and if the resulting embryo survives, which is rarer still, the virus becomes part of the genome and will be transmitted as yet another trait, just like the eye color or predisposition to asthma. Its name is now endogenous retrovirus ( EVR ).
When the sequence of the human genome was mapped, the researchers discovered something they had not anticipated: our genetic material is littered with fragments of retroviruses.
The human genome uses less than 2% to make all the proteins necessary for life. However, 8% corresponds to broken or disarmed retroviruses that millions of years ago managed to get into the DNA of our ancestors. Like dinosaurs, these viral fragments are fossils but in exchange for being buried in the sand, they reside within us, carrying a record of millions of years. Since they do not serve any purpose or cause harm, they were called “junk DNA.”
Fossil viruses in the human genome
What do those fossils do within the genetic material?
Thierry Heidmann, director of the Gustave Roussy Institute of Virology in Paris, revived one of those retroviruses. Combining the tools of genetics, virology, and evolutionary biology, he and his colleagues took a virus that was extinct millions of years ago, imagined how the fragmented parts must have lined up, and put them together. After resuscitating the virus, the group introduced it into human cells and recorded how it inserted into cellular DNA. Infection occurred quickly. Heidmann named it Phoenix virus, in a clear reference to the myth.
The experiment was reproduced by other researchers in various laboratories around the world, with different viruses. Paleo virology was born. The derived results opened the way to the clarification of various fields of research, from evolution to cancer, through understanding important aspects — which had not been possible to address before — such as the functioning of HIV and its devastating action in human beings. Heidmann notes: “The Phoenix virus will help to understand how the AIDS virus operates, but more than that it will help to understand how we operate, and how we evolve. Many people study other aspects of human evolution. I argue that viruses are equally important in shaping the way we are today. Take, for example, the processes of gestation and birth ”.
Would we be oviparous without viruses?
Heidmann and other researchers suggest that without endogenous retroviruses, mammals would never have developed the placenta, which protects the fetus and gives it enough time to mature.
The placenta is a powerful factory that generates 70% of the energy necessary for the formation of the fetal brain and made us viviparous, an evolutionary success that placed us above fish, reptiles, and birds. It is possible that without the collaboration of the RVEs we would still be hatching eggs.
A hundred million years ago, the embryos of ancestral mammals, instead of growing in a shell, became parasites. In the state of a ball of cells, they began to implant in the layers of the uterus. The placenta is a modified egg, which is characterized by cell fusion, a characteristic shared with RVEs
If Darwin raised his head!
Robin Weiss, professor of viral oncology at the University of London, in 1968 found RVEs in the embryos of healthy chickens. When he suggested that they were not only benign but may also have played a critical role in the development of the placenta in mammals, the molecular biologists laughed. “When I submitted the results on a new endogenous envelope, suggesting the existence of an integrated retrovirus in normal cells, the manuscript was roundly rejected,” says Weiss. “One of the reviewers ruled that my interpretation was impossible.” Weiss, who is responsible for how much is known about how the AIDS virus interacts with the biological system, was not discouraged. On the contrary, he continued with his work. “If Charles Darwin reappeared today, he might be pleasantly surprised to learn that we share common ancestry not only with monkeys but also with viruses,” he said recently.
The RVEs found in all organisms that have been studied, in varying amounts, indicating a correlation between complexity and “fossil record” viral greater as the body is placed higher on the evolutionary scale.
What better demonstration that evolution has been a process that has taken millions of years and that can be traced like a path full of breadcrumbs, these molecular ones, which can lead to the very origins of life on this planet.
EPPUR SI MUOVE
