We protect ourselves by wearing clothes, sleeping inside, applying various substances on our body, following hygienic practices and taking medication. Yet these are mere additional measures. Our greatest security cover is our immunity. Because the most dreadful things that still get inside are minuscule biological entities. Viruses for instance.
Our immune system has tools to identify a biological entity as a foreign element. Basically, it knows its own molecular constituents very well and marks anything that does not match the pattern as “non-self”. Once a non-self entity is recognized, response follows and that’s when we feel the symptoms of sickness. We all have experienced this during common cold and flu. Our internal security guards are the white blood cells patrolling the streets of the body by riding on the blood plasma. They are constantly on the lookout for anything suspicious.
Now a dilemma presents itself during reproduction when a female body bears a fertilized egg inside which develops into an embryo and later into a fully-grown baby. The fertilized egg cell has half “foreign” genetic elements – those contributed by the father. That’s enough for the alarms inside the mother’s body to go off and trigger the response mechanism. The easiest solution to this problem is to eject the embryo outside the body, which is what the egg-laying animals do. Animals like us however have evolved a special provision to let that “foreign” body grow inside the female body. The evolutionary invention that made this possible is placenta.
Placenta has brought in enormous advantages to mammals like us humans. An egg ejected outside of the body has limited resources supplied with just before ejection. There is no scope of augmenting the resources once the egg is laid. The life inside the eggshell must consume that limited supply for survival and growth till it comes out and faces the harsh exterior environment. It has to eat whatever is available in the yolk and egg white. It breathes through the tiny gap between the very thin membranes, which then exchanges oxygen and carbon dioxide through the pores of the eggshell. In comparison, an embryo inside the mother’s womb leads a royal life. There is a regular supply of nutrients and oxygen from the mother’s body. All this because of the special organ placenta which performs the dual role of protecting the embryo/fetus as well as the mother and acting as a conduit for resources supply. That scientists are analyzing information, that I am writing this article and that you are reading and comprehending the story are all thanks to the placenta that once bundled us inside the womb and acted as our first interface to the exterior world which was just mother all around. I say this because our brain size too is attributed to our huge growth inside mother’s body made possible by the placental interface.
Where does this placenta come from? Clearly, egg-laying animals lack one. This feature apparently became integral part of a particular group of mammals some 50 million years ago. The key to the formation of placenta lies in a specific type of proteins known collectively as syncytin. How a protein is constructed from amino acids is prescribed in genes. Thus the question of origin of placenta reduces to where the syncytin genes originated from. Hold your breath… the source of syncytin genes has been traced back to a virus named HERV-W. Yes, you’ve read it right, a virus. To be precise, a retrovirus – a group of viruses to which also belongs HIV. To be more precise, an endogenous retrovirus – a genetic element of a retrovirus that has become an integral part of the host genome.
Animals, plants and other similar organisms are usually infected by RNA viruses – viruses whose genetic material is RNA. A virus usually encloses its genetic material in a protein capsid. With the protein coating gone as the virus enters a cell, its naked RNA uses the cell’s machinery to replicate itself and manufacture protein coatings. The copies are then released as the cell bursts. When the copies of the virus come outside of the body because of, say, sneezing or coughing, they can be taken in by other bodies and the chain reaction progresses.
Retroviruses are a step ahead. They enter as RNA and use their enzyme ‘reverse transcriptase’ to transform into DNA. Our cells’ DNA resides inside the nucleus where the retroviral DNA enters and becomes part of our genome by using another of its tool called ‘integrase’. Now if such a virus integrates with a germ cell (egg or sperm), it acquires the capability of being transmitted via heredity. It is then said to be endogenised because it then forever becomes part of DNA of that species – an evolutionary milestone. Believe it or not, but 45% of our genome is composed of such elements which were acquired via endogenisation! Such elements are like “fossil” viruses and their study constitutes a fascinating new discipline called paleovirology.
So you see, as much as we think of viruses as enemies of mankind, in the larger scheme of things of nature, they may not be deemed as having a sinister motive. They can be thought of, rightly, as change agents that bring about evolutionary additions to the living beings. That does not however mean that we stop taking measures to prevent pandemics or even a small infection. We as humans have a responsibility to look after ourselves. Nature will look after itself its own way, maybe via agents such as viruses. It always helps though to stretch one’s perspective a little further for better understanding of problems we face as the “highest” species.
Let me end the article by citing a quote from Cordingley’s book which emphasises the vastness and scope of viruses in the biosphere:
“…they are not simply life’s pathogens. They are life’s obligate partners and a formidable force in nature on our planet. As you contemplate the ocean under a setting sun, consider the multitude of virus particles in each milliliter of seawater; flying over wilderness forestry, consider the collective viromes of its living inhabitants. The stunning number and diversity of viruses in our environment should engender in us greater awe that we are safe among these multitudes than fear that they will harm us.” – Michael G. Cordingley, Viruses: Agents of Evolutionary Invention, Harvard University Press (2017)