Why You Wouldn't Exist Without Viruses

There are an estimated quadrillion quadrillion individual viruses which is 10 to the 31st power that exist on this Earth with us representing more than all the stars in the universe but they are not all bad since only a small few have the ability to cause us harm and the vast majority do not affect us at all.

The human genome contains around a hundred thousand fragments of viral DNA where around eight percent of our DNA is not human at all but viral and in recent years scientists have started to see that these viral gene sequences are not simply leftover genetic baggage strange and purposeless but are sequences that code for essential proteins in human development representing traces of infection and many others that exist as viral fossils within our genome.

To invade a cell a virus recognizes and binds to the cell via a receptor molecule on the cell surface and then breaks in where once inside the viral genes are expressed, viral proteins are created and new virus particles are assembled and are ready to be released where in some cases the exiting viruses leave the host cell intact so it can continue cranking out more virus particles but sometimes the viruses cause the host cell to burst killing it.

Retroviruses are viruses that have an RNA genome and work backwards from how other viruses work where instead of using DNA to make RNA which is then used to make viral proteins, retroviruses insert RNA to make DNA using reverse transcriptase where this DNA then makes its way to the nucleus and inserts itself into the host cell’s DNA thus changing the genome of the host cell and HIV for example is a retrovirus that incorporates itself into the genome where it lives as a template for creating more HIV viruses.

Some viruses like HIV, measles and the herpes simplex virus can also spread directly between two cells that are in contact with each other in a process called cell to cell transmission where this is particularly insidious as it enables viruses to evade the body’s immune response and viruses that can spread in this way have genes that code for proteins that force host cells to fuse together allowing the viruses to jump from cell to cell.

If retroviruses infect reproductive cells meaning the cells that produce eggs or sperm the virus inserts its DNA into the heritable genome of the host where once a retrovirus has embedded itself in this way it is known as an endogenous retrovirus which at first forces cells to make more retroviruses that can infect other cells but over the generations the viral DNA mutates and endogenous retroviruses eventually lose the ability to infect new cells.

Much of the viral DNA within our genomes is therefore thought to be dormant evidence of our ancestors past infections but nothing that affects us now but for some endogenous retroviruses the story is more complicated where scientists have realized that even after being disabled and no longer being able to fully replicate itself some retrovirus sequences can still make some of their proteins.

Over time two of these viral proteins became essential in human development where evolution co-opting viral genes for human needs with syncytin one and syncytin two being viral fusion proteins that originally enabled cell-to-cell viral transmission but were repurposed for placental development demonstrating evolution’s opportunistic use of available genetic material.

In viruses the syncytin proteins allow them to fuse host cells together so they can spread from one cell to another in the cell to cell viral transmission but now the proteins allow placental cells to fuse together to form the syncytiotrophoblast which is a structure that determines which substances cross the placenta such as nutrients and oxygen and which substances do not such as certain maternal hormones and toxins being both life-giving and protective.

Without syncytin the placenta does not form correctly and if it had never been introduced into the human gene pool the placenta would not have evolved as it is today where without it internal pregnancy would have developed very differently or perhaps not at all demonstrating that viral sequences act as raw material for new adaptations where lines of code that should be useless to us turned into something valuable.

Around five days after fertilization the embryo has differentiated into two distinct cell types being the inner cell mass which will develop into the fetus and eventually become the baby, and trophoblasts which will develop into the placenta and external membranes where around the seventh day the blastocyst starts to implant onto the uterine wall where it will remain attached until birth and at this point the syncytiotrophoblast starts to form around the developing placenta.

This reminds us that evolution has no plan where there is no single track on which it operates but rather it is a machine of randomness and variability and in all likelihood our history of past retroviral infections and even current ones will continue to shape our evolution as a species behind the scenes.