No Human Beings without Viruses


Without viruses, there would be no human beings today

Reference News Network reported on March 1 that the US "National Geographic" magazine published an article in the February issue of "How Viruses Shape Our World". The full text is as follows:


Imagine what a virus-free earth would be like for Humans

The result is not as clear as you think. In fact, we live in a world of viruses. There are so many types of viruses that are countless. 

In addition to the huge number, the virus also has a significant impact. 

For life on earth, including humans, many viruses are not only harmless, but also bring evolutionary advantages.


Coronavirus and humans
Structure of Coronavirus

Without the virus, it is difficult for us to survive; without the virus, we cannot get out of the primitive mud. 

For example, there are two segments of deoxyribonucleic acid (DNA) derived from viruses, and they are now found in the genomes of humans and other primates. 

Surprisingly, without these two pieces of DNA, there would be no pregnancy. Even more surprising is that another type of viral DNA helps to pack and store memories. 

There are other genes from viruses that promote embryonic development, regulate the immune system, or fight cancer. Their significance is only just now known.


Facts have proved that viruses have also played a key role in triggering major evolution. If all viruses are eliminated, the biodiversity of the earth will collapse suddenly, as if all the nails of the beautiful wooden house were suddenly removed.


"What is a virus" is difficult to define

To recognize the ever-changing viruses, we must first solve the most basic problem: what is a virus and what it is not. 

It is relatively easy to figure out what viruses are not. They are not living cells. Cells contain multiple sophisticated mechanisms, including the synthesis of proteins and the execution of specific functions according to the cell type. 

Bacteria are also cells, with similar characteristics, but their structure is much simpler. Viruses are different from them.


Viruses are easy to describe, but difficult to define. Each virus particle contains a piece of genetic instructions. 

The instruction is either DNA or another information-carrying molecule ribonucleic acid (RNA), which is wrapped in a protein shell called a "capsid." Some viruses have an envelope outside the capsid, which protects the virus and helps it attach to cells. 

After the virus enters the cell, it will occupy the cell's "three-dimensional printing" mechanism that converts genetic information into protein. Only in this way can it replicate itself.


If the host cell is unlucky, many new virus particles will be produced. New viruses spewed out and the cells were reduced to wreckage. 

It is this kind of damage caused by the new coronavirus to human respiratory epithelial cells, which also explains to a certain extent how the virus becomes a pathogen.


But if the host cell is lucky, the virus may just stay in this stable residence, or go dormant, or reverse engineer its tiny genome into the host's genome, and then wait for the opportunity. 

The latter possibility has multiple effects on genome mixing, evolution, and even our sense of identity as human beings.


Cells and viruses love and kill each other

Viruses bring innovation, but cells are responding with innovations in their own defenses, so this is an "arms race" that is moving toward an increasingly complex. 

Many scientists assume that viruses have achieved major evolution through "virus pickpocketing", that is to say, the virus in the organic matter it infects, fishes a DNA here and a DNA there, and then applies the stolen gene fragments to itself Genome.


But Patrick Fortel of the Pasteur Institute in Paris suggested that perhaps more common is reverse theft, in which cells obtain genes from viruses. Scientists such as Fortel also hold a more thorough view: viruses are the pacesetters that contribute to genetic diversity.


Horizontal gene transfer

This view holds that in the past billions of years, viruses have stored new genetic material into the genome of cell organisms, thus enriching the evolutionary options of cell organisms. 

This peculiar process is a phenomenon called "horizontal gene transfer". Horizontal gene transfer refers to the lateral flow of genes between different gene sequences, while vertical gene transfer is a more common way of inheritance from parent to offspring.


Structural Image of Coronavirus

Fortel et al. proposed that viral genes have been "overwhelmingly" flowing to the cell genome, which may help explain some major evolutions, such as the origin of DNA, the origin of cell nuclei in complex organisms, the origin of cell walls, and perhaps even the tree of life The differentiation and so on.


There is a retrovirus among viruses whose RNA genome works in the opposite way to the conventional one. 

The conventional method is to use DNA to generate RNA, which then acts as a messenger to instruct the "stereoprinter" to synthesize proteins. 

But retroviruses use their own RNA to generate DNA and then integrate the DNA into the genome of the infected cell.


Some retroviruses infect germ cells, thereby embedding their DNA in the host's heritable genome-which brings about a major evolution. These embedded gene fragments are called "endogenous" retroviruses. If they are combined with the human genome, they become "human endogenous retroviruses" (HERV).


8% of human genes come from viruses


8% of the human genome is the DNA of this virus. In the process of evolution, retroviruses inserted them into our genetic sequence. Each of us carries 1/12 of HERV, and one of the genes with a relatively profound impact is syncytin-2.


A certain gene initially helped the virus to fuse with the host cell, and later entered the genome of ancient animals. This gene is then transformed to produce a similar protein, which helps cells to fuse, form a special structure, and eventually evolve into a placenta, opening up a new possibility for some animals: pregnancy in the body. This innovation has a profound impact in the history of evolution, allowing females to conceive offspring wherever they go, instead of leaving the developing offspring vulnerable and helpless as insects and birds leave their eggs in their nests.


This gene was first derived from an endogenous retrovirus, and was eventually replaced by other similar genes that are more suitable for this role. Over time, the new reproduction model continues to improve, and the placenta also evolves. Among such genes obtained from viruses, there is syncytin-2. There are two types of syncytin that help human cells fuse and form the placental layer near the uterus. This unique structure is responsible for connecting the mother and fetus, absorbing nutrients and oxygen, expelling waste and carbon dioxide, and possibly protecting the fetus from the mother’s immune system. Evolution to implant viral elements into humans is an effective design that is almost miraculous.


Studies have also found that there is evidence that the viral fragments produced by another human endogenous retrovirus, HERV-K, exist in the initial human embryos, or play an active role in protecting the embryos from viral infection, or Is it helpful to control fetal development, or both.


If 8% of your and my genome are retroviral DNA, then the notion that humans are unique (not to mention supremacy) may not be as conclusive as we believe.


Is the virus a blessing or a curse?


Of course, this evolutionary flexibility also has a downside: Sometimes viruses may change their hosts, jump from one organism to another, and become pathogens for new and unfamiliar hosts. This process is called "spillover." The transmission of viruses from animal hosts to humans is the source of most human infectious diseases.


In the original host, the virus may wait quietly for thousands of years, and the number and impact are very limited. The virus may reach an evolutionary compromise with its natural host, in exchange for its own safety by not causing trouble. But in new hosts, such as humans, the virus does not necessarily comply with the original agreement. If the virus not only replicates, but also spreads between people and groups of dozens of people, it constitutes an outbreak; if it sweeps across a certain community or country, it will form an epidemic; if it rages around the world, it will cause a major plague . This is the case with the new coronavirus.


Therefore, the virus has to ask and pay. It is difficult for viruses to return to their place on the tree of life. Perhaps the reason is that their life course is not completely tree-like. The tree diagram is just our traditional way of interpreting evolution, because Darwin has become the golden rule. But even as great as Darwin, he knew nothing about horizontal gene transfer. In fact, he knew nothing about genes or viruses.


Now we realize that everything in the world is quite complicated. Even a virus that is so simple at first glance is so complicated. If we see that the complex performance of viruses makes us humans understand the intricacies of nature more clearly, and if we think that the virus genes in our bodies can partly dispel our sense of indifference, then I ask you to judge whether viruses are a blessing or a curse.



Cover of the February issue of National Geographic magazine

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