What is the virus？ Why stay away from wild animals？

　　The medical staff are working. Xinhua news agency

　　Production of antiviral drugs. Xinhua news agency

　　editorial comment/note

　　How did the virus originate? What are scientists doing in developing drugs against Covid-19? It is the top priority for Chinese researchers to develop effective clinical drugs for Covid-19, improve the cure rate and reduce the mortality rate. On February 15th, Zhang Xinmin, director of the Biological Center of the Ministry of Science and Technology, introduced that the drug screening and treatment program had made positive progress at the press conference held by the the State Council Joint Prevention and Control Mechanism on the latest progress of drug research and development and scientific research in Covid-19. The Ministry of Science and Technology organized a national excellent scientific research team to screen more than 70,000 drugs or compounds by computer simulation screening and in vitro enzyme activity test. At present, The scientific research team has focused on a few drugs, and a number of drugs, such as chloroquine phosphate, lundixivir, and farpiravir, have successively carried out clinical trials. At present, some drugs have initially shown good clinical efficacy. This issue focuses on the origin of the virus, drug research and development, and psychological anti-epidemic.

　

　　The structure of most viruses is very simple. Basically, it is a shell made of protein wrapped with some genetic material, which is no different from dead things at ordinary times. Once infected cells, they inject their own genes into the host cells and use the host cells to replicate themselves to produce new viruses. Is the virus a living thing? Where did they come from? Below, let’s take a look at three hypotheses about the origin of the virus.

　　Hypothesis 1 Ancient macromolecules Some macromolecules, such as RNA, DNA and protein, begin to replicate themselves independently or cooperatively, and this stage can be called "molecular life". However, in a peaceful molecular life, there is a different kind, that is, some molecular life, including LUCA, the latest common ancestor of all living things, which has developed a structure that has changed the law of life, that is, cells. Cells with membrane structure can better protect delicate core macromolecules such as RNA and protein, which greatly enhances the adaptability of these organisms, which means that they will rub their original molecular life on the ground. There is a hypothesis (The Virus-First Hypothesis) that the virus is the "adherents" of the primitive molecular life world. This hypothesis was once very popular, after all, the structure of viruses is so simple, even crude, and their differences from cell life are so great.

　　Some viruses can form crystals after purification, which is unimaginable for cell biology. More solid evidence comes from "Virusoid", which is an RNA molecule. Pseudovirus can’t infect cells directly, but it can infect viruses. To be exact, it can copy itself and spread itself by hitchhiking when some viruses infect cells, which can cause some diseases such as human hepatitis D.

　　Hypothesis 2 The gene of "defection" Genes can also defect? Yes, in The Progressive Hypothesis, genes will do anything to keep themselves alive.

　　There is a small circular DNA called "plasmid" widely in bacteria. These genes are basically a group of wage earners and temporary workers. Bacteria can absorb them from the environment for their own use at any time, and they can also drive them away at any time.

　　In many bacteria, in addition to their original DNA, there are often some small fragments of circular DNA, which are called plasmids. So in the long evolution, some plasmids learned one thing: we should not work all our lives! These plasmids changed from wage earners to wage earners, which in turn hijacked their bacterial boss and took away all the nutrients of bacteria to replicate themselves. Over time, some plasmids become viruses.

　　Bacteria often suffer from a kind of virus infection called "phage". Some people think that phage comes from plasmid. The cells and bacteria of human beings and all animals and plants are very different. We all belong to "eukaryotes". There are no plasmids like bacteria in the cells, but there are still some genes waiting to move. They refused to stay on the chromosome, but jumped around in the nucleus, running to this chromosome for a while and running to that chromosome for a while. However, these naughty genes do have a name of sage like type — — Transposon (transposon)

　　According to the detection of molecular biology, many transposons have very similar gene sequences to viruses, and the mechanism of integrating themselves into the chromosomes of host cells is also highly similar. In particular, the "Retrotransposon" among them is almost as similar as some viruses, and the only difference is that these transposons cannot migrate from cell to cell like viruses.

　　However, the origin of viruses is still confusing here, because they don’t follow the evolutionary model of general organisms at all.

　　The general process of virus infection is: inject your own genes into the host cells, then use the host cells to copy your own genes, and make all kinds of materials needed to construct virus particles. Finally, manipulate the cells to package all parts of virus particles together with the virus genes into new virus particles and release them to infect other cells. But in this process, the virus is exchanging genes with various organisms all the time.

　　For example, when a virus directs a host cell to package virus particles, it sometimes packages some host cell DNA, or accidentally leaves a little bit of its own gene in the host cell. There is a gene in mammals to prevent the maternal immune system from attacking the fetus, that is, a virus accidentally landed in our cells more than 100 million years ago. When the virus spreads across species, it often leads to the transfer of genes from one species to another.

　　Hypothesis 3 Decaying cellular organisms. After entering the 21st century, a series of discoveries began to make scientists more aware that there are other possibilities for the origin of the virus.

　　In 2003, scientists discovered a very unreasonable virus — — "Mimivirus", the size of this virus has reached 0.4 to 0.5 microns, which is almost the same as bacteria under the microscope. The coronavirus that caused this epidemic is considered to be a relatively large type of virus, and its size is less than 0.1 micron.

　　In 2008, scientists discovered the second big virus, and named it "Mamavirus". Since then, one kind of "Giant Virus" has appeared in human vision one after another. By the time Pandoravirus was discovered in 2013, the record of the largest virus was even more than 1 micron.

　　Pandora virus, the largest known virus, looks almost the same as a bacterium.

　　Since then, the boundaries between viruses and some single-celled organisms have become blurred. For example, the structure and genes of Bacteroides virus are very similar to those of a single-celled organism called archaea. The only difference is that Bacteroides virus has lost a part of the key genes that independently complete cell division, so it has to parasitize in the cells of other organisms and use the cells of the host to grow and reproduce.

　　So there is the third hypothesis of the origin of the virus (The Regressive Hypothesis), which holds that the virus is essentially a degenerate creature. Some single-celled organisms gradually degenerate most of their cell structures in the long-term parasitic life, and finally become this "living dead" general appearance, and the flow like bacteroides virus is an archaea that has just begun to degenerate.

　　Why do deadly viruses always come from wild animals? Let’s analyze it from the perspective of evolution.

　　The "best" virus does not cause too serious symptoms (otherwise it will kill the host itself), but it should not be too gentle (after all, the host is often infected with other viruses at the same time, so it is still necessary to grab resources). Therefore, in the long evolution, this game will promote the virus to finally reach a certain tacit understanding with the host. For example, human beings and rhinoviruses that may cause the common cold belong to this relationship.

　　However, the virus will mutate, and some mutations will cause the host of the virus to change. Without the tacit understanding of long-term running-in between the virus and the new host, there will be the problem of "not paying attention to it", and some of them will bring fatal diseases to the host.

　　In the long-term evolution, human beings have reached a perfect tacit understanding with the viruses that accompanied them all the way from ancient ancestors, and the viruses from livestock, such as measles and flu, are not perfect, but they are somewhat tacit, so they rarely cause serious epidemics. Only the virus from wild animals has nothing to do with human beings, so almost all the diseases that caused the great plague came from wild animals. Therefore, I hope everyone can stay away from wild animals, including stray animals, and don’t raise or eat wild animals.

　　(Author: Tang Cheng Author: Center for Excellence and Innovation of Brain Science and Intelligent Technology, China Academy of Sciences)