Friends who understand evolution should know that as long as they trace back, any two creatures can trace back to a common ancestor, and the ancestors can continue to trace back to the common ancestor of the ancestor, and finally build an evolution that is sufficient to encompass all living beings, ancient and modern tree.

However, the virus does not seem to be on this tree. The structure of most viruses is very simple. Basically, a protein-made shell encloses some genetic material, which is usually no different from dead things. Once they infect a cell, they inject their own genes into the host cell and use the host cell to replicate itself. Generate new viruses. This weird lifestyle makes it difficult for us to place it anywhere in the tree of evolution.

Is the virus a creature? Where do they come from?

Let's take a look at the three hypotheses of virus origin.

Hypothesis 1: The revenge soul of ancient elements

The story of the virus may have to be told from ancient times. On that inanimate earth, the ocean occupies the vast majority of the world, and under the immense sea there are hundreds of millions of years of continuous volcanic activity. By some coincidence, the first organic macromolecules in history were created in the sea water, which lit the star fire of life.

Slowly, there are some macromolecules, such as RNA, DNA, and proteins that start to replicate themselves independently or in cooperation with each other. This stage can be called "molecular life". However, in a peaceful molecular life, a heterogeneity has arisen, that is, some molecular life including the recent common ancestor "LUCA" of all living things, they have developed a structure that changes the law of life. That's the cell. Cells with membrane structures can better protect core macromolecules such as delicate RNA and proteins, which greatly enhance the adaptability of these organisms, which means that they will rub the original molecular life on the ground and rub.

There is a hypothesis (The Virus-First Hypothesis) that the virus is the survivor of the primitive molecular life world. They created a parasitic lifestyle at the last moment, which in turn used the cellular structure of the enemy, like a group of ancient The spirit of vengeance is eternally fighting against the race that has taken its home.

This hypothesis was very popular for a time, after all, the structure of viruses is so simple, even simple, that the difference between them and cell life is so great.

More solid evidence comes from "Virusoid," which is an RNA molecule. Mimivirus cannot infect cells directly, but it can infect the virus. In fact, when some viruses infect cells, they can copy themselves and spread themselves by hitchhiking, which can cause diseases such as human hepatitis D. .

However, the story of the origin of this bloody virus is no longer perfect with the deepening of molecular biology ...

Hypothesis 2: The genes of "defection"

Can gene brow with big eyes also betray? Yes, in this Progressive Hypothesis, genes can do everything in order to let them pass on.

A small piece of circular DNA called "plasmid" widely exists in bacteria. These genes are basically a group of temporary workers. The 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 the original DNA (red part), there are often small fragments of circular DNA (orange), which are called plasmids (source: drawn by the author)

So in the long evolution, some plasmids have learned one thing: we don't work for a lifetime! These plasmids changed from wage earners to two or five earners, which in turn hijacked their bacterial bosses, taking away all the bacteria's nutrients to replicate themselves. Over time, some plasmids become viruses.

The cells and bacteria of humans and all animals and plants are very different. We all belong to "eukaryotes". There are no bacteria-like plasmids in the cells, but there are still some genes that are ready to move. They don't want to stay on the chromosome, but they jump left and right in the nucleus, and then run on this chromosome, and then on that chromosome. However, this mischievous gene has a name for a fairy bone—transposon.

According to molecular biology tests, many transposons have gene sequences very similar to viruses, and the mechanisms by which they integrate themselves into host cell chromosomes are also highly similar. In particular, the "virus-like retrotransposons" (Retrotransposon) are similar to some viruses to an outrageous degree. The only difference is that these transposons cannot migrate between cells like a virus.

However, the origin of the virus is still confusing here, because they do not follow the evolutionary pattern of ordinary organisms at all.

The general process of virus infection is: inject your own genes into the host cell, then use the host cell to copy your own genes, and create various materials required to construct the virus particle, and finally manipulate the cell to combine the various parts of the virus particle with the virus gene Packaged together into new virus particles, released to infect other cells.

But in the process, the virus is exchanging genes with various organisms at all times.

For example, when the virus instructs the host cell to package the virus particles, it sometimes encloses the DNA of some host cells, or accidentally leaves a little of its own genes in the host cell. One of the genes in mammals that prevents the mother's immune system from attacking the fetus is a virus that accidentally landed in our cells for more than 100 million years. And when the virus spreads across species, it often causes genes to be transferred from one species to another, so the "transgenic" thing is left to nature.

However, it is precisely because the virus's genetic modification is so frequent that one has to rethink it. Is the virus really a defected gene? Will we get it wrong from the start? What about those transposons may not be the genes of the restless room, but the viral genes that accidentally stay in the cell?

Hypothesis 3: Viruses may be sinking cellular organisms

In the 21st century, a series of discoveries began to make scientists more aware of the possibility of the origin of the virus.
 
In 2003, scientists discovered a very unreasonable virus-"Mimivirus". The size of the virus reached 0.4 to 0.5 micrometers, which was almost the same as bacteria under a microscope. The coronaviruses that caused the outbreak are considered to be a relatively large class of viruses, and their size is less than 0.1 micron.

In 2008, scientists discovered the second major virus and named it "Mamavirus". From then on, "Giant Virus" appeared one after another in human vision, until 2013 The "Pandoravirus" discovered in the year brushed the record of the largest virus to more than 1 micron.

Since then, the boundaries between viruses and some single-celled organisms have begun to blur. For example, the structure of "mimicidal viruses" is very similar to genes and a type of single-celled organisms called "archae". Bacterial viruses have lost some of the key genes that autonomously complete cell division, so they have to be parasitic in the cells of other organisms and use the host cells to grow and reproduce.

Then there is the third hypothesis of the origin of the virus (The Regressive Hypothesis), which believes that the virus is essentially a fallen organism. In the long-term parasitic life, some single-celled organisms gradually degrade most of the cell structure, and finally become like a living dead person, and the likes of mimics are archaea that have just begun to fall.

Viruses have left a deep mark on our evolution

So, should the virus be the ancient soul of revenge, the gene of defection, or the fallen creature? All three hypotheses are reasonable, and there are also problems that cannot be explained. Perhaps the origin story of the virus is far more complicated than all these hypotheses, and may not even have a single source.

We may never know where the virus came from, but the virus really left a deep mark on our evolution.

Due to the extremely powerful mutation ability of the virus, we can never predict when and where a virus that is deadly to us will emerge. In the end, many organisms have to take some measures to defeat the mutation by mutation.

For example, sexual reproduction. The most direct consequence of sexual reproduction is that the genes of each of our offspring can be rearranged, and as far as possible, all offspring can be protected from a virus. The arms race between viruses and organisms has also been the strongest promoter of evolution. Viruses have also broken the barriers to reproductive isolation and allowed genes to flow between different organisms.

Therefore, although viruses do not belong to any branch of the tree of evolution, they seem to flow around the tree of evolution and ghosts, leaving their traces on this tree.

So why do deadly viruses always come from wild animals? Let's analyze it from an evolutionary perspective.

The "best" virus does not cause too severe symptoms (otherwise it kills the host and is finished), but it should not be too mild (after all, the host is often parasitic with other viruses at the same time, which should be done when robbing resources (Vicious hands still have to). Therefore, in a long evolution, this game will cause the virus to reach some tacit understanding with the host. For example, humans and rhinoviruses that may cause a common cold belong to this relationship.

However, viruses can mutate, and some mutations can cause the host of the virus to change. The virus and the new host do not have a tacit understanding for a long period of time, and the problem of "no light or heavy" will occur, and some of them will cause deadly diseases to the host.

In the long-term evolution, human beings have reached a perfect understanding with the viruses that have accompanied them from ancient ancestors. The viruses from domestic animals, such as measles, flu, etc., have not been perfected, but they are somewhat tacit, Rarely causes a very serious outbreak. The virus from wild animals has nothing to do with us, so almost all the diseases that caused the plague came from wild animals.

Therefore, I hope that everyone can stay away from wild animals, including stray animals, and do not raise and eat wild animals, otherwise the viruses on these wild animals may not be allowed to take you to try the knife.

From Chinese Academy of Sciences

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