The first theory holds that virus is one of the most primitive life forms in the evolution of life on earth. Viruses have both the properties of chemical macromolecules and some biological characteristics. This seems to imply that in the long transition from inorganic nature to life, the virus is in the transition position from abiotic to biological, that is to say, the virus just fills the gap between chemical macromolecules and primitive cell organisms. Mainly, the environment in which life forms on the earth evolved from inorganic matter to organic matter, and then to macromolecular life. This theory is based on the pure hypothesis of life origin theory and molecular evolution theory, and lacks any evolutionary evidence.
According to the second theory, virus is a kind of degraded life substance of advanced microorganisms, and some genes of microbial cells are discarded in the course of life, which makes them lose their independent self-reproduction ability and eventually degenerate into viruses. The basis of this hypothesis is that there are some intermediate forms of obligate intracellular parasitism between bacteria and viruses parasitic in the intracellular environment, such as rickettsia and chlamydia (two dependent living cell organisms). It is speculated that low-level bacteria parasitic in cells degenerate into organisms like rickettsia, then into organisms like chlamydia and then into viruses. If the hypothesis is true, we should find small cell organisms that are parasitic in the cells of animals and plants and bacteria that can be infected by viruses, but this is not the case. Moreover, the virus has not been found in rickettsia and chlamydia. It can be seen that the evidence of this hypothesis is insufficient.
The third theory holds that the virus comes from the nucleic acid of normal cells and becomes a virus by accidentally leaving the cells, which is the popular endogenous theory of virus origin at present. Most of the support of this theory comes from indirect evidence from some experiments: the similarity between virus and plasmid, plasmid originally belongs to a part of a cell, but it can leave the cell at any time and pass from one cell to another; There are many kinds of DNA viruses, such as λ phage in bacterial viruses, cauliflower mosaic virus in plant viruses, hepatitis B virus, adenovirus, herpes virus and papillomavirus in animal viruses. All or part of the DNA of these viruses can combine with the chromosomes of their parasitic cells, thus becoming a part of the cells, which is the reverse process of cell nucleic acid escape; By using nucleic acid molecular probe technology, it is found that the sequence of the integration region that many viral DNA can bind to the cell chromosome has great homology with the flanking sequence of the integration chromosome, especially in some retroviruses (the pathogen of AIDS-human immunodeficiency virus), the oncogene V-onc is highly homologous to the proto-oncogene C-onc in the cell. There are a wide range of reverse transcription mobile genetic factors in normal cells, such as Ty factor in yeast cells, Copia-like factor in Drosophila, IAf and gene in vertebrates, and reverse transcription repeats, such as human Alu factor and Kpn factor, suggesting that normal cells contain RNA-mediated DNA synthesis reaction, which is consistent with the replication behavior of retrovirus nucleic acid. These indirect evidences can partly explain the origin of DNA virus, but it is very difficult to explain the origin of RNA virus.
1999 The1/International Congress on Virology provided new evidence for the origin and evolution of viruses at the gene level: a genetic unit called "inversion" was found in an archaea (the third kind of organism between prokaryotes and eukaryotes), which is a self-replicating nucleic acid molecule with only one gene and is of great importance. Its gene sequence is highly homologous to the allele of the host cell, and it is also highly similar in codon usage frequency. According to the chronological study of this archaea, it is estimated that the reverser existed 400 million years ago. With the evolution of archaea, transformants capture genes from bacterial gene pool, expand their genetic information, increase their biological functions, and produce the most primitive infectious virus particles, while the original virus adopts different replication strategies, which leads to the evolution of different viruses. It can be seen that the origin of life begins with bacteria, which is the cradle of virus evolution. The basic speculation is that the earliest virus on earth may be a retrovirus with RNA as its genome, and then it will develop into a retrovirus with DNA as its genome (hepatitis B virus belongs to this category), and finally a DNA virus will appear, and RNA virus may have its own independent evolutionary path.
The above three theories of virus origin and the relevant evidence of virus evolution enlighten people that the origin of viruses is complex and diverse, and different viruses have different origins, at least between DNA viruses and RNA viruses. This conclusion needs to be accumulated in more research data.