All viruses, including those that infect humans, are known as obligate intracellular parasites which is a very technical way of saying that they require host cells to reproduce. However, this reproduction is not as simple as it may be for living organisms. Once a virus is able to enter a cell, it may begin using the proteins of that cell to replicate its genome. Some viruses, like HIV, originally use their own proteins to convert their genomes into the same double stranded DNA material that is used by the organisms that they infect. They can then insert this material into the genome of the host. It is only after these steps that the viruses will allow their genetic material to be replicated by the proteins of their host organism and passed on to other cells. Any virus that undergoes this process in order to reproduce is referred to as a retrovirus and it is retroviruses that have had the most impact on human evolution.
When a retrovirus infects a cell that will give rise to a host's offspring, the virus will be passed on to the following generations and continue being passed down for the foreseeable future. It is believed that retrovirus' reproduction processes, more specifically the integration of their genomes into host genomes, are the origin of about eight percent of human genetic material. Much of this material is ancient and the result of infections in extremely distant ancestors. There is even some evidence to suggest that some viral genomes began placing themselves into the DNA of vertebrates anywhere from 460 to 550 million years ago. This would mean that retroviruses originally infected the cells of fish that would go on to evolve into Homo sapiens.
New and Old Genes
Because retroviruses have been in existence since the days before humans, this has allowed them to shape the human body. A prime example of this is the gene ERVW-1 which originated as the genome of an ancient retrovirus that entered the cells of primates around 25 million years ago. The gene codes for a protein, syncytin-1, that is essential in the formation of the placenta. However, this is one of the rarer types of interaction between human and viral genomes. More commonly, retroviruses have altered the function or expression of already existing genes. This occurred with the gene AMY1C which, at some point, had a retroviral genome inserted into its promoter sequence. As promoters are used to control expression of genes, it is believed that this event changed the way that the gene was used. The AMY1C gene is responsible for the creation of amylase, a digestive enzyme. It is thought that the insertion of a viral genome in its promoter allowed for the gene to be expressed in the parotid gland which is located within the mouth.
The retroviral DNA molecules that have gone on to become essential parts of the human genome are preserved and protected from mutations. The majority of retroviral genomes that have been inserted into the human genetic material are no longer infectious and those genomes that go on to provide benefits for human beings remain intact through the generations.
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