Viruses are nothing more than protein and genetic material, yet some scientists now believe they may have played a crucial role in evolution. Retroviruses are a special category of viruses whose genetic material is RNA, not DNA. When RNA transcriptase transcribes their genetic information from RNA to DNA, that new viral DNA becomes incorporated into the DNA of the host cell. The host cell then copies the viral DNA along with its own genome during DNA synthesis, and when it divides the viral DNA is passed on to its daughter cells as well.
Darwin did not know about genes or viruses, so the mechanisms of all of this would surprise him. However, he would not be surprised by how elegantly this new information supports his understanding of common ancestors. The presence of viral DNA allows scientists to better trace the evolutionary history of species: the more viral fragments species share, the more recently they diverged. This technique can be used to investigate the relationships among species and test how closely they’re related.
Aside from being clues to ancestral lineage, these viruses have played an active role in evolution. For example, the biggest difference between the chimpanzee and human genomes is that chimpanzees have copies of a DNA sequence from a virus known as PtERV. The version of the gene TRIM5α that humans have codes for a protein that destroys the PtERV virus, protecting us from infection. But the other version of this gene, which does nothing against PtERV, completely protects against HIV. Studying these viruses and their role in evolutionary history helps explain why humans are susceptible to HIV and apes, our closest relatives, are not.
I had no idea that so much of our DNA is the remains of old endogenous retroviruses. Only 2% of our genome contains the information used to create the proteins necessary for life, while 8% is viral DNA that has been passed down from infected ancestors; I wish the article has explained where the other 90% of junk DNA comes from. I was also surprised that some retroviruses cause cancer. The article says that “experiments with mice and chickens have shown that they can block new infections by viruses with a similar genetic structure,” leading me to wonder if one reason chance of cancer is partially genetic is because endogenous viruses that protect against infection from similarly structured cancer-causing viruses would be passed from parent to offspring.
Although I understand the benefits of reconstructing viruses for research, there’s risk in creating something with an unknown function. John Coffin, a molecular biologist at Tufts University who has studied the role of endogenous retroviruses in human evolution, says “there are many more viruses that are far more dangerous than these,” but the problem is that you don’t really know how dangerous a virus is until it’s infected something. The opportunities for bioterrorism– as demonstrated by the researchers who built a polio virus and infected mice just “to prove it can be done”– worry me, but on the other hand the best way to prevent bioterrorism may be to have non-terrorist scientists researching the viruses so that we understand the risks and are better prepared to protect against potential threats.