An endogenous retrovirus or ERV is a genetic scar produced by a failed viral infection. Geneticists can tell its of viral origin because it will contain base pairs associated exclusively with viruses. If the ERV happens to insert in a portion of a cell genome that goes onto contribute to an egg or sperm that creates a new person, and if that same stretch of DNA containing the ERV is part if an allele that ultimately gets selected for widely, eventually the entire population will share it. If the population then splits over time, both descendant groups will have it in the same place. If you and a stranger have a unique ERV occupying the exact same segment in both your respective genomes, it is powerful evidence you are closely related.
Humans and chimps share a number of ERVs, which is more than just a smoking gun that we share a relatively recent common ancestor, it’s a video tape of evolution pulling the trigger. So it was with great interest that a new study comparing ERVs in anatomically modern humans to Neandertal DNA as well as poorly understood contemporary referred to as Denisovans has been released:
In this latest study, the researchers compared genetic data from fossils of Neanderthals and another group of human ancestors called Denisovans to data from modern-day cancer patients. In the end, they found evidence of Neanderthal and Denisovan viruses in the modern human DNA. This suggested that the viruses originated in our common ancestors more than half a million years ago. Currently, the scientists are looking to further investigate these ancient viruses, which belong to the HML2 family of viruses, for possible links with cancer and HIV. More specifically, they’re looking at whether these ancient viruses affect a person’s risk of developing diseases such as cancer.
I’ve already seen a couple of articles pop up with the “scientists baffled” undertone that new evidence shows we are related to neanders and denisovans. Scientists aren’t baffled at that. Finding multiples identical typos in two old copies of War and Peace would make a good case they were printed by the same publisher at the same time. But there would be little doubt they were written by the same author before that. What is fascinating is there was a time, not that long ago in evolutionary scales, when the hominid population was more like Middle Earth, with many different kinds of humans, and less like modern Earth with just the one clade.
Whether or not we could interbreed and produce fertile offspring is also a fascinating debate. The smart money says not only could we, but that we probably did, and may still carry traces of those ancient, now extinct humans deep in our modern DNA.