Human DNA carries the artifact of an infection so ancient that it predates humanity itself. The brain-attacking disease, known as bornavirus, infected proto-hominids forty million years ago. It may provide evidence of how our evolutionary ancestors dealt with infection.
Keizo Tomonaga of Osaka University in Japan led a team of scientists in the hunt for signs of bornavirus in the genomes of a range of animals, including humans, other primates, elephants, and marsupials. They discovered several fragments of bornavirus in human DNA, including two genes that were somehow affected by the pathogen.
Although bornavirus is not the first disease to be passed from generation to generation, all previous examples observed in vertebrates were retroviruses. While retroviruses take over their host cells to reproduce, bornavirus is a neurotropic virus, meaning it attacks but does not control the nerve cells. Rather, it reproduces in the cellular nuclei of its hosts. Bornavirus's name derives from the town of Borna, Germany, where the first documented incidence of the virus was observed in horses in 1885.
Beyond demonstrating a new way for viruses to latch onto the human genome, bornavirus may have evolutionary implications for humans. The wave of infection that struck our protohominid ancestors forty million years ago may have led to either genetic mutation or adaptive innovation. On the other hand, bornavirus might have led to the introduction of other inherited illness, or it might have warded off attacks from other pathogens. As per usual in evolutionary biology, there are countless more questions than answers, but this serves as a useful reminder that humanity's evolutionary history depends on far more species than just our own.