This ongoing merging makes the koala retrovirus different than just about any other virus on Earth today known to science. But it is hardly unique in the history of life. When scientists look at the genomes of humans and other animals, they see stretches of DNA that bear indisputable hallmarks of viruses. In many cases, two closely related species will share the same viral DNA at the same spot in their genomes. That shared remnant of an ancient parasite tells scientists that the virus infected the common ancestor of both species. We share viral DNA with other primates, indicating that our common ancestors were invaded by viruses starting some 60 million years ago.
Aris Katzourakis of the University of Oxford has tallied up all of the invasions of retroviruses into the genomes of our ancestors. His latest total is 31. Each of those invasions may well have caused a devastating outbreak akin to what retroviruses are doing to koalas today—perhaps spreading an immune-crushing cancer that nearly brought our early primate ancestors to extinction.
Eventually, these outbreaks ended, and the viruses became trapped in their hosts. But they didn’t lose all their viral powers. They could still parasitize their host’s genome. Sometimes a cell would make an extra copy of the viral genes and then insert them back elsewhere in its genome. As a result, our 31 viral invasions gave rise to 100,000 separate chunks of virus DNA. Altogether, they make up at least 8 percent of the human genome.
In their lingering twilight, our endogenous retroviruses can still be dangerous to our health. When a new copy of their DNA gets inserted into our genome, it can disrupt an essential gene. And cancer cells often switch the virus genes back on, probably using them to the cancer’s own advantage. Some viral proteins help tumors escape the immune system’s notice, for example.
On the other hand, evolution has domesticated some of these virus genes for our own benefit. One virus gene makes a protein that’s essential for placentas to join to the uterus wall. And we use other virus genes to fight off free-living viruses. Some virus proteins are produced in our brains, although no one knows what, if anything, they are doing there.
It’s been millions of years since we last acquired a new endogenous retrovirus. The outbreaks that helped build the human genome are ancient history. But that doesn’t mean that a 32nd retrovirus won’t worm its way into our genome. New retroviruses—such as HIV, which jumped from chimpanzees to humans in the early 1900s—infect our species fairly often. We can’t predict which virus species will slip into eggs and sperm and provide us with the next piece of the human genome, but here is one fact that’s pretty unsettling to ponder: If you put a koala retrovirus in a dish with human cells, it can easily infect them. Koalas may not just be a guide to our past. Perhaps they will be a part of our future.