[Many] viruses bring adaptive benefits, not harms, to life on Earth, including ours.
We couldn’t continue without them. We wouldn’t have arisen from the primordial muck without them. There are two lengths of DNA that originated from viruses and now reside in the genomes of humans and other primates, for instance, without which—an astonishing fact—pregnancy would be impossible.
There’s viral DNA, nestled among the genes of terrestrial animals, that helps package and store memories—more astonishment—in tiny protein bubbles. Still other genes co-opted from viruses contribute to the growth of embryos, regulate immune systems, resist cancer—important effects only now beginning to be understood.
Viruses, it turns out, have played crucial roles in triggering major evolutionary transitions. Eliminate all viruses, as in our thought experiment, and the immense biological diversity gracing our planet would collapse.
A virus is a parasite, yes, but sometimes that parasitism is more like symbiosis, mutual dependence that profits both visitor and host. Like fire, viruses are a phenomenon that’s neither in all cases good nor in all cases bad; they can deliver advantage or destruction. Everything depends: depends on the virus, on the situation, on your point of reference. They are the dark angels of evolution, terrific and terrible. That’s what makes them so interesting.