From Alex Barash at Slate:
The not-quite life forms have a bad rap. But they’re a reliable sign of life, and it would be exciting to find them in space.
For one thing, viruses are an excellent indicator for life itself: Wherever there’s life on Earth, there are viruses, too, and almost invariably in far greater numbers. Some scientists think that’s been true from the very beginning. While we know that RNA, the genetic material that makes up some viruses, came before DNA, the genetic material required by everything else, the fact that all modern viruses depend on cells to reproduce has led to something of a chicken-or-the-egg scenario. The NIH’s Eugene V. Koonin has spent decades investigating the evolution of life. In a landmark paper from 2006, he made the case for a “virus world”—an origin story in which viruses predated cells, only becoming intracellular parasites (losing the ability to replicate independently in the process) once they had bacteria and other lifeforms to prey on. It’s hard to know for sure whether this is how it played out—it’s clear that viruslike entities came before cells as we know them today, but no one has been able to confirm that modern viruses are their direct descendants. But even if the nature and extent of viruses’ involvement in the origins of life is still contentious, their role as drivers of current evolution is indisputable. As one report from the American Academy of Microbiology put it, “Without viruses, life on Earth would be very different, or perhaps there would be no life at all.”
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Of course, finding viruses in outer space wouldn’t be the same as finding life. Stedman and his colleagues class them as indirect evidence of life, and the debate over what a virus actually is, considering its need to co-opt cellular machinery in order to reproduce, continues. (Renowned immunologist Sir Peter Medawar’s working definition was said to be “a piece of bad news wrapped up in a protein.”) Still, the researchers contend that “if a virion (or a viruslike particle) were to be unequivocally detected in an extraterrestrial sample, very few people would claim that this would not be evidence for life—wherever that sample was from.” This makes sense: If these hypothetical viruses, unlike the ones we’ve found so far on Earth, are actually self-sustaining, they do meet NASA’s criteria for a life form; if not, it means that an actual living organism must be nearby to play host. More.
Actually, it’s not clear that RNA came first. Nor is it clear that viruses precede life. A good case can doubtless be made for viruses being part of the scrap heap of existing life.
But no matter. If you think you can find viruses in space, boldly go.
See also: Why viruses are not considered to be alive
Another stab at whether viruses are alive
Phil Sci journal: Special section on understanding viruses
Why “evolution” is changing? Consider viruses
The Scientist asks, Should giant viruses be the fourth domain of life? Eukaryotes, prokaryotes, archaea… and viruses?
and
Are viruses nature’s perfect machine? Or alive?