The search for extraterrestrial life is ramping up among eccentric billionaires and government officials alike. ( NASA, as Slate has argued, is pivoting to astrobiology.)  One possibility for what might be out there that’s, relatively speaking, one of the most plausible theories has so far also been overlooked: space viruses.  While some might dismiss microscopic phages as considerably less thrilling than fluorescent green humanoids, they’d actually be a hugely exciting first find. 

All of this might sound wild—astrobiology is, after all, the kind of scientific specialization that gets name-dropped in Star Trek—but it’s the direction we’re taking.  Congress has ordered NASA to take on “the search for life’s origins, evolution, distribution, and future in the universe.”  Here on Earth, all of those big ideas are tied to viruses.  The problem is that viruses’ importance to life on Earth isn’t necessarily widely known—among scientists or members of the public.  Ken Stedman, the co-chair of NASA’s Virus Focus Group, is hoping to raise viruses’ profile and repair their reputation—last week, he published a review in which he and two other scientists, Aaron Berliner of UC–Berkeley and Tomohiro Mochizuki of the Tokyo Institute of Technology, lay out the case for how, where, and why we should be looking for these tiny not-quite life forms. 

It’s a convincing argument.  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.” 

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. 

That line gets at something important: Contrary to popular belief, some viruses can be a major force for good.  Far from harming everything they touch, the genes they insert into their hosts can actually allow them to inhabit areas that would otherwise be totally inhospitable.  For example, once infected, some cyanobacteria—ocean-bound bugs that rely on sunlight for energy—are able to undergo photosynthesis and generally thrive under conditions in which uninfected bacteria couldn’t.  While uninfected cells can sustain serious damage or even die if sunlight gets too intense, these bacteriophages, in seeking to protect themselves, also protect their hosts.  In fact, we have these virus-infected cells (and specifically the genes the viruses provide) to thank for 5 percent of all oxygen on Earth. “ Viruses have a bad rap,” Stedman said. “ Unfortunately, when most people think about viruses, they think about things that make you sick, but that’s a vanishingly small proportion of the viruses on our planet—and maybe on other planets.” 

In allowing organisms to adapt to new environments, viruses have shaped (and continue to shape) countless ecosystems.  And viruses don’t only play a huge role on our planet; they’re also here in huge numbers. “ I think the really crucial thing to know about viruses is that there are literally astronomical numbers of them,” said Stedman—1031, according to the latest estimate. “ That’s a ridiculously, insanely large number [10,000,000,000,000,000,000,000,000,000,000, for those keeping track], and the data are very good on that.”  This means individual viruses outnumber cellular organisms by a staggering 10 to 1—and a single milliliter of ocean water can hold up to 10 million viral particles.  It’s no wonder that Stedman wants to see what Europa’s oceans might be hiding. 

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. 

Unfortunately, it might be a while before we have the technology necessary to search for viruses in space—Stedman acknowledges that transmission electron microscopes are unlikely to be loaded onto a spaceship anytime soon, and scanning electron microscopes, the more rocket-ready alternatives, don’t always yield images at a high enough resolution to spot them.  But with the right instrumentation in place, he believes there could be viruses waiting to be found just a few planets away.  His own ideal experiment would entail sticking a TEM underneath the ice on Europa (widely held to be the best shot at extraterrestrial life within our solar system).  When asked about Dragonfly, one of the two possible mission concepts for NASA’s mid-2020s launch, he said it was “distinctly possible” that the voyage might yield evidence of viruses on Titan—it’s just a matter of the equipment we’ll be able to fly there. 

“If we have a way to detect them—and that’s a really big if—you find viruses first,” said Stedman, adding, “I think that would tell us something fundamental about life itself.”  If we find life in space that’s also associated with viruses, that will lend strength to the viruses-are-essential-to-life hypothesis and perhaps tell us something about why.  And if we find life that isn’t, we’ll know it’s totally unlike anything we’ve found on our own planet.  That seems like a pretty strong case for getting started.