Two robotic rovers, Opportunity and Curiosity, are currently roaming across the surface of Mars, looking for evidence that the planet was once warm and wet enough to sustain life. Overhead, the Mars Reconnaissance Orbiter, working in parallel, is taking high-resolution pictures of the planet’s surface. Before these machines were launched into space, they were meticulously and repeatedly wiped down with an alcohol solution, and some of their parts were heated to at least two hundred and thirty degrees Fahrenheit, in order to avoid contaminating the planet with bacteria from Earth. If some sort of microbial life exists on another world, the last thing you want to do is seed the place with terrestrial microbes—if only to prevent “discovering” alien life that isn’t alien at all.

Yet, in most cases, sterilizing probes sent to Mars is a waste of time and money, according to Alberto Fairén, a Cornell astronomer who has worked on both Curiosity and Opportunity. Writing in Nature Geoscience, Fairén and his co-author Dirk Schulze-Makuch, of Washington State University, argue that Earth bacteria likely couldn’t survive on Mars—or, if they could, they’ve already been living there for millions of years.

Whether or not the surface of Mars was hospitable billions of years ago—and a growing body of evidence suggests that it was—it is drier today than the driest desert on Earth. Most of the heat-trapping atmosphere vanished into space long ago, and because Mars has no substantial magnetic field, the surface is constantly blasted by cosmic rays. “We conclude,” Fairén and Schulze-Makuch write, “that, if Earth life cannot thrive on Mars today, our concerns about forward contamination of Mars with Earth organisms are unwarranted.”

We know that chunks of Mars been blasted off of its surface by incoming asteroids; some of them wandered through space and eventually fell to Earth as meteorites. In 1996, NASA scientists claimed they’d found evidence of fossilized Martian bacteria in just such a rock. The claim was later refuted, but it is generally accepted that living bacteria, if they existed in the first place, could have survived the trip.

It is also likely that chunks of Earth have also been blasted into space and wandered over to Mars. And since we know that bacteria have existed here for at least 3.8 billion years, they presumably could have hitched a ride. If bacteria had made the trip during a time when Mars was more habitable, they could have thrived, and, as conditions became more brutal, it’s possible that they could have adapted to the harsher environment. In this case, Fairén and Schulze-Makuch argue, there is no point trying to avoid contamination, since it has already happened. “We cannot see,” they write, “how our current program of Mars exploration might pose any real threat to a possible Martian biosphere.”

However, Catharine Conley, NASA’s planetary-protection officer, is already at work co-authoring a formal rebuttal, to be published in Nature Geoscience in a few weeks. In an e-mail conversation, she agreed that Earth bacteria could, in principle, have migrated to Mars. But, she wrote, “if organisms on Mars are related to us, they got there quite a while ago and are interesting to science in their own right, so planetary protection becomes even more important—it’s more difficult to distinguish cousins, than just neighbors.”

Fairén agreed that for missions designed specifically to search for life—none of which exist at the moment—“sterilization to some extent could be in order, because if not we can have ‘false positives.’ If we find life on Mars, we need to be sure that the life we are seeing did not come onboard the same spacecraft looking for it.”

The conversation raises another possibility: that it’s easier to get a rock from Mars to Earth than vice versa, in part because it’s easier to escape Mars’s relatively low gravity and in part because it’s easier for a space rock to migrate toward the Sun, in the Mars-to-Earth direction, than it is to go the other way.

Planetary scientists consider it entirely plausible, therefore, that life on Earth didn’t begin on Earth. There’s no concrete evidence at this point, but, in theory, the bacteria that began to multiply here billions of years ago, which eventually evolved into multi-celled creatures—and, ultimately, into us—might have travelled here. The search for Martians, which began in earnest with the astronomers Giovanni Schiaparelli and Percival Lowell, at the turn of the end of the nineteenth century, might have been, in a sense, a waste of time. The Martians may have been here all the time.

Michael Lemonick is a senior staff writer at Climate Central and a lecturer at Princeton University; his most recent book is “Mirror Earth.”