If We Found Life On Mars, What Would That Mean For The Fermi Paradox And The Great Filter?

Last week, NASA called a press conference for the Perseverance rover team to make an exciting announcement; the "clearest sign of life that we've ever found on Mars". Further analysis continues on the potential biosignatures, attempting to rule out non-biological explanations, but while we wait there is plenty of time for speculation on what such a discovery might mean.

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As well as telling us about alien life and how it evolves on other planets, discovering alien life on Mars could provide helpful clues to what is going on in the wider universe. One mystery on which it may provide further clarity is the Fermi paradox, or the age-old question "where is everybody?", and even whether humanity will go on to survive and become an advanced civilization. 

In short, the Fermi paradox is this: If there are all those stars and planets out there where intelligent life could evolve, why do we not see any sign of them throughout the universe? 

One popular way scientists have attempted to analyze this is through an idea known as the "Great Filter". Proposed by Robin Hanson of George Mason University, the argument goes that given the lack of observed technologically advanced alien civilizations, there must be a great barrier to the development of life or civilization that prevents them from getting to a stage where they are making big, detectable impacts on their environment that we can witness from Earth.

The Great Filter interpretation of Fermi’s great silence asserts that Npq is not a very large number, where N is the number of potentially life-supporting planets in the observable universe, p is the probability that a randomly chosen such planet develops intelligent life to the level of present day human civilization, and q is the conditional probability that it then goes on to develop a technological supercivilization visible all over the observable universe. Evidence suggests that N is huge, which implies that pq is very small.

The question, according to that idea, is why? Is life itself unlikely to form, or multicellular life, or complex animals, or intelligent animals, or do intelligent civilizations inevitably destroy themselves (by conflict or some hidden dangers in physics or AI?) before they can contact others like them? What hard step, or Great Filter, prevents us from seeing signs of others out there in the cosmos?

What life on Mars will tell us about the cosmos heavily depends on the type of life we find. For instance, we could find that life there is related to life on Earth, or that it is completely unrelated.

"With mounting evidence for the viable transfer of microorganisms between the two planets, this would be evidence for panspermia, and establish the presence of a common biosphere in which the two planets participate. Should Martian microorganisms show a different chiral specificity than that of Earth life then this would indicate separate origins of the two neighbouring life forms, thereby strongly implying that life occurs widely throughout the cosmos," a paper on the topic explains. "Any one of these possible outcomes would be a paradigm-breaking event."

If Martian life were found to be related to Earth life, it would confirm panspermia at least between planets that share a solar system, and could imply that life is delivered through the cosmos on chunks of rock and ice. Further investigation would be necessary, if this is the case. But if life transfers through the Solar System fairly easily, but not beyond it, such a discovery wouldn't tell us a lot more about life in the universe on the whole, other than it is hardy enough to be transferred between planets.

Assuming that Martians are unrelated to Earthlings, then we would have more useful information to plug into our Great Filter concept. First off, finding unrelated life on our nearest planet would make it unlikely that the first step is a hard barrier to cross. In fact, according to philosopher Nick Bostrom, the level of life could narrow down where the Great Filter takes place. 

In an article published in the MIT Technology Review in 2008, he argued that we can assume that the Great Filter likely takes place after whatever level of life we were to find on Mars. So if it were single-celled, it would make it more likely that the difficult step comes at multicellular life, and so on. 

Bostrom believes that in order to narrow down where the filter takes place, we should look at life on Earth to see which steps are improbable. "One criterion is that the transition should have occurred only once," he wrote. 

"Flight, sight, photosynthesis, and limbs have all evolved several times here on Earth, and are thus ruled out."

He also argued that evolutionary features that took a long time to occur even after prerequisites were met would indicate that this evolutionary step was improbable, e.g. the original emergence of life. The step from animals to humans took place over a relatively short time period, geologically speaking, suggesting it's a weak candidate for a Great Filter event.

What would really suck, according to Bostrom, would be if we were to find vertebrate life on Mars, as it would mean that the Great Filter looms menacingly in our future.

"Such a discovery would be a crushing blow. It would be by far the worst news ever printed on a newspaper cover," Bostrom wrote. "This is why I’m hoping that our space probes will discover dead rocks and lifeless sands on Mars, on Jupiter’s moon Europa, and everywhere else our astronomers look. It would keep alive the hope for a great future for humanity."

Though a philosophical paper, separate teams studying the idea with Bayesian statistics have come to similar conclusions. For now, we must wait for more work on the potential biosignatures, and the exciting possibility of finding life on another planet, before we begin to worry what that might mean for our own eventual extinction.