Perseverance, the Fermi Paradox, and the Great Filter.
Over these past few weeks, we’ve all been stunned by the beautiful images returned by the Perseverance rover on Mars. One of that mission’s main purposes is to find traces of past life on the Red Planet, and the rover has already started traveling around Jezero Crater in pursuit of that goal.
For me as an astrobiologist, no discovery would be more exciting. Yet there are other ways of looking at it. In a 2007 essay, Nick Bostrom, Director of Oxford University’s Future of Humanity Institute, wrote that while the discovery of life on Mars would be of tremendous scientific significance, it would be really bad news for the future of the human species.
A re-evaluation of Bostrom’s argument seems timely now that we’re actually getting closer to determining if life ever existed on Mars. Why would a “yes” answer freak out Bostrom? As often happens when considering the possibility of extraterrestrial life, it comes back to the Fermi Paradox—also called the Great Silence. Despite our best research efforts, we have not found any firm signs of intelligent alien life, even though there are myriads of planets out there, many of them likely habitable. A huge number of these must have formed well before our own Solar System, so if the evolution of technically advanced alien species is not incredibly hard, shouldn’t there be evidence of advanced aliens all around us? But there is not.
That means there must be a “Great Filter”—a kind of evolutionary hurdle that prevents most, or maybe all, life forms from becoming a “cosmic” species. And this Great Filter—also proposed by economist Robin Hanson—must be very effective. In Bostrom’s words: “There are billions of potential germination points of life, and you end up with a sum total of zero alien civilizations that developed technology to the point where they become manifest to us Earthly observers.”
We don’t know at what evolutionary step this Great Filter comes into play. It may be at the very beginning, making it exceedingly rare for life to originate in the first place. It could come at any of the major evolutionary transitions, such as the “invention” of the eukaryotic cell or multicellular life. Maybe the hurdle of technological advancement is very hard to clear. Or perhaps the filter lies in our future—the scariest scenario, because it means that doom might be just ahead of us!
If we were to find life on Mars that has an origin independent from Earth—not just our long-lost microbial “cousins”—it would lead us to believe there are probably millions of planets all over the galaxy where life originates, and therefore, that the Great Filter must be located later in the evolutionary timeline. If we find eukaryotic life on Mars or even simple multicellular life like nematodes, it would mean that neither of these evolutionary transitions are the Great Filter either. William Bains and I have argued that once life originates, there are multiple ways for it to achieve these types of transitions given enough time, although the rise of technological intelligence may be rare since it has happened only once in Earth’s 4.5-billion year history.
Thus, if Perseverance or other follow-up missions discover evidence of alien life on Mars, this implies that the Great Filter happens at the point where humans became technologically advanced, or that it lies in our future. If the former is true, that makes our species truly special. Could we really be that unique? I have my doubts when I see other intelligent species on our planet, some of which, such as octopi, apes, and crows, could be said to be in a kind of pre-technological stage. On the other hand, in writing the book The Cosmic Zoo with William, I couldn’t escape the feeling (yes, I know, scientists and feelings) that there is something very special about us humans.
But if the latter is true, and we technological humans are not incredibly rare in the galaxy, the outlook for our species, and indeed for life everywhere, is very gloomy. We’re talking about an existential threat—bigger than Coronavirus or even climate change—that could set us back decades, or even hundreds of years. It would be something capable of taking out all (or nearly all) technologically advanced species. This is why Bostrom hopes we don’t find alien life on Mars, or anywhere else. It would mean we’ve already made it through the Great Filter—perhaps when life first arose on our planet, against great odds.
But that would also leave us in a barren, almost lifeless universe. This is a real possibility, but personally I don’t think it’s likely. My preferred scenario is that we are indeed a truly exceptional species. Or that technologically advanced aliens are in fact all around us, and for some reason we haven’t seen them. In the end, though, the universe is as it is, and not as we wish it to be.