Philosopher and director of the Future of Humanity Institute at Oxford University, Nick Bostrom has a fascinating article in the current issue of Technology Review in which he hopes that Mars turns out to be a barren lifeless rock. Why? Because that would mean that the "Great Filter" is more likely behind us than ahead of us. So far the Search for Extraterrestrial Intelligence (SETI) has been a dud. Bostrom argues that there may be two explanations for this.
First, the evolution of intelligent life may be extremely unlikely despite the fact that the galaxy's stars are orbited by billions and billions of planets. Some "filter," say the development of prokaryotic eukaryotic cells, must be passed before intelligence evolves. If intelligence is extremely hard to evolve that would explain why we have encoutered no ETs nor overheard them chattering among the stars.
The alternative explanation for the silence of skies is more sobering. As Bostrom explains it:
The more disconcerting hypothesis is that the Great Filter consists in some destructive tendency common to virtually all sufficiently advanced technological civilizations. Throughout history, great civilizations on Earth have imploded–the Roman Empire, the Mayan civilization that once flourished in Central America, and many others. However, the kind of societal collapse that merely delays the eventual emergence of a space-colonizing civilization by a few hundred or a few thousand years would not explain why no such civilization has visited us from another planet. A thousand years may seem a long time to an individual, but in this context it's a sneeze. There are probably planets that are billions of years older than Earth. Any intelligent species on those planets would have had ample time to recover from repeated social or ecological collapses. Even if they failed a thousand times before they succeeded, they still could have arrived here hundreds of millions of years ago.
The Great Filter, then, would have to be something more dramatic than run-of-the mill societal collapse: it would have to be a terminal global cataclysm, an existential catastrophe. An existential risk is one that threatens to annihilate intelligent life or permanently and drastically curtail its potential for future development. In our own case, we can identify a number of potential existential risks: a nuclear war fought with arms stockpiles much larger than today's (perhaps resulting from future arms races); a genetically engineered superbug; environmental disaster; an asteroid impact; wars or terrorist acts committed with powerful future weapons; superintelligent general artificial intelligence with destructive goals; or high-energy physics experiments. These are just some of the existential risks that have been discussed in the literature, and considering that many of these have been proposed only in recent decades, it is plausible to assume that there are further existential risks we have not yet thought of.
The study of existential risks is an extremely important, albeit rather neglected, field of inquiry. But in order for an existential risk to constitute a plausible Great Filter, it must be of a kind that could destroy virtually any sufficiently advanced civilization. For instance, random natural disasters such as asteroid hits and supervolcanic eruptions are poor Great Filter candidates, because even if they destroyed a significant number of civilizations, we would expect some civilizations to get lucky; and some of these civilizations could then go on to colonize the universe. Perhaps the existential risks that are most likely to constitute a Great Filter are those that arise from technological discovery. It is not far-fetched to imagine some possible technology such that, first, virtually all sufficiently advanced civilizations eventually discover it, and second, its discovery leads almost universally to existential disaster.
So where is the Great Filter? Behind us, or not behind us?
If the Great Filter is ahead of us, we have still to confront it. If it is true that almost all intelligent species go extinct before they master the technology for space colonization, then we must expect that our own species will, too, since we have no reason to think that we will be any luckier than other species. If the Great Filter is ahead of us, we must relinquish all hope of ever colonizing the galaxy, and we must fear that our adventure will end soon–or, at any rate, prematurely. Therefore, we had better hope that the Great Filter is behind us.
What has all this got to do with finding life on Mars? Consider the implications of discovering that life had evolved independently on Mars (or some other planet in our solar system). That discovery would suggest that the emergence of life is not very improbable. If it happened independently twice here in our own backyard, it must surely have happened millions of times across the galaxy. This would mean that the Great Filter is less likely to be confronted during the early life of planets and therefore, for us, more likely still to come.
You really should treat yourself to Bostrom's thoughtful article "Where Are They?: Why I hope the search for extraterrestrial life finds nothing."
Hat tip to Darrell Brookstein.