THE IDEA OF life on Mars has been with us for nearly 300 years, ever since early astronomers saw what they believed to be polar icecaps through their primitive telescopes. If NASA's Phoenix lander successfully touches down on Mars this afternoon, it will become part of a long experiment to determine whether the planet was ever habitable, and whether it contains any traces of life, extinct or still active.
Discovering traces of life on Mars would be of tremendous scientific significance: The first time that any signs of extraterrestrial life had ever been detected. Many people would also find it heartening to learn that we're not entirely alone in this vast, cold cosmos.
They shouldn't. If they were wise, they'd hope that our probes discover nothing. It would be great news to find that Mars is a completely sterile planet.
On the other hand, if we discovered traces of some simple extinct life form - a bacterium, some algae - it would be bad news. If we found fossils of something even more advanced, like a trilobite or even the skeleton of a small mammal, it would be horrible news. The more complex the life we found, the more depressing. Scientifically interesting, yes, but dire news for the future of the human race.
Why? To understand the real meaning of such a discovery is to realize just what it means that the universe has been so silent for so long - why we have been listening for other civilizations for decades and yet have heard nothing.
. . .
Aliens may visit us in books and films and in rumors in Internet chat rooms, but it's a fact that there has been no objective evidence for the existence of any extraterrestrial intelligent civilization. We have not received any alien visitors, nor have our radio telescopes detected their signals. As far as we can determine, the night sky is empty and silent.
We know that the universe contains many stars. There are some 100 billion of them in our galaxy alone, and the observable universe contains billions of galaxies. Thanks to recent astronomical discoveries, we now know that it's common for these stars to have planets. Many of these solar systems are much older than our own. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero alien civilizations that developed technologically to the point where we earthly observers can detect them.
So what's stopping them? Perhaps the most compelling theory is that there is some kind of barrier - what the economist and polymath Robin Hanson called a "Great Filter" - that prevents the rise of intelligent, self-aware, technologically advanced, space-colonizing civilizations. This filter would be one or more highly improbable steps along the path that starts with the creation of a planet and ends with a race capable of colonizing the galaxy.
Somewhere between those two points, the Great Filter operates, and it must be powerful enough that even with all the billions of possible starting worlds on which life might evolve - all those rolls of the cosmic dice - one ends up with nothing: no aliens, no spacecraft, no signals, at least not in our neck of the woods.
The important question for us, however, is just where on the long timeline of development this Great Filter might be located. Is it behind us, in our distant past, or somewhere ahead of us in the decades or millennia to come?
Consider first the possibility that the filter is in our past, somewhere between the creation of our planet and emergence of digital technology. We tend to take it for granted that the evolution of life was inevitable because, well, here we are. But perhaps it's extremely improbable that even simple self-replicating organisms should emerge on an Earthlike planet. Perhaps that very first step could be the Great Filter in which almost all planets get stuck.
Or perhaps it comes later, during the transition from the most basic life form into something more complex. For example, it took 1.8 billion years for life on Earth to evolve from prokaryotes, the most basic organism, into eukaryotes - still very simple, but with the addition of a membrane-enclosed cell nucleus. That immense span of time suggests that some extraordinary, improbable coincidence, some bit of amazing luck, might have been required in order for the simplest kind of life to become just a little bit more complex. This step is a good candidate for a Great Filter. Others include the rise of multicellular organisms or sexual reproduction. Each of these steps took a very long time, suggesting that they might have required a huge amount of evolutionary trial and error, combined with a huge amount of luck.
So one possibility is that the Great Filter is behind us. If so, this also explains the absence of observable aliens. Why? Well, if the rise of intelligent life is sufficiently improbable, then it follows that we are likely the only such civilization in our galaxy, and perhaps even in the entire observable universe.
But it may be that the Great Filter is ahead of us, in our future. That would mean that there is some great improbability that will prevent humanity - and perhaps any technological civilization - from traveling to other parts of the galaxy and making its presence known to others.
Throughout history, great civilizations on Earth have imploded - the Roman Empire, the Mayan civilization that once flourished in Central America - but here we are hypothesizing a more drastic termination, the extinction of the intelligent species itself, or at least the permanent destruction of its potential for further development. Would the event be nuclear war? Environmental disaster? A deadly superbug? Probably not; we might recover from any of these, eventually. The kind of collapse that merely sets a civilization back a few hundred or a few thousand years would not help explain why no such civilization has visited us from another planet.
There are 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 could still have arrived here hundreds of millions of years ago.
Obviously, we must hope that the Great Filter is behind us rather than ahead of us. If the Great Filter is ahead us, we have still to confront it. The kind of risk we are talking about here is called an "existential risk" - one that would either cause the extinction of Earth-originating intelligent life or destroy its potential for future development. It could be a war fought with powerful future weapons; badly programmed superintelligent machines; even a high-energy physics experiment gone awry.
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 too will go extinct before reaching technological maturity - we have no reason to think that we will be any luckier than most other species at our stage of development.
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 that it will end prematurely.
. . .
Now what has all this got to do with finding life on Mars? Consider the implications of discovering that life had evolved independently on another planet in our solar system. That discovery would suggest that the emergence of life is not a very improbable event. If it happened independently twice here in our own backyard - indeed, on the only other planet we have closely examined - it must have happened millions of times across the galaxy. This would mean that the Great Filter is less likely to occur in the early life of planets and is therefore more likely still to come.
If we discovered some very simple life forms on Mars in its soil or under the ice at the polar caps, it would show that the Great Filter almost certainly exists somewhere after that period in evolution. If we discovered a yet more advanced life form, such as some kind of multicellular organism, this would be even worse news for us. And if we discovered the fossils of some very complex life forms, like a vertebrate mammal, we would have to conclude that the probability is overwhelming that the bulk of the Great Filter is ahead of us. Such a discovery would be a crushing blow.
Yet most people reading about the discovery would be thrilled, not realizing that they were looking at the worst news ever displayed on the front page of a newspaper. They would not understand the implications of the finding. If the Great Filter is not behind us, it is ahead of us, meaning that the human species is doomed to fail ever to reach technological maturity.
So this is why we should hope that our space probes will discover dead rocks and lifeless sands on Mars, and also on Jupiter's moon Europa, and everywhere else our astronomers look. It would keep alive the hope for a great future for humanity.
Even if we are the only intelligent species that has ever evolved in our galaxy, and perhaps in the entire observable universe, it does not follow that our survival is not in danger. Nothing in the above reasoning precludes the Great Filter from being located both behind us and ahead of us. It might be extremely improbable that intelligent life should arise on any given planet, and very improbable that intelligent life, once evolved, should succeed in becoming advanced enough to colonize space.
But if Mars is indeed found to be barren, we would have some grounds for hope that all or most of the Great Filter is in our past. In that case, we may have a significant chance - if we play our cards right - of one day growing into something almost unimaginably greater than we are today.
In this scenario, the entire history of humankind to date is a mere instant compared with the eons of history that lie still before us. All the triumphs and tribulations of the millions of peoples who have walked the Earth since the ancient civilization of Mesopotamia would be like mere birth pangs in the delivery process of a kind of life that hasn't really begun.
Imagine the tremendous responsibility of those who find themselves present and called upon to midwife the conception of such a future. And that is where we are, you and me.
Nick Bostrom is director of the Future of Humanity Institute at Oxford University. His homepage is nickbostrom.com. This article was adapted from a lecture written for BBC radio, a version of which also appeared in Technology Review.