If it is just us, seems like an awful waste of space. - Carl Sagan
In the summer of 1950, Enrico Fermi and co-workers Emil Konopinski, Edward Teller, and Herbert York had a casual lunchtime conversation at Los Alamos National Laboratory. Legend has it that during the lunch, Fermi, seemingly disturbed by a persistent thought, suddenly exclaimed: Where is everybody? "Somehow ... we all knew he meant extraterrestrials", York recalled some time after, adding that Fermi "followed up with a series of calculations on the probability of earth-like planets, the probability of life given an earth, the probability of humans given life, the likely rise and duration of high technology, and so on. He concluded on the basis of such calculations that we ought to have been visited long ago and many times over".
Fermi's puzzling conclusion is now known as the Fermi paradox, and refers to the apparent contradiction between the various high probability estimates for the existence of extraterrestrial civilizations elsewhere in the galaxy and the lack of evidence for it. The detailed argument was fully developed in a 1975 paper by Michael H. Hart, and includes the following points:
There are billions of stars in the galaxy that are similar to the Sun, and many of these stars are billions of years older than the Solar system.
With high probability, some of these stars have Earth-like planets, and if the Earth is typical, some may have already developed intelligent life.
Some of these civilizations may have developed interstellar travel, a step the Earth is investigating now.
Even at the slow pace of currently envisioned interstellar travel, the Milky Way galaxy could be completely traversed in a few million years.
Thus, why haven't we seen any convincing evidence for extraterrestrial intelligent life yet? If interstellar travel is possible, even the "slow" kind nearly within the reach of Earth technology, then it would only take from 5 million to 50 million years to colonize the galaxy. This is relatively brief on a geological scale, let alone a cosmological one. Since there are many stars older than the Sun, and since intelligent life might have evolved earlier elsewhere, the question then becomes why the galaxy has not been colonized already. Even if colonization is impractical or undesirable to all alien civilizations, large-scale exploration of the galaxy could be possible by probes. These might leave detectable artifacts in the Solar System, such as old probes or evidence of mining activity, but none of these have been observed.
As it usually happens, the Fermi paradox is nothing like a genuine inconsistency, but rather an odd fact about our place in the universe that needs to be explained. And explanations in this case abound. First of all, the origin of life on Earth is very poorly understood. The details and likelihood of abiogenesis, the natural process by which simple organic compounds arise from non-living matter, are largely unknown. For this reason, one simple resolution of the "paradox" could be that the conditions needed for the evolution of life, or at least the evolution of biological complexity, are rare or even unique to Earth. Under this assumption, called the rare Earth hypothesis, the evolution of biological complexity requires a host of fortuitous circumstances that are unlikely to arise anywhere else in the time the universe has existed. This would make the emergence of life on Earth, and especially the appearance of eukaryotic cells, sexual reproduction and the Cambrian explosion, a one-time, almost miraculous event (even though clearly not impossible).
Having said that, some scientists believes that life exhibits self-organizing properties, and its emergence is therefore inevitable given enough time. If this was true, the universe would be teeming with life: it is hypothesized that there are 11 billion potentially habitable Earth-like planets in the Milky Way alone, some of them much older than Earth. However, even if complex life is common, intelligence (and consequently civilizations) might not be. In such a scenario, the great filter to intelligent life would be intelligence and not life itself. Many biologists used to believe that evolution was progressive and had a direction that led towards so-called "higher organisms". This idea of "progression" and "higher organisms" in evolution is now regarded as misleading, as we now understand that natural selection has no intrinsic direction and organisms are selected for either increased or decreased complexity in response to local environmental conditions. Although there has been an increase in the maximum level of complexity over the history of life on Earth, the large majority of organisms has always been small and simple and the most common level of complexity appears to have remained relatively constant. All of this is consistent with the widely held belief that evolution is a blind process with no purpose and that the rise of intelligent life on Earth is presumably an environmental fluke, a chance occurrence that is far from inevitable.
Moreover, even if we concede that once life develops it will naturally evolve towards higher intelligence, the universe might simply be too young. On Earth, life has existed for 4 billion years, but intelligent life has only arisen with the genus Homo about 3 million years ago. This means that for most of its existence, Earth was a wild planet. If other uninhabited planets developed similarly to Earth, they are more likely not to bear intelligent life yet. It is of course very hard to extrapolate the timescale needed for the emergence of complex intelligent life based on just the one example that we have, but if the rise of the genus Homo was typical, then the natural conclusion is that the universe is not old enough to harbour a large number of alien civilizations. This of course begs the question of why we exist at such a special point in time, in the middle of the cosmic transition to sentience. If consciousness is going to dominate the history of the universe in the future, why were we born so early, at the very beginning of this great new era? It would be a strange coincidence, but nothing is fundamentally wrong with that.
Another possibility is that alien intelligent life, while widespread throughout the galaxy, is simply too alien to allow for meaningful two-way communication. Aliens are generally depicted as humanoids in movies and books, but there is no reason to believe higher intelligence can only arise in human form. One just needs to take a look at the biological diversity here on Earth to have a vague idea of what alien life might look like. Most probably though, the biological spectrum of allowed intelligent life forms is so huge that we are only scraping the surface of what's possible. The physiology of alien life might create a considerable communication barrier; it was speculated, for example, that an alien species might have a thought process orders of magnitude slower (or faster) than ours. A message broadcast by that species might well seem like random background noise to us, and therefore go undetected. Alien life, though highly intelligent, might also be physically unable to send electromagnetic signal throughout the universe, and thus be a silent observer.
Finally, I would like to dwell on one interesting yet ominous resolution of the paradox. During the Second Iberoamerican Graduate School on Astrobiology of 1995, an interesting debate arose between astrophysicist Carl Sagan and evolutionary biologist Ernst Mayr about the probability of extraterrestrial intelligent life. Carl Sagan, one of the enthusiastic pioneers of the search for extraterrestrial intelligence (SETI), argued that the universe is so vast that it would be unthinkable for Earth to be the only planet harbouring sentient beings. Ernst Mayr took a quite different view, saying that we should pay attention to the one example that we have, namely Earth.
In the history of life on Earth there have been several billion species with different degrees of evolutionary success, and it turns out that the success of a species, as determined by the number of individual members that survive, is inversely related to intelligence. The most successful species are those that mutate very quickly like bacteria, or those that have a fixed ecological niche like beetles. These species do very well no matter what happens because they are best at responding to changes in the environment. As we move up the scale of what we call intelligence, survival becomes much more hazardous. Mammals are not doing very well in the evolutionary arena, apart from the few species that we have domesticated, like cows and chickens. Even humans have been very sparse throughout the world until very recently, a matter of a few thousand years, which is meaningless in evolutionary terms (Mayr also pointed out that the average life expectancy of a species is about a hundred thousand years, which is roughly the length of time that modern humans have existed). And despite the great success humans enjoyed in the past few millennia, the prospects for organized human existence in the future, in light of the combined existential threats of nuclear war and climate change, don't look so bright. The problem, as pointed out by many people, is that any advanced civilization will eventually reach a critical point in technological advancement when it will gain the power to end itself. For these reasons, his conclusion was that intelligence is basically a lethal mutation.
Judging from how our species is dealing with the combined threats of global warming and nuclear war, it almost looks like Mayr actually had a point. Should we perhaps adopt Lars Von Trier's apocalyptic view of the human being as inherently evil and celebrate our own demise, one that would surely benefit every other species on Earth? I will not do it. I want to believe there is another way, one in which human beings can coexist with their environment and intelligent life in the cosmos is more than just a passing accident.
The choice is ours.