We live, Douglas Adams once observed, in the uncharted backwaters of the unfashionable end of the western spiral arm of the galaxy where “our utterly insignificant little blue-green planet” orbits a small unregarded yellow sun. As a summation of the peripheral importance of Homo sapiens, these words – from The Hitchhiker’s Guide to the Galaxy – could not be bettered.The inner nerd is thrilled. An article opening with a quote from Douglas Adams! But the Hitchhiker's Guide to the Universe is filled with alien species, some of them are actually intelligent. Where are they?
This notion is more seriously known as the principle of mediocrity and it stems directly from the ideas of Copernicus, who replaced our old self-important, geocentric view of the universe with one that makes our planet a mere adjunct of the sun. Thus humanity’s place in the heavens was displaced, making us neither central nor special.The problem with the "principle of mediocrity" is that is only true if there is intelligent life on other planets. Otherwise, earth is anything but mediocre. And right now, it looks unique.
If our circumstances are unremarkable, they are – by definition – going to be repeated round the universe, particularly when it comes to the prevalence of life. For if living beings are common on our boringly typical planet, then the heavens should also abound with life.They may "abound with life," but not apparently intelligent life.
Astronomers have come up with a wide range of explanations to account for this absence. Some say intelligent creatures may be common but will quickly destroy their worlds when they develop technology that will inevitably run out of control. Hence we never spot them. Others say complex life is actually extraordinarily rare because our planet is just about the only one in the galaxy that could support it. Only our special orbit round a special star with a special single moon above our heads has created the incredibly unlikely circumstances for the appearance of life.