The narrow range of conditions required to produce human beings suggests that intelligent life is probably exceedingly rare elsewhere in the universe.
In the past millennium, the human species finally completed its long campaign of exploring planet Earth. Driven by curiosity, hunger, greed, religion, or hostile forces, wanderers stumbled onto virtually every region of the planet. When unknown lands were reached, they were usually already inhabited. Even when barren of human inhabitants, the newly discovered lands often provided environments that were at least marginally suitable for habitation. New immigrants could use their brains, tools, and agriculture to survive in all but the harshest deserts, the driest islands, the bleakest polar regions, and the highest mountains.
Given the heady success of our species, it has seemed natural to imagine that people, or at least comparable beings, would flourish on other worlds in the vast universe. We have expected that as we reach out in space we will find comparable life-forms, or, if not, at least worlds suitable for human habitation. It is widely believed both that advanced life is destined to dominate planets and that it is commonplace throughout the Milky Way and even the universe at large. Thousands of novels, movies, and television shows suggest that pathways leading to advanced life in the cosmos are easy to follow. It is likely, they suggest, that complex life-forms either evolve readily on planets or arrive as colonizing spacefarers from distant worlds. Those holding this view imagine that just as early explorers on Earth encountered native inhabitants, so too shall space explorers encounter native inhabitants on nearby planets.
Our task is to ask if such dreams are likely to be true. Or are they just the wishful thinking of a lucky, confident, and overachieving species?
A special place
Perhaps advanced life is not common. Perhaps Earth is a highly unusual place, and life comparable to humans and even the simplest animals is exceedingly rare elsewhere. In our recent book, Rare Earth: Why Advanced Life Is Uncommon in the Universe (Copernicus Books, 2000), my colleague, paleontologist Peter Ward, and I describe why Earth is likely to be rare. The book outlines the "rare Earth hypothesis," the proposition that microbial life similar to bacteria might be common in planetary systems but that advanced life like animals will be highly uncommon. The intent was to suggest that Earthlike conditions and advanced life-forms are likely to be much rarer than generally believed.
In fact, the cosmic rarity of planets similar to Earth is unknown. Life has not been detected elsewhere, so it is possible that our planet is unique. As far as we know, its advanced life-forms could be alone in the vastness of space. This extreme case seems highly unlikely, given that the universe contains nearly a trillion galaxies, each containing up to a few hundred billion stars. While it seems unlikely that we are alone, it is highly probable that we are rare.
Even if we are not unique, we might never know. The universe has many stars and even more planets, but they are separated by truly vast distances. Rarity can mean permanent isolation. If we are rare enough, we will be forever isolated in our little bubble of space and time. If the neighbors are too far away, then for all practical purposes we will effectively be alone. Unless advanced life is so common that it exists around the few thousand nearby stars, we may never have the opportunity to contact or even detect other intelligent species.
The idea that we might be rare seems preposterous and presumptuous. To assume that humans and planet Earth are unique goes against the "principle of mediocrity," the commonsense expectation that we are more likely to be common than rare. From our everyday experience, it seems logical to expect that we should be fairly common beings living on a fairly common planet under reasonably common environmental circumstances. …