According to the dominant theory among biologists, life originated on our planet when peculiar combinations of chemicals—known to form spontaneously under the proper circumstances—came together in just the right way to form cells; from these cells, the theory holds, all subsequent life evolved. Astronomers, writes Ethan Siegel, have meanwhile concluded that “there are there are nearly 1022 potentially habitable, Earth-like planets containing the right conditions and ingredients for life.” It would therefore stand to reason that not only did life evolve on some of these, but on some it evolved into an intelligent form capable of technological advancement. Where, then, are these aliens?
While many scientists are optimistic that it may be easy to create a simplified form of life, we’ve never successfully done so, nor have we witnessed it happening. We have yet to detect any life-form that didn’t originate on Earth. And as far back as we’re capable of tracing it, all life on Earth goes back to a single, universal common ancestor. Life might be common in the universe, but until we detect a second example where life arose from non-life, we cannot know. . . .
When we ask the big question—“Where is everybody?”—it’s worth keeping a great many possibilities in mind. Aliens might be plentiful, but perhaps we’re not listening properly. Aliens might be plentiful, but they might self-destruct too quickly to maintain a technologically advanced state. Aliens might be plentiful, but they may choose to remain isolated. Aliens might be plentiful, but they might purposely choose to exclude Earth and its inhabitants from their communications. Aliens might be plentiful, but the problems of interstellar transmission or travel might be too difficult to overcome.
But there’s another valid possibility that we must keep in mind as well: aliens may not be there at all. The probability of the vital leaps [necessary to produce life, let alone advanced civilizations] is enormously uncertain. If even one of these three steps is too cosmically improbable, it may well be that in all the universe, there’s only us.