FOR centuries, speculation about the existence of life elsewhere in the universe was the preserve of philosophers and theologians. Then, 50 years ago last month, the question entered the scientific sphere when a young American astronomer named Frank Drake began sweeping the skies with a radio telescope in hopes of picking up a signal from an extraterrestrial civilization. Initially, his quest was considered somewhat eccentric. But now the pendulum of scientific opinion has swung to the point where even a scientist of the stature of Stephen Hawking is speculating that aliens exist in other parts of our galaxy.

The search for extraterrestrial intelligence is predicated on the assumption, widely held today, that life would emerge readily on Earth-like planets. Given that there could be upward of a billion Earth-like planets in our galaxy alone, this assumption suggests that the universe should be teeming with life.

But the notion of life as a cosmic imperative is not backed up by hard evidence. In fact, the mechanism of life’s origin remains shrouded in mystery. So how can we test the idea that the transition from nonlife to life is simple enough to happen repeatedly? The most obvious and straightforward way is to search for a second form of life on Earth. No planet is more Earth-like than Earth itself, so if the path to life is easy, then life should have started up many times over right here.

Searching for alternative life on Earth might seem misconceived, because there is excellent evidence that every kind of life so far studied evolved from a common ancestor that lived billions of years ago. Yet most of the life that exists on Earth has never been properly classified. The vast majority of species are microbes, invisible to the naked eye, and scientists have analyzed only a tiny fraction of them. For all we know, there could be microbes with other ancestral origins living literally under our noses — or even inside our noses — constituting a sort of shadow biosphere, containing life, but not as we know it.