When I was a student in the 1960s, almost all scientists believed we are alone in the universe. The search for intelligent life beyond Earth was ridiculed; one might as well have professed an interest in looking for fairies. The focus of skepticism concerned the origin of life, which was widely assumed to have been a chemical fluke of such incredibly low probability it would never have happened twice. “The origin of life appears at the moment to be almost a miracle” was the way Francis Crick described it, “so many are the conditions which would have had to have been satisfied to get it going.” Jacques Monod concurred; in his 1976 book Chance and Necessity he wrote, “Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance.”

Today the pendulum has swung decisively the other way. Many distinguished scientists proclaim that the universe is teeming with life, at least some of it intelligent. Biologist Christian de Duve went so far as to call life “a cosmic imperative.” Yet the science has hardly changed. We are almost as much in the dark today about the pathway from nonlife to life as Charles Darwin was when he wrote, “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.”

There is no doubt that SETI—the search for extraterrestrial intelligence—has received a huge fillip from the recent discovery of hundreds of extrasolar planets. Astronomers think there could be billions of Earth-like planets in our galaxy alone. Clearly, there is no lack of habitable real estate out there. Yet because we do not know the process that transformed a mishmash of chemicals into a living cell, with all its staggering complexity, it is impossible to calculate the probability that life has actually arisen on these planets.

Carl Sagan once remarked that the origin of life cannot be that hard, or it would not have popped up so quickly once Earth became hospitable. It is true that we can trace the presence of life on Earth back 3.5 billion years. But we cannot draw any statistical significance from a sample of one.

Another common argument is that the universe is so vast, there just has to be life out there somewhere. But what does that statement mean? If we restrict attention to the observable universe, there are probably 1023 planets. Yes, that is a big number. But it is dwarfed by the odds against forming even simple organic molecules by random chance alone. If the pathway from chemistry to biology is long and complicated, it may well be that fewer than one in a trillion trillion planets ever spawns life.

Affirmations that life is widespread are founded on a tacit assumption that biology is not the upshot of random chemical reactions but the product of some kind of directional self-organization that favors the living state over others—a sort of life principle at work in nature. There may be such a principle, but if so we have found no evidence for it yet.

Maybe we do not need to look far. If life really does pop up readily, as Sagan suggested, then it should have started many times on our home planet. If there were multiple origins of life on Earth, the microbial descendants of another genesis could be all around us, forming a possible shadow biosphere. Nobody has seriously looked under our noses for life as we do not know it. It would take the discovery of just a single “alien” microbe to settle the matter.