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. The 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 non-life to life as 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 extra-solar planets. Astronomers think there could be billions of earthlike planets in our galaxy alone. Clearly there is no lack of habitable real estate out there. But habitable implies inhabited only if life actually arises.

I am often asked how likely it is that we will find intelligent life beyond Earth. The question is meaningless. Because we don’t know the process that transformed a mish-mash of chemicals into a living cell, with all its staggering complexity, it is impossible to calculate the probability that it will happen. You can’t estimate the odds of an unknown process. Astrobiologists, however, seem more preoccupied with the chances that microbial life will eventually evolve intelligence. Although biologists can’t do the math on that either, at least they understand the process; it is Darwinian evolution. But this is to put the cart before the horse. The biggest uncertainty surrounds the first step—getting the microbes in the first place.

Carl Sagan once remarked that the origin of life can’t be that hard or it would not have popped up so quickly once Earth became hospitable. It’s true that we can trace the presence of life on Earth back 3.5 billion years. But Sagan’s argument ignores the fact that we are a product of the very terrestrial biology being studied. Unless life on Earth had started quickly, humans would not have evolved before the sun became too hot and fried our planet to a crisp. Because of this unavoidable selection bias, we can’t 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’s 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 less 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 sort 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 don’t 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 sort of 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.