From Scientific American:
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, …