What can be said about the chances of life starting up on a habitable planet? Darwin gave us a powerful explanation of how life on Earth evolved over billions of years, but he would not be drawn out on the question of how life got going in the first place. “One might as well speculate about the origin of matter,” he quipped.Put bluntly, there's no more evidence now than there was in the 1960s that there's life elsewhere in the universe. Claims that there is are based more on wishful thinking than on empirical evidence. Davies continues:
In spite of intensive research, scientists are still very much in the dark about the mechanism that transformed a nonliving chemical soup into a living cell. But without knowing the process that produced life, the odds of its happening can’t be estimated.
When I was a student in the 1960s, the prevailing view among scientists was that life on Earth was a freak phenomenon, the result of a sequence of chemical accidents so rare that they would be unlikely to have happened twice in the observable universe. “Man at last knows he is alone in the unfeeling immensity of the universe, out of which he has emerged only by chance,” wrote the biologist Jacques Monod.
Today the pendulum has swung dramatically, and many distinguished scientists claim that life will almost inevitably arise in Earthlike conditions. Yet this decisive shift in view is based on little more than a hunch, rather than an improved understanding of life’s origin.
The underlying problem is complexity. Even the simplest bacterium is, at the molecular level, staggeringly complex. Although we have no idea of the minimal complexity of a living organism, it is likely to be very high. It could be that some sort of complexifying principle operates in nature, serving to drive a chaotic mix of chemicals on a fast track to a primitive microbe. If so, no hint of such a principle has been found in laboratory experiments to re-create the basic building blocks of life.This is, of course, the big obstacle facing any naturalistic account of the origin of life. No one has been able to come with a plausible explanation as to how the enormous complexity of a living cell could ever have arisen through purely mechanistic processes.
On the other hand, if life arose simply by the accumulation of many specific chemical accidents in one place, it is easy to imagine that only one in, say, a trillion trillion habitable planets would ever host such a dream run. Set against a number that big — and once you decide a series of unlikely accidents is behind the creation of life, you get enormous odds very easily — it is irrelevant whether the Milky Way contains 40 billion habitable planets or just a handful. Forty billion makes hardly a dent in a trillion trillion.So why are scientists striving so earnestly to find evidence of life on planets elsewhere in the galaxy? Why do they keep reassuring each other that it just has to be out there? Perhaps it's because they need to find life elsewhere to bolster their faith in metaphysical naturalism. The likelihood of life emerging on earth solely through chance and chemistry is deemed so low that many people believe that something more than chance and chemistry, an intelligent agent, must also have been at work. But if it can be shown that life does occur throughout the galaxy it becomes much easier to think that it's not so improbable after all and that no intelligent agent is necessary to account for it.
In other words, the search for life in the galaxy is spurred by much the same sort of motivations that drive people to search for Noah's ark. Just as the ark's discovery would provide strong confirmation of the truth of the Old Testament narrative, the discovery of alien life would provide strong confirmation of the truth of the naturalist narrative. I wonder if it's not a sign of a deep-seated insecurity surrounding the truth of the narrative that compels people to seek such confirmations.