I’m halfway through Paul Davies’s book The Eerie Silence, about the Search for Extraterrestrial Intelligence (SETI) project and the wider scientific and philosophical issues involved. One of the ways of investigating the probability of extraterrestrial life is to look at the vexed question of the probability of life on earth, and chapter 2 of the book is entitled, “Life: Freak side-show or cosmic imperative?”
Was there a high probability that any life, let alone intelligent life, would develop on earth? The answer is: we haven’t a clue. And that’s because we still have almost no understanding about how life developed on this planet in the first place; and we don’t even know if it started here anyway – it may have started on Mars and migrated on materials that got dispersed into the solar system and then fell to earth.
We simply don’t know how life began. As Charles Darwin said:
We might as well speculate about the origin of matter.
This lack of knowledge isn’t reflected in the ‘cosmic imperative’ mood of the scientific and journalistic moment. Many thinking people, in other words, believe that given the vastness of the universe the emergence of life must be almost inevitable. Alan Boss of the Carnegie Institution in Washington declared in 2009:
If you have a habitable world and let it evolve for a few billion years then inevitably some sort of life will form on it… It would be impossible to stop life growing on these habitable planets… There could be one hundred billion trillion Earth-like planets in space, making it inevitable that extraterrestrial life exists’ [25-26].
The flaw in this probability argument is obvious even to a non-scientist like myself. Boss uses the word ‘evolve': if you let a habitable world ‘evolve’ then life is bound to emerge. That would be true if we had any evidence that a ‘world’ evolves. But we don’t. Life evolves, once it is started – we know that. But we can’t use an assumption about the progress of evolution within life as an argument that life itself, at its beginnings, is the result of a pre-life evolutionary process. We have no idea what such a process might involve, or any evidence that it took place, or any indication of what the probability of it taking place might be.
George Whitesides, Professor of Chemistry at Harvard University, gives the alternative view, which Paul Davies himself accepts. First of all he seems sceptical:
How remarkable is life? The answer is: very. Those of us who deal in networks of chemical reactions know of nothing like it… How could a chemical sludge become a rose, even with billions of years to try? … We (or at least I) do not understand. It is not impossible, but it seems very, very improbable .
But it’s not so much scepticism as a humble awareness of the impossibility of speaking about a high probability of life emerging when we know so little about what would or would not make it probable in the first place.
How likely is it that a newly formed planet, with surface conditions that support liquid water, will give rise to life? We have, at this time, no clue, and no convincing way of estimating. From what we know, the answer falls somewhere between ‘impossibly unlikely’ and ‘absolutely inevitable’. We cannot calculate the odds of the spontaneous emergence of cellular life on a plausible prebiotic earth in any satisfying and convincing way’ .
All we know is that it has happened at least once.