This post is one in a series covering, and expanding on, topics in the book The Copernicus Complex (Scientific American/FSG).
The conversation usually goes like this:
Do you think we're alone in the universe?
Answer A) :
No, absolutely not. It's a huge universe, we're not at the center or central in any way and it would be the height of vanity to think humans or Earth are in any way special or significant.
We might be. There's never been any firm evidence of extraterrestrial life, and our galaxy is old enough that intelligent civilizations should have spread everywhere by now.
I've lost track of how many times I've had this exchange, with scientists, with family, with random people at pubs and (once) on a heavily forested hillside in Norway with a pair of slightly suspicious looking hikers who had probably just hidden a body somewhere. But the fascinating thing is how we tend to fall into either camp A or camp B, and how strongly we feel about our answers.
There are some caveats. The 'we' in the original question is a loaded word. It can be taken to mean 'technologically intelligent life' (i.e. as modern humans like to think they are), or it can be taken to mean 'slime' - the single-celled microbial life that represents, and has always represented, the bulk of living matter on Earth. If the definition of 'we' gets refined using these categories the answers can change a bit. In fact both A and B responses tend to converge to a tepid middle-ground, along the lines of saying that there could be lots of microbial-type life in the cosmos, where it sits slime-like under a favorite rock rather than building pan-galactic empires, while more complex life is either very rare (as in the so-called Rare Earth hypothesis) or never makes it very far into interstellar space (part of an idea called the Great Filter).
This is an awfully unsatisfactory state of affairs, a set of answers that are enormously influenced by our interpretation of events here on Earth. The impasse would be broken if we could detect life with an independent origin elsewhere - either in the solar system or farther beyond - yet that's a challenge that remains unmet.
It may not stay that way for much longer, between our exploration of Mars and our ambitions for exploring places like comets and icy moons, we really do seem to be getting closer to examining local possibilities for life. And with a stunning array of exoplanet detections, and near-term cataloging of all the best neighboring targets with upcoming missions like TESS, we should be able to apply the next generation of space and ground based 'super observatories' to make crude measurements of the properties of a few potential Earth-analogs. But this is an optimistic overview. In all of these examples, the non-detection of life (whether as fossils or as chemical signatures) is unlikely to eliminate the possibility of life in these places - we simply won't be able to be that thorough.
So, unfortunately, in ten years time we're quite likely to still be having the A vs B conversation. On the face of it the compromise solution - that microbial-type life may be common, but life like us isn't - seems like a decent answer. However, there's a catch.
Most of this argument hinges on the idea that Earth's complex-celled, multi-cellular life (everything from nematodes to sheep) only exists because of a sequence of very specific, but low probability, events - including the way and location in which the Earth formed (with water, with plate tectonics), to the presence of a large moon (keeping our spin axis from varying too much), to a chance merger of two equally simple, single-celled, organisms giving rise to complex, eukaryotic life 2 billion years ago. Thus the odds of a planet making creatures like us is vanishingly unlikely, and so this simply can't have happened in many other places, even in a universe of hundreds of billions of galaxies and nearly 14 billion years old.
Except this is a very specific interpretation of the facts, after the events (a post-hoc analysis). To use an analogy, imagine that you're woken up one morning by the telephone ringing. You answer and it's a distant cousin just calling to let you know their new number. Later, as you walk to work, a bus honks at traffic and you glance up to see some of the digits of that phone number on its side. At lunchtime a torn piece of newspaper gets stuck on your foot, with the headline that the national lottery has a record prize draw. Back at work a colleague insists that you participate in a meeting where the word 'prize' gets used extensively. On your way home you stop at a newspaper stand and decide to buy a lottery ticket. The next morning you discover that you've won the huge jackpot!
What do you make of this? Your natural instinct is to look back at the events of the previous day, marveling at how, step-by-step, you were led to this point by a series of unlikely events. Taken altogether, you reason, this was incredibly improbable, it's as if the cosmos has singled you out to win!
Yet this isn't true at all. Someone, somewhere, was going to win the lottery draw. And whoever they were, in whatever circumstances, they would be having the same thoughts. The events of the previous day, week, or year would all take on new meaning in light of the outcome. There would be numbers that they'd seen, choices made, steps taken, random occurrences that appeared to lead up to this point.
Of course some of these events were necessary, but they would also be entirely different if another person had won. The point being that it would be hugely irrational to claim that the chain of events that led to you winning was the only way this could have happened.
All of which circles back to the idea of the 'rarity' of life's trajectory here on Earth. The fact is that our perspective is not unlike the lottery winner's. It's easy to look back at 4 billion years of bio-geo-chemical evolution and say that organisms like us were a highly improbable outcome. But in truth we don't know whether or not that's actually true, and we can't know because we have no information about how things have played out across billions of other worlds across the galaxy. In a sense, we have no idea how many other winning lottery tickets are out there in the cosmos, or how they were picked.
And that raises a new question (one that I will tackle in a later post, Part 2), about why we have no idea, and how complex, even technological, life elsewhere could have remained unseen to this point...