The nature of life is one of the most fascinating but challenging issues in science. In fact, it’s been that way since the start of recorded human history, before we even thought of ‘science’ as a distinct pursuit among human intellectual pursuits. (Which it arguably isn’t or shouldn’t be. Except that our past four centuries having been dominated by a specific dissociation of science from natural philosophy, boxing it into the enterprises of nation-states and industrialization. Phew, there, I said it.) 

Today we still don’t have a complete, agreed upon, picture of how life is initiated; how living systems might originate in an environment. Nor do we have a complete picture of how life and a planetary environment become as intimately entwined as they are on the Earth. Perhaps the most honest - and hugely unsatisfying - answer to both puzzles at the moment is that these things clearly do happen, and must happen for Earth to be the way it is.

Nonetheless, we can say with some certainty that life is when a system successfully propagates versions of itself into the future, by whatever means work in the face of a complex and variable environment (that itself might include other living systems). And when we say ‘work’ we mean ‘work well enough’. There is a bit of a misconception that can infect our thinking, one that says that organisms can be ‘perfect’ in the sense of adaptation to their environments. You’ll have heard phrases like this in breathless nature documentaries or rushed science reporting – where it is hinted that a species is a miraculous piece of finely-tuned engineering or intuitive genius. Such-and-such a characteristic is perfectly tuned or balanced, or the animal knows when and where to do something because this will ensure the survival of its species.

No, not really. Behind the curtains of Darwinian selection is the simple fact that what we see of organisms merely reflects properties that combine to enable those organisms to propagate themselves (meaning their genetic material) into the future with a finite probability. If these properties were not the way they are the propagation might or might not fail. There can be, and often is, enormous attrition within generations or across generations. This is, literally, a numbers game.

Which brings us to what I think are some of the most critical unanswered questions in our efforts to characterize places in the cosmos where life might be. 

While we often talk about the ‘habitability’ of environments, like rocky exoplanets, that term is woefully inadequate. It is a crude conflation of ideas, that while convenient in a very broad way, may be hindering our ability to really tackle the more fundamental questions.

For instance, when we evaluate whether a particular world might harbor life we should really be looking at a layering of probabilistic phenomena. First might be the root frequency (the rate) at which ‘origin events’ (abiogenesis) would be expected on a planet as a function of time. Second might be the frequency, or probability with which a given origin event continues into the future – again as a function of time or planetary age. In other words, the capacity of a planet to sustain life is potentially a very different issue to that of the capacity to initiate life. 

It’s very hard to know whether these two phenomena are going to be correlated. Maybe some places are great at constantly triggering origin events but lousy at maintaining them for eons, or vice versa. These two probabilities could be wildly different.

The circumstances of the Earth tend to bias us to what could be an extreme viewpoint. Life on this planet appears to have an unbroken lineage going back at least 3.5 billion years. It’s tempting to see this as a yardstick for what a habitable planet ‘should do’. But maybe it isn’t. Maybe a good run for life on an average planet is only a few million years, or even less. We simply don’t know.

I’m not sure there’s a reason to imagine that the history of life plays out in any very consistent way across the cosmos. Even if the underlying molecular mechanisms are indeed universal and the energetic requirements from a planet (geochemical and climatological) are what we suspect. Evolution does not care. If something works well enough, it will be there in the future. If something doesn’t work well enough it will be filtered out. Life is not engaged in a ‘struggle’. That’s a convenient and misleading anthropomorphism. If things don’t work well enough life simply goes away.

What does this mean for our efforts to find other biologically active planets in the universe? In some ways it doesn’t impact those searches, or indeed the ways in which we attempt to gauge the suitability of other worlds for harboring life. We are stuck using our template of the Earth and what we think happens to planetary conditions in varying circumstances – based on very limited data from our history and from the solar system. Until we get much more detailed information about rocky exoplanets most bets are off, and we need these crude templates to make tough choices about which worlds to study. 

But our ignorance about the nature of life is going to become a critical issue when we do eventually learn more about these other worlds. We may not know their ages well (stellar and system ages can be horribly poorly constrained), and we will still not know what the root probabilities are for life’s origins and life’s persistence. Nor will we know how many worlds have had or will have life sprawling across them – if we’re lucky we will simply see the worlds that are at present ‘good enough’.