Life
Photo by heatherwalker / Getty Images.
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’.
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