Was life on Earth a fluke? Perhaps not. New research suggests that the emergence of life on Earth, while far from a guaranteed outcome, was in fact a probable event – although the odds of intelligent life evolving remain rather less favourable.
In a new study, astronomer David Kipping from Columbia University examines these likelihoods as a way of loosely exploring the timeless question of whether humanity might be alone in the Universe.
Despite all our technological advancements and scientific prowess, we haven't yet found any evidence of life beyond Earth, let alone signs of advanced extra-terrestrial civilisations resembling our own, human breed of intelligent life.
Of course, a lot of this conjecture is based around a particular assumption: that it makes sense for life (and intelligent life) to exist elsewhere, given the mind-boggling number of habitable worlds estimated to exist out there in the cosmos.
Given that excess of exoplanets – and given how we emerged here on Earth – then surely other life-forms will similarly spring up on other orbs too, right?
Well, maybe not. At least, we don't yet have any real data to know one way or the other.
The only firm evidence we have that life exists anywhere, in fact, is Earth, and even life on Earth may not have been destined to thrive, let alone evolve to the point where you can read these words.
"Despite having no observational data concerning non-terrestrial life, we are in possession of stronger constraints when it comes to life on Earth," Kipping writes in his paper.
"Until this situation changes, inferences concerning the existence of life elsewhere in the Universe must unfortunately rely heavily on this single data point."
In his new work, Kipping examines what the chances of life and intelligence emerging on Earth might have been, using a statistical method called Bayesian inference. Put very simply, this is a type of statistics that uses probability to take into account subsequent information - giving it an advantage over comparing strict sets of figures.
It's not the first time researchers have employed this technique as a means of theoretically quantifying the odds of life arising on other, Earth-like planets, but Kipping made some changes to the formula.
In the study, Kipping ran calculations based on the evidence that while life is thought to have emerged quickly in Earth's planetary history, intelligent life only recently showed up – roughly 4 billion years later.
With this in hand – and some elaborate formulae – Kipping weighed up a number of evolutionary possibilities for life on Earth: life is common and often develops intelligence; life is common but rarely develops intelligence; life is rare but often develops intelligence; life is rare and rarely develops intelligence.
Looking at all these potential outcomes, Kipping says the case for life taking hold on Earth quickly is strong.
"On this basis, we can conclude that even with the most conservative date for the emergence of life, a scenario where abiogenesis occurs rapidly is at least three times more likely than a slow emergence," Kipping explains, noting that if you accept the earliest purported evidence for the beginning of life on Earth, the odds become even greater.
"If the more ambiguous evidence for an earlier start to life is confirmed, then this would increase the odds to a factor of nine, representing relatively strong preference for a model where life would consistently emerge rapidly on Earth, if time were replayed."
If Kipping is right, statistically speaking, the conditions on Earth were well-suited for life to spring up like it did – but that doesn't mean intelligent life was necessarily assured its place afterwards.
"The possibility that intelligence is extremely rare and Earth 'lucked out' remains quite viable," Kipping writes.
"Overall, we find a weak preference, 3:2 betting odds, that intelligence rarely emerges given our late arrival."
While that means the odds may have been against intelligent life evolving on Earth, as the researcher acknowledges, it's not too far off a 50:50 chance, and in any case, all of this is entirely theoretical.
Can we apply these odds to the search for life beyond Earth? Yes and no, Kipping explains, emphasising that the "analysis purely concerns the Earth, treating abiogenesis as a stochastic process against a backdrop of events and conditions which might be plausibly unique to Earth".
That said, if another planet just happened to be virtually identical to Earth in terms of its planetary conditions and evolution, then maybe – just maybe – we can now fathom how probable life on that strange, familiar world might be.
The findings are reported in PNAS.