Odds of life
A NEW STUDY HAS PROVIDED SOME INTRIGUING ODDS ON THE EMERGENCE OF LIFE BASED ON EARTH’S OWN EVOLUTION
A new study has provided some intriguing odds on the emergence of life based on Earth’s own evolution
What is the likelihood that intelligent life could arise on another Earth-like planet under similar circumstances? This thought experiment offers the opportunity to imagine if the clock went back to zero on Earth – or an identical twin of our home planet – and deduce what the odds are that an intelligent species would evolve. This recent study was conducted by Dr David Kipping of Columbia University, New York, and it utilises a statistical tool known as Bayesian inference.
There have been many astronomers throughout time that have tried to find any indication of whether we are the only intelligent life to exist throughout the universe or whether there are other civilisations and species out there. Frank Drake’s equation, devised in 1961, is one famous example of how astronomers have tried to calculate the number of possible intelligent extraterrestrial life forms mathematically, but it also demonstrates how many different parameters it depends on and how little is known about each parameter.
The statistical model used by Kipping allows some leeway for interpretation, but at the same time relies upon some very precise and key points. In a nutshell, Bayesian analysis combines prior beliefs and information to inform the probability of certain outcomes. However, an interesting aspect of this model is that it can change when new data becomes available. This is not an undisputed fact about life and intelligence coming into fruition elsewhere in the cosmos, but instead an important part of the conversation that will be updated and referred back to as time goes on.
“The technique is akin to betting odds,” explains Kipping. “It encourages the repeated testing of new evidence against your position – in essence a positive feedback loop of refining your estimates of the likelihood of an event.”
This thought experiment essentially repeats the Earth’s evolution after the planet’s formation, trying to figure out how likely it is for life and intelligence to arise again. For this to happen, Earth’s history – as we currently understand it, at least – needs to be taken into account.
Earth has a defined habitable window that started approximately 4.2 billion years ago after a collision with a Mars-sized protoplanet called Theia, which formed our Moon. It took about 300 million years for Earth’s oceans and atmosphere to return after the collision. Although it’s impossible to look into the future, astronomers have calculated that in 900 million years, the Sun’s luminosity will start to increase, ending Earth’s habitability window and preventing the evolution of simple microbial life. In between these events, life began from chemical processes within inanimate organisms, known as abiogenesis, and intelligence slowly followed.
There has been much discussion about when life first began on Earth, and there are a few guesses. The strongest evidence, given by microfossils in the Earth’s rock, show life was around 745 million years
“THERE HAVE BEEN MANY ASTRONOMERS THAT HAVE TRIED TO FIND ANY INDICATION OF WHETHER WE ARE THE ONLY INTELLIGENT LIFE TO EXIST”
after the planet became habitable, so 3.5 billion years ago.
The other, more disputed version of when abiogenesis arose on Earth is based on carbon-13depleted zircon deposits found in Western Australia. This evidence says life was present just 300 million years after the planet first became habitable, so 3.9 billion years ago.
The next data point, when intelligent life arrived, is a bit more ambiguous. This is more of a prior assumption: one person may think that intelligence first reared its head when the first cities were built, while someone else could argue that it was when the first early hominids used fire. This parameter is a bit more subjective. Either way the difference in time is only a few hundred million years or so, and therefore it is relatively negligible when considering the timeframe of multiple billions of years. In the case of this study, signs of intelligent life arrived very recently, and very late on in the habitable window, approximately 4.1 billion years after the first signs of life.
At this point, Kipping narrows down the probabilities into four possible scenarios: life is common and so is intelligence, life is common but intelligence is rare, life is rare but intelligence is common or the lonely universe scenario, which says life is rare and so is intelligence.
At this point it is best to think of the universe as a corner of a huge cosmic casino. There’s a stellar game of blackjack going on in one corner and dark matter slots over in another. But in this game Kipping has been trying to figure out the odds of intelligent life emerging again on Earth-like planets using Bayesian statistical inference, which has interesting implications for long-sought-after alien life elsewhere in the universe.
From his complex calculations – based on an assumption that intelligence is rare and using the more disputed carbon-13-depleted zircon evidence – life is nine times more likely to arrive in a rapid fashion as opposed to being a rare and slow process. When Kipping applied the calculations to the microfossil evidence, the odds were still in favour of life being common, but by a factor of three. Either way it is clear that life is more likely to reemerge, given the chance, than to not.
Intelligence is a different question though. As previously mentioned, it seems as if the gap between life arriving and intelligence following suit is vast. There are many factors that can help or hinder it, and to take all of it into account is simply impossible. But with the same information and prior assumptions thrown into the Bayesian formula, Kipping has arrived at betting odds of
3:2 that intelligence is rare compared to it being common. This is not as favourable.
“THE SEARCH FOR INTELLIGENT LIFE IN WORLDS BEYOND EARTH SHOULD BE BY NO MEANS DISCOURAGED”
David Kipping
Based on this statistical model, Kipping recommends you put your chips on the scenario that abiogenesis is common and that its genesis can occur over a relatively short period of time – but intelligent life is not as lucky. However, Kipping does indicate that these are not overwhelming odds. In fact, it is remarkably close to being 50:50. This does not mean that half the amount of intelligent life would emerge, but if you had ten identical Earth-like planets, intelligent life would emerge on five of them.
Although this is an Earth-specific scenario, it does have implications about life arising on other planets. “The analysis can’t provide certainties or guarantees, only statistical probabilities based on what happened here on Earth,” Kipping says. “Yet encouragingly, the case for a universe teeming with life emerges as the favoured bet. The search for intelligent life in worlds beyond Earth should be by no means discouraged.”
With astrobiology and exoplanet research gaining more momentum as technologies improve, space exploration missions become more ambitious and assumptions change, this model can be adjusted and the odds could improve. There is also more work to be done understanding technosignatures and the lifetime of civilisations in other planetary systems. Once again this points to the fact that more parameters must be scrutinised in order to hazard a clearer mathematical guess at alien intelligence being elsewhere in the universe.