Statisticians say intelligent life on other planets is ‘exceptionally rare’, and that human-like civilisations are unlikely to exist away from Earth.
In a new paper, Oxford researchers theorise that, for life to have evolved the same way elsewhere in the universe, it would have taken longer than the whole of Earth’s projected lifespan.
Evolution on Earth from the Big Bang up until the current day has involved a series of what they call ‘evolutionary transitions’ that were helped by chance.
These include the emergence of primitive life from non-living matter (known as abiogenesis) and eukaryotic life (with cells that have a nucleus enclosed), the evolution of sexual reproduction, multicellularity, and intelligence itself.
If intelligent life does exist on other planets, it would have needed to go through comparable evolutionary transitions – and to say that life does exist on other planets carries a burden of proof that science has not yet provided.
Intelligent life in the Universe is exceptionally rare, assuming that intelligent life elsewhere requires analogous evolutionary transitions, scientist say. Pictured, an artists’s impression of an Earth-like planet in another galaxy
TRANSITIONS THAT LED TO LIFE ON EARTH
Abiogenesis – the the emergence of primitive life from non-living matter.
Eukaryogenesis – emergence of eukaryotic cells (those with cells that have a nucleus enclosed).
Multicellularity – organisms made up of more than one cell.
Evolution of sexual reproduction
Emergence of cooperation and societies
The research draws on the Fermi paradox, which asks why scientists are predicting so many extraterrestrial civilisations, but as yet are still to find evidence for them.
‘It took approximately 4.5 billion years for a series of evolutionary transitions resulting in intelligent life to unfold on Earth,’ the experts, from Oxford University’s Future of Humanity Institute, say in their paper.
‘In another billion years, the increasing luminosity of the Sun will make Earth uninhabitable for complex life.
‘Together with the dispersed timing of key evolutionary transitions and plausible priors, one can conclude that the expected transition times likely exceed the lifetime of Earth, perhaps by many orders of magnitude.
‘In turn, this suggests that intelligent life is likely to be exceptionally rare.’
The researchers used a special statistical technique called a ‘Bayesian analysis’ to determine the probability of events in Earth’s history – a ‘chain of multiple evolutionary transitions’ – happening elsewhere.
‘Our methods were basically statistics,’ Dr Anders Sandberg at the Future of Humanity Institute told MailOnline.
Eukaryotic life – with cells that have a nucleus enclosed – took over a billion years to emerge from prokaryotic precursors. Pictured, cell of a eukaryote
‘We made use of the assumption that what happened on Earth is typical for what happens on other planets – not the exact times, but that there are some tricky steps life needs to get through in sequence to produce intelligent observers.’
For example, eukaryotes – organisms with a nucleus – needed more than a billion years ago to emerge from their nucleus-less prokaryotic predecessors.
This was a far less probable event than the development of multicellular life, which is thought to have originated independently over 40 times.
The fact that some transitions occurred only once in Earth’s history suggests a remarkable stroke of luck that resulted in intelligent Earthlings today.
Also, only a few planets that have ‘very lucky’ biospheres manage to get through the steps to intelligent observers before their sun becomes a red giant – a dying star in the last stages of stellar evolution.
The team quote American evolutionary biologist Stephen Jay Gould, who said that if the ‘tape of life’ were to be rerun, ‘the chance becomes vanishingly small that anything like human intelligence’ would occur.
‘What we added was a statistical approach that allows us to get estimates of just how unlikely the steps could be,’ Dr Sandberg told MailOnline.
‘We feed in data about when things happened on Earth and a guess of how many steps there were, and in return we get the most likely levels of difficulty.
‘[These] turn out to indicate that, yes, we are an unlikely planet.’
Arriving at the opposite conclusion – that life in the universe isn’t rare at all – would require evidence for much earlier transitions than the ones that occurred on Earth or multiple instances of transitions.
The classic version of this argument stems from the work of Australian theoretical physicist Brandon Carter, who sought to explain why intelligent life emerged so late in Earth’s history.
It is unknown how abundant extraterrestrial life is, or whether such life might be complex or intelligent
Earth is 4.5 billion years old and in another billion years, the increasing luminosity of the Sun will likely destroy Earth’s ability to support complex life, due to increased surface temperatures.
But humans have only existed on Earth for about the last 6 million years.
‘[Carter] pointed out that there is no reason to think the tricky steps on average may take much more time than planets remain habitable,’ Dr Sandberg said.
‘There might well be entirely different kinds of life and minds but they are likely as hard (or harder) to evolve than us.’
Dr Sandberg said that the study fits into what one could call ‘armchair astrobiology’ as it deals with probabilities.
‘Just because we got our results doesn’t mean it is a waste of time to look at the actual universe,’ he said.
‘Data will always trump ever so careful reasoning and statistics.’
WHAT IS THE FERMI PARADOX?
The Fermi Paradox questions why, given the estimated 200bn-400bn stars and at least 100bn planets in our galaxy, there have been no signs of alien life.
The contradiction is named after its creator, Italian physicist Enrico Fermi.
He first posed the question back in 1950.
Fermi believed it was too extraordinary that a single extraterrestrial signal or engineering project has yet to be detected in the universe — despite its immense vastness.
Fermi concluded there must a barrier that limits the rise of intelligent, self-aware, technologically advanced space-colonising civilisations.
This barrier is sometimes referred to as the ‘Great Filter’.
Italian physicist Enrico Fermi devised the so-called Fermi Paradox in the 1950s. It explores why there is no sign of alien life, despite the 100 billions planets in our galaxy
If the main obstacle preventing the colonisation of other planets is not in our past, then the barrier that will stop humanity’s prospects of reaching other worlds must lie in our future, scientists have theorised.
Professor Brian Cox believes the advances in science and engineering required by a civilisation to start conquering the stars ultimately lead to its destruction.
He said: ‘One solution to the Fermi paradox is that it is not possible to run a world that has the power to destroy itself and that needs global collaborative solutions to prevent that.
‘It may be that the growth of science and engineering inevitably outstrips the development of political expertise, leading to disaster.’
Other possible explanations for the Fermi Paradox include that no other intelligent species have arisen in the universe, intelligent alien species are out there — but lack the necessary technology to communicate with Earth.
Some believe that the distances between intelligent civilsations are too great to allow any kind of two-way communication.
If two worlds are separated by several thousand light-years, it’s possible that one or both civilisation will be extinct before a dialogue can be established.
The so-called Zoo hypothesis claims intelligent alien life is out there, but deliberately avoids any contact with life on Earth to allow its natural evolution.