Scientists 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 somewhere else, 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.
But if intelligent life on other planets exists, it would have needed comparable evolutionary transitions to exist – 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
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.’
KEY DISCOVERIES IN HUMANITY’S SEARCH FOR ALIEN LIFE
Discovery of pulsars
British astronomer Dame Jocelyn Bell Burnell was the first person to discover a pulsar in 1967 when she spotted a radio pulsar.
Since then other types of pulsars that emit x-rays and gamma rays have also been spotted.
Pulsars are essentially rotating, highly magnatised neutron stars but when they were first discovered it was believed they could come from aliens.
‘Wow!’ radio signal
In 1977, an astronomer looking for alien life in the nigh sky above Ohio spotted a powerful radio signal so strong that he excitedly wrote ‘Wow!’ next to his data.
In 1977, an astronomer looking for alien life in the nigh sky above Ohio spotted a powerful radio signal so strong that he excitedly wrote ‘Wow!’ next to his data
The 72-second blast, spotted by Dr Jerry Ehman through a radio telescope, came from Sagittarius but matched no known celestial object.
Conspiracy theorists have since claimed that the ‘Wow! signal’, which was 30 times stronger than background radiation, was a message from intelligent extraterrestrials.
Fossilised martian microbes
In 1996 Nasa and the White House made the explosive announcement that the rock contained traces of Martian bugs.
The meteorite, catalogued as Allen Hills (ALH) 84001, crashed onto the frozen wastes of Antarctica 13,000 years ago and was recovered in 1984.
Photographs were released showing elongated segmented objects that appeared strikingly lifelike.
Photographs were released showing elongated segmented objects that appeared strikingly lifelike (pictured)
However, the excitement did not last long. Other scientists questioned whether the meteorite samples were contaminated.
They also argued that heat generated when the rock was blasted into space may have created mineral structures that could be mistaken for microfossils.
Behaviour of Tabby’s Star in 2005
The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astonomers since being discovered in 2015.
It dims at a much faster rate than other stars, which some experts have suggested is a sign of aliens harnessing the energy of a star.
The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astonomers since being discovered in 2015 (artist’s impression)
Recent studies have ‘eliminated the possibility of an alien megastructure’, and instead, suggests that a ring of dust could be causing the strange signals.
Exoplanets in the Goldilocks zone in 2015
In February this year astronomers announced they had spotted a star system with planets that could support life just 39 light years away.
Seven Earth-like planets were discovered orbiting nearby dwarf star ‘Trappist-1’, and all of them could have water at their surface, one of the key components of life.
Three of the planets have such good conditions, that scientists say life may have already evolved on them.
Researchers claim that they will know whether or not there is life on any of the planets within a decade, and said ‘this is just the beginning.’