Where did life on Earth begin? That sounds like a relatively straightforward question. Surely on Earth began... well, on Earth, right? We know this planet had the right conditions for life to arise, and we can see evidence stretching back more than 4 billion years, almost to its formation.
An origin on the planet seems like a simple answer. Yet for years, some scientists have favoured an idea that life didn't start on Earth, but elsewhere in the cosmos.
In this scenario, life (or at least the ingredients for it) would have been delivered to Earth on comets or asteroids, seeding the planet with inhabitants – a process called panspermia.
If it occurred, it could – and arguably should – have taken place on other planets too, perhaps suggesting alien life is common in the Universe, and maybe even hinting that we ourselves were originally aliens to the planet we now call home.
That idea is gaining traction thanks to a pair of missions – NASA’s OSIRIS-REx and Japan’s Hayabusa2 – that, in the past few years, have returned rocks from asteroids back to Earth.
Analysis of those samples suggests that some of the building blocks of life are present in the asteroids, raising the prospect that those same building blocks, and perhaps even life itself, could have been delivered to Earth.
It’s “absolutely” plausible, says Dr Jason Dworkin, project scientist on the OSIRIS-REx mission at NASA’s Goddard Space Flight Center in Maryland. “Early Earth had a bombardment of material.”
When such an idea was first proposed decades ago, “everybody laughed,” says Prof Paul Davies, a theoretical physicist and astrobiologist at Arizona State University. “Now we can look back and say they contained a germ of truth. They weren’t totally crazy.”
So where are we right now with the idea and what does the future hold?
A book suggests life began beyond Earth
The origins of panspermia date back many years, but it was popularised in the 1970s by the British astronomers Sir Fred Hoyle and Prof Chandra Wickramasinghe in their book Diseases from Space.
In it, they suggested that certain diseases, such as influenza, could have originated from beyond Earth.
The book wasn’t well-received. “It sort of pushed Hoyle off the edge of the scientific community,” says Davies. “He was widely regarded as having become crazy in his old age.”
One of their ideas was that asteroids and comets could have been incubators for life, being full of organic material that could have been life’s building blocks.
As the objects orbited a star, they would evaporate, releasing the material held inside, which could seed planets in a solar system, such as our own. Such an idea remains controversial today.
"I'm still very sceptical about that particular scenario," says Davies.
The bold idea gets presidential approval
However, the possibility that asteroids and comets could contain the building blocks of life (and perhaps even primitive microbial life) and directly delivered to planets became more alluring.
The idea reached a fascinating peak in 1996 when scientists said they had found traces of microfossils of bacteria inside a meteorite from Mars that had landed in Antarctica a decade prior.
Known as Allan Hills 84001, the meteorite sparked wild claims that alien life had originated on Mars and could have been transported to other planets, such as Earth.
Such was the clamour around the rock that it prompted President Bill Clinton to make an announcement from the White House Rose Garden on the significance of the discovery.
“Today, rock 84001 speaks to us across all those billions of years and millions of miles,” he said. “It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into the Universe that science has ever uncovered.”
The signs of life in the meteorite were later refuted and today it’s thought that non-biological processes led to the shapes inside the rock being misidentified as microfossils.
Nonetheless, the announcement had a wide-ranging impact at the time, particularly for Davies, who, in January 1996, had suggested at a conference in London that life could be transported between Earth and Mars or vice versa.
“There were very few people prepared to take what I was saying seriously – I was even denounced at the conference banquet,” he says. “And then I was rescued by none other than Bill Clinton.”
Mars actually could have been a good place for life in the Solar System to begin, says Davies, because it’s smaller than Earth and would have cooled quicker following its formation 4.5 billion years ago. “So it was ready for life sooner,” he says.
Today, we can see evidence that Mars was warm and wet, with lakes, seas and rivers on its surface about 3–4 billion years ago, but the possibility that life was present there remains an unanswered question.
The fact that we have found Martian meteorites on Earth, though, shows that material can be transferred between the planets by impact events.
Could it be that life actually did start on Mars, or elsewhere, before being transferred to Earth?
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How life could be delivered from other planets
In our Solar System, the distance between the planets is quite large. That makes the chances of material being blasted off one world by an impact and drifting to another relatively slim.
On Earth, of the 75,000 or so known meteorites, only about 300 are confirmed to have come from Mars. No meteorites from Venus have been found on Earth, nor from Mercury.
The possibility of transferring material between worlds is more promising in other solar systems, however. In 2017, planetary scientist Prof Fred Ciesla at the University of Chicago and colleagues suggested that for one system in particular, called TRAPPIST-1, the idea was particularly encouraging.
In this system, which lies about 40 light-years from Earth, seven Earth-sized rocky planets orbit a star much smaller than our Sun, called a red dwarf.
But they’re arranged in much tighter orbits than the planets of our Solar System – all of the TRAPPIST-1 planets would fit inside the orbit of Mercury.
That should make the transfer of material between planets in that system more frequent, says Ciesla.
“It’s a very compact system,” he says. “So the efficiency of something being launched from one to the other is much greater.”
In their 2017 work, Ciesla and his colleagues suggested that the prospect of lithopanspermia in the TRAPPIST-1 system – where life itself is transported between planets – was plausible.
The team calculated that 10 per cent of material ejected from a habitable zone planet in the TRAPPIST-1 system would reach another habitable zone planet in just 100 years.
“We conclude that transferring solids between habitable zone planets in TRAPPIST-1 is up to [10,000–100,000 times] faster [than our own Solar System],” they wrote.
The arrival of life from asteroids and comets
If the transfer of life-bearing rocks between the worlds of our Solar System is somewhat unlikely, what about another type of delivery – not from other planets, but leftover material around the Sun?
Many millions of asteroids and comets orbit the Sun today and we know that these periodically hit planets.
In fact, Earth's seas and oceans are thought to have originated from water delivered by these bodies, following a molten phase after a collision with a Mars-sized object that created the Moon around 4.5 billion years ago.
That makes asteroids and comets a potentially preferable place for the building blocks of life to a rise in the Solar System.
“Earth went through a tremendously dramatic event and the entire planet melted, so all of the early organics were lost,” says Dworkin. Could rocks from space have subsequently carried the ingredients of life, or even life itself, to Earth?
Once deemed implausible, recent results from two spacecraft are now suggesting that might well have been the case – and the race is on to find out for certain.
Finding the ingredients for life inside an asteroid
In December 2020, a spacecraft called Hayabusa2 from the Japanese space agency (JAXA) returned to Earth with about 5g (0.1oz) of samples it had collected from an asteroid, called Ryugu, more than a year prior.
The spacecraft had fired a ball bearing-sized projectile into the asteroid, kicking up material into a collector and returning it to Earth for a landing in the Australian outback.
Three years later, in September 2023, a NASA spacecraft called OSIRIS-REx followed suit with its own samples from an asteroid called Bennu.
Its extendable arm had been able to gather much more material than its Japanese counterpart, using a burst of nitrogen gas to scoop up more than 120g (4.2oz) of material. The samples touched down in the Utah desert inside a capsule.
Results from both missions have now shown that organic material, and the building blocks of life, are present on both asteroids, including carbon, ammonia, salts and more.
“It’s very clear that they do contain organics,” says Davies. “It wouldn’t have been a surprise to Fred Hoyle – he was saying all this in the early 1970s.”
In January 2025, scientists analysing OSIRIS-REx’s samples said they had found 14 of the 20 amino acids that life on Earth uses to make proteins, and the basic constituents of DNA and RNA.
The asteroid also seems to have been wet at some point in the past, with channels of water running through the rock.
“We see evidence of veins of water that left behind salt deposits as it dried out,” says Dworkin. “Think of it as mud.”
Noticeably absent, however, is life itself. Bennu is thought to be the fragment of a larger asteroid that came from the outer Solar System, which begs the question, “What was Bennu’s parent body missing that Earth had?” says Dworkin.
“We know that the salts formed around room temperature, so it was a mild environment,” heated by radioactive isotopes in the asteroid. “It was an environment you would expect maybe would be good for life, but it didn’t happen.”
Studies of the OSIRIS-REx samples are in their infancy. “We’ve only looked at about 14 per cent of the total samples,” says Dworkin, with scientists only planning to look at about 25 per cent in total for the time being.
“The other samples are being held for years so that people not yet born can use techniques not yet invented to answer questions we haven’t even thought of,” he says.
But the findings so far suggest that just having the right conditions and ingredients for life doesn’t mean you’ll necessarily create life.
“It’s got to be harder than just mixing the chemicals together, otherwise life would have formed on Bennu,” says Dworkin. “Bennu is a pantry full of ingredients, but it wasn’t quite the right conditions to make a cake. On Earth, we have cake and we don’t know why.”
Life from among the stars
There also remains the possibility that the transfer of life isn’t limited to within worlds of a single solar system, but perhaps between stars.
We’ve found two objects passing through our Solar System – ‘Oumuamua in 2018 and Comet Borisov in 2019 – that were moving so fast they must have originated elsewhere.
Could life, or its building blocks, survive the journey on such a body to seed worlds of different stars?
“The fact we’re finding that stuff can be kicked out of one planetary system and make its way to another shows that this isn’t limited to just within one planetary system,” says Ciesla. “There’s merit to the idea of life making its way from one planetary system to another.”
The odds of a life-bearing rock making its way across interstellar space and hitting another planet to seed it with life seem slim, however.
“When you put the numbers in, it looks exceedingly unlikely,” says Davies. More likely panspermia is “restricted to within one planetary system unless there are exceptional circumstances,” he says.
Whether life itself, not just the building blocks of it, could survive a journey between stars is also uncertain.
“There are some species that can lie dormant for a long period of time and still survive,” says Ciesla. “There’s a potential there, but it isn’t something people are necessarily talking about.”
More plausible is the spread of life within a single solar system, such as ours.
The future search for the origins of life
In the future, Ciesla says he would like to see a mission return material from a comet, not just an asteroid, to tell us more.
“Comets are just like asteroids, they’re the building blocks of planets, but they form further out in the Solar System,” he says. “Understanding the pieces that were available to be delivered to a potentially habitable planet is a question I’d be really excited to see us pursue.”
Ongoing missions to Mars and the icy moons of Jupiter could give us further clues, too. NASA’s Perseverance rover is on Mars right now, scooping up samples intended to be returned to Earth in the next decade.
The hope is that those samples might contain evidence of microbial life that originated on Mars back when the planet was warm and wet.
Meanwhile, NASA's Europa Clipper mission will arrive at Jupiter in 2030 to tell us more about this intriguing moon that seems to have an ocean beneath its icy shell.
“People have speculated that life could take hold there,” says Ciesla. “The ingredients seem to be there. The question again is whether or not the recipe was followed.”
What is certain is that the prospect of life beginning elsewhere in the Solar System seems increasingly plausible. With that, there are important implications for our origins – whether we were a fluke resulting from an ingredient-laden asteroid hitting Earth at just the right time – and the possibility of life elsewhere.
“We have to contend with the fact we really don’t know where life began,” says Davies. “And it’s entirely likely it didn’t begin on Earth.”
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