Where did life on Earth begin? That sounds like a relatively straightforward question. Surely life 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 them 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 to 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?