Did life on Earth come from outer space?

In August, Nasa's Curiosity rover, the largest and most expensive robot ever to be sent to the surface of another planet, will arrive on Mars. It has several goals, including studying the climate and geography of the Red Planet, and providing information for a possible future manned mission. But Curiosity will also be looking for signs of life and clues as to where, as well as how, life began - still one of science's greatest unsolved mysteries.

Ever since Charles Darwin suggested that life may have begun in a "warm little pond", the idea of the primordial soup has taken hold in the public and scientific imagination. The hypothesis states that the chemical building blocks of life mixed together until a chance event combined them into a replicating being. That replicator, which would have "lived" around four billion years ago, is the great-to-the-Nth-power grandparent of every organism.

The primordial soup explanation has had rivals, but most of them centre around a similar idea: a replicating process starts on Earth, and once it is up and running, Darwinian evolution acts on the offspring of that original ur-organism. In his book The Blind Watchmaker, Richard Dawkins championed Graham Cairns-Smith's theory that clay crystals, which preserve information as they grow from solution, were the first replicators; Nick Lane, a biochemist at University College London, proposes that they arose by a hot vent (rather than a warm pond) in the deep ocean.

All the various theories rely on a single event kickstarting the process on our planet. In recent years, however, a radical alternative has gained new attention.

Prof Chandra Wickramasinghe, the director of the University of Buckingham's Centre for Astrobiology, argues that life may not have arisen on Earth at all, but spread from solar system to solar system, carried on interstellar bodies such as comets and asteroids. This hypothesis, known as "panspermia", states that life is transferred from one place in the galaxy to another. It would suggest that wherever life began, on Earth or elsewhere, it would have spread to other places - and so we should expect life on other planets in the solar system.

The evidence, he insists, is growing. Recent discoveries of "extremophiles" - microorganisms that survive in volcanoes, in space, or even in nuclear reactors - suggest that life can evolve to be hardy enough to survive almost anywhere it finds itself.

Only 40 years ago, such creatures were unheard of; in fact it was believed that life was impossible above 60C. Now they have been found at 120C. Similarly it was believed that life could not exist below a few metres below the seabed, because there is not enough energy to sustain it: now it has been found at a mile deep.

One extremophile, Deinococcus radiodurans, found in the cores of nuclear reactors, is capable of resisting very high levels of radiation - 1,000 times that which would kill a human - as well as dehydration, exposure to a vacuum and acid. In 2010 a bacterium called GFAJ-1 was found that used arsenic instead of phosphorus in its metabolic process, a previously unheard-of biochemical innovation - a finding so outlandish that several scientists refuse to believe it. The same year, a colony of cyanobacteria were left on the outside of the International Space Station for 18 months, by turns freezing and boiling, and bombarded by radiation. They survived, apparently quite happily. Other strains are being found every week.

That's encouraging for scientists looking for life in the harshest of environments - like the rest of the universe. "The first probes to Venus found microscopic particles in the clouds that were exactly the right size for bacteria," says Wickramasinghe. The Venusian surface is totally inhospitable, temperatures (450C) and pressures (90 times those here), believed to be far too high even for extremophile life to survive. But in the clouds, roughly 40 miles above the surface, the conditions are just right for microorganisms to live and to replicate. "We can envisage aerobiology [cloud-dwelling microorganisms] similar to the aerobiology that exists on Earth," Professor Wickramsingh says.

The implications of panspermia and the hardiness of extremophiles go beyond our local neighbourhood. Our solar system orbits the centre of the galaxy about once every quarter of a billion years. Life on Earth has therefore been on 16 trips around the galaxy. If Prof Wickramasinghe is right, then life from Earth - whether or not it arose here - will have spread to millions of other planetary systems.

This, in turn, suggests that there could be more interesting life in the cosmos than a sort of omnipresent bacterial pond scum. The genes that allowed the development of complex, multi-cellular life - such as frogs and oak trees and dinosaurs and us - will have been seeded on millions of planets. Even if the evolutionary steps from bacteria to animal are extraordinarily unlikely, as many have argued, the sheer number of places where it might happen would be huge.

Panspermia, though attracting more interest in recent years, is far from universally accepted. For a start, it does not answer the fundamental question of how life arose out of inanimate matter. And one obvious problem is that space is a deeply inhospitable environment with DNA-ruining radiation sleeting through it. In addition, any bacteria would have to survive entry to a planet's atmosphere, and then hope that the habitat they had landed in was suitable for their biology.

In response, Prof Wickramasinghe points to the success of extremophiles in Earth's more brutal habitats; he also suggests that there is such a vast amount of living matter floating around the universe that it is inevitable that some of it will land in appropriate habitats.

Other astrobiologists are less convinced. Dr Lewis Dartnell of UCL is careful to say how important Wickramasinghe's ideas have been: "He's done some phenomenal stuff; a book of his got me into the subject in the first place. But it's on the fringe of what's accepted in the community," he says. For example, his claims about the aerobiology of Venus are based on the undisputed fact that bacteria exist in the lower atmosphere of Earth. "But Wickramasinghe claims that he's found cells living in the stratosphere, which is a big claim, and hasn't been independently repeated to demonstrate that it wasn't just contamination."

Similarly, the numbers may not add up for interstellar transport of life. "You simply can't get a rock blasted off a planet with sufficient force to escape the sun's gravity that then slows down enough to land on another planet in another solar system without being utterly destroyed," says Dr Dartnell. Most significantly, it's not clear that we need to invoke panspermia. "At the moment there's no reason to think life didn't arise on Earth, so why try to construct scenarios whereby life originates elsewhere and needs to be delivered?" he asks.

But if Professor Wickramasinghe is right, then it suggests that life is far, far older than originally thought - not a mere four billion years, but perhaps just a fraction of a per cent younger than the universe itself. "We're working on a model that puts the origin of life in the first 10 to 30 million years after the Big Bang," says Prof Wickramasinghe.

That would be almost as soon as the first supernovae started to create the heavy elements required for life. This model, he thinks, would allow a greater exchange of life between a vast number of planets when the universe was smaller than today. "For the most complex of organisational systems in the entire universe to develop in a minuscule pond on the Earth when other, much bigger systems were available is, I think, unlikely." Telescopes focused on distant stars have found evidence of organic molecules in the very early cosmos, he says.

It's speculative, certainly. But it's a profoundly different look at life's origins. "If we ever find reasons to believe that terrestrial life isn't native, then panspermia will become very important," says Dr Dartnell.

The last few decades have shown just how hardy life is, as new extremophiles are found almost daily. What is not known is how likely it is to arise in the first place, and while we have only a single example - the genesis of life on this planet - we cannot extrapolate to elsewhere. But if the panspermists are right, then we will probably find life just about everywhere we look - and Curiosity may well make the discovery of the century.