The recent discovery that exotic microbes teem in the rocks hundreds of metres beneath the floor of the Pacific Ocean looks set to fuel the controversy over where and when life began. And it will also considerably boost hopes for life on Mars.
When H G Wells wrote War of the Worlds in the 1890s, belief in life on the Red Planet was widespread. Astronomers even produced maps showing networks of canals they thought had been built by water- deprived Martians. These fanciful notions were knocked on the head in the 1960s when the first space probes to visit Mars revealed a barren, cratered terrain.
The coup de grace came in 1976 when the US space agency Nasa landed two Viking spacecraft on the surface to search for signs of biological activity. Data showed a freeze-dried desert bathed in ultra-violet radiation. Scooped up Martian dirt was analysed, and not a single bacterium was found. Mars was red and dead.
In recent years, however, sentiment has begun to shift. Several forthcoming Mars missions, notably Britain's Beagle 2 designed by Prof Colin Pillinger of the Open University, will seek out telltale signs that Mars has, or at least once had, some form of life. So what has changed?
Life as we know it requires liquid water, and photographs of Mars show river valleys and flood channels. There are hints of lakes and even a small ocean. All are dried up now, but results from the Mars Odyssey probe suggest an abundance of ice locked up in the form of permafrost. The water is still there, but frozen.
Three and a half billion years ago it was a different story. Mars had a thick atmosphere that created intense greenhouse warming. It also had extensive volcanoes. Running water was in abundance. Such warm, wet conditions were ideal for life. On Earth, the oldest microbes are found clustering around volcanic vents on the ocean floor. Mars probably had similar environments billions of years ago that could have hosted such organisms.
Another key factor in the reappraisal of Mars is the discovery that life on Earth extends deep into the crust. The new results by Prof Stephen Giovannoni and colleagues at Oregon State University, Corvallis, confirm the existence of a pervasive hidden biosphere that may be kilometres deep.
This subterranean life thrives without sunlight, exploiting dissolved gases and fluids percolating up from the torrid depths. The primary producers are microbes that can convert inorganic substances directly into living material using chemical energy alone.
The significance of this discovery for Mars is that, though the surface is hostile to life, the warmer subsurface may be more congenial. Miles down, liquid water aquifers might harbour hardy organisms of the sort found beneath the sea bed in the Pacific. And even if Mars is dead today, life could have clung on underground for billions of years.
In fact, Mars may even have been a more favourable planet than Earth for life to get going in the first place. For a start, it was spared the colossal impact that created the moon and melted Earth's crust. Scientists now recognise that cosmic impacts have played a major role in the story of life.
For the 700 million or so years that followed the formation of the solar system 4.5 billion years ago, a barrage of giant asteroids pounded the planets. On Earth, these impacts would have swathed the globe with incandescent rock vapour, boiling the oceans and sterilising the rock beneath.
On Mars, life could have been shielded from the bombardment by taking up residence in the deep subsurface. Being smaller than Earth, the Red Planet cooled quicker, rapidly dissipating the fiery heat of its formation. When Earth's crust was still a searing hell, the Martian subsurface could have been comfortable for heat-tolerant microbes of the sort now living near volcanic vents.
Ironically, the same impacts that threatened early life in the solar system might also have served to propagate it. A comet slamming into Mars would blast billions of tons of rocks into space. A few per cent of this ejected material will eventually hit Earth. A couple of dozen meteorites are known to have come from Mars, and estimates suggest that, on average, about one Mars rock per month reaches our planet.
It seems inevitable that, if there were once abundant microbes on Mars, some of them would have reached Earth by hitching a ride on Martian meteorites. Cocooned inside a large rock, they would be screened from the worst effects of radiation and protected from burning up as they plunged into the Earth's atmosphere.
Experiments using superguns in New Mexico and at the University of Kent confirm that bacteria could withstand the shock of being blasted off Mars. Moreover, the Martian meteorites collected so far were not subjected to lethal temperatures during their violent ejection. It seems likely that at least some resilient microbes could have made the journey unscathed.
This raises the tantalising prospect that Mars may be the cradle of terrestrial life. One of the puzzles about life on Earth is that it established itself so quickly after the bombardment abated 3.8 billion years ago. In Western Australia there are 3.5 billion-year-old rocks containing fossil bacteria. Nearby are stromatolites, thought to be the product of ancient microbial mat-building. There is even a hint of life in Greenland rocks as old as 3.85 billion years, although those claims are contentious.
Clearly life didn't spring into existence ready-made in the form of fully- fledged bacteria, so there must have been an extended even earlier period of evolution. But given the battering Earth's surface took from asteroids and comets, it looks more and more likely that this pre-history took place beyond our planet.
If life began on Mars and evolved to the point of bacteria by, say, four billion years ago, then it had plenty of chance to infect our planet. A rain of microbe-laden Martian debris would have fallen on Earth throughout the bombardment. Possibly Mars life took root here many times, only to be exterminated by the next big impact. Eventually, some of these incoming colonists would have flourished, probably by spreading into the sheltered subterranean depths where microbial life still lurks today. >From this precarious niche, they evolved into the vast diversity of life we see today, from mouse to man.
If this theory is right, then history has turned full circle since H G Wells penned his alien scare story. Far from being our enemies, the Martians are our ancestors.
Paul Davies is in The Australian Centre for Astrobiology at Macquarie University in Sydney. This article is based on his Michael Faraday Prize lecture "The Origin of Life" to be given at The Royal Society on January 27, before scientists and media gather at an event sponsored by The Daily Telegraph and Novartis.
File Date: 01.28.03