American chemist Stanley Miller surveys the contents of a flask where primitive gases have been zapped by an electric spark. For many years, scientists assumed this was how life started on Earth: now, many have doubts.
	photo - Hencoup Enterprises

Chicago chemist Stanley Miller blazed the trail in the early 1950s by zapping a noxious brew of gas and water with electricity. Others have tried different formulae. Unfortunately, after fifty years of experimentation, the results are disappointing. Some of the simpler building blocks of life, like amino acids, are readily made, but the building itself - a living cell - remains as elusive as ever.

Deep-sea Life

Now a radical new theory could explain the lack of progress. Scientists increasingly suspect that the tepid pool scenario is wrong, and that life didn't start on the Earth's surface at all. This change of mind stems from the startling discovery of bizarre micro-organisms, dubbed superbugs or extremophiles, that inhabit some of our planet's most extreme environments.

In the late 1970s, a research submarine called Alvin was sent to the bottom of the Pacific Ocean to explore a string of volcanic vents known as 'black smokers'. These chimneys on the sea floor spew forth superheated water rich in dusky chemicals. The investigators were astonished to find many exotic life forms clustered around the searing effluent in pitch darkness, including weird-looking crabs and giant tube worms.

Most remarkable of all were bacteria that inhabited the hot zone itself, thriving in the disgorging fluids at temperatures in excess of 100^oC. Previously, nobody believed that any life could survive above the normal boiling point of water. These superbugs lie at the base of the food chain. They are primary producers, turning inorganic material from the hot vents directly into biomass, without the need for sunlight.

Subterranean Bugs

Alvin's discovery turned out to be just the tip of the iceberg. In the 1980s, Thomas Gold, an astrophysicist from Cornell University, New York, supervised an oil drilling project in Sweden. On examining the rock cores from several miles down, Gold was amazed to find unmistakable signs of life.

At the time, the idea that something could live so deep in the Earth's crust was laughed at. There have been many stories about life in the underworld, from the Greek fable of Orpheus to Jules Verne's Journey to the Centre of the Earth. Few scientists took the possibility seriously. But Gold's claim was confirmed when other drilling projects yielded similar results. Biologists in the United States began extracting live microbes from miles beneath North Carolina and the Columbia River basin. Environmentalists became concerned that subterranean superbugs might eat through containment vessels of buried nuclear waste, causing leaks.

Meanwhile, the international Ocean Drilling Programme retrieved rocks from far below the seabed, and the story was the same. The basalt of the ocean floor is teeming with microbes too. Some of them have been cultured in the laboratory by John Parkes of the University of Bristol, who believes that some species can withstand temperatures as high as 169^oC. It is beginning to look as if there may be as much biomass inside the Earth as there is on the surface.

Our Ancestors?

The full significance of the deep-living microbes became clear only after their innards were analysed, following the pioneering work of Karl Stetter at the University of Regensburg in Germany. By sequencing the superbug's genes, microbiologists can construct a sort of family tree linking them with normal bacteria. The results came as a complete surprise. It turns out that the oldest and deepest branches of the tree of life are all occupied by heat-loving superbugs. In other words, the microbes residing deep within the Earth are among the world's oldest surviving organisms. In effect, they are living fossils, having changed little since the dawn of time.

 

	Mono Lake in California is thick with salt and - in places - almost boiling. Yet it thrives with microbes, closely related to the earliest life-forms on Earth.
	photo - Jerre Goldin

To some researchers these discoveries spell out a fascinating message. It suggests that life was incubated in the volcanic depths of the Earth, in pressure-cooker conditions, and migrated to the cooler surface zone only much later. This theory neatly meshes with what we know about the Earth's history. The Solar System is four and a half billion years old. For almost a billion years after the planets formed they were pounded mercilessly by giant asteroids and comets. A record of this primordial violence is etched on the face of our nearest neighbour in space - the Moon - which is pockmarked with countless large craters.

The biggest impacts would have blasted away the Earth's atmosphere and swathed the globe in incandescent rock vapour. The heat pulses were fierce enough to boil the oceans dry and sterilise the exposed land to a depth of half a mile or more. Not even superbugs could survive such cataclysmic episodes unprotected. Yet, paradoxically, there are traces of relatively advanced life in ancient rocks from Greenland dated at over 3.85 billion years - a time before this massive cosmic bombardment had abated. But if the 'comfort zone' of heat-loving microbes was deep enough, they could shelter from the cosmic barrage in the torrid strata of the Earth's crust, beyond the reach of even the fiercest heat pulses.

Superbugs on Mars

 

	Mars today is a frozen desert « yet dried-up channels and crater erosion hints that the Red Planet was once warmer and drier, with rivers and possibly oceans. Was this, too, a cradle of life?
	photo - NASA

If the theory is right, and life did begin deep within the Earth, it may also have got going beneath the surfaces of other planets too. Mars is an obvious candidate. When two Viking space probes landed there in 1977 they scooped up some dirt to test for biological activity. No clear evidence was found, and most scientists pronounced the Red Planet dead. With hindsight, this outcome wasn't surprising, since the surface of Mars is a freeze-dried desert bathed in ultra-violet radiation that would prove lethal to almost all known organisms. However, beneath the hostile surface, conditions may be more congenial for life. Geothermal heat will have melted the permafrost to create reservoirs of liquid brine similar to those beneath the Earth's sea bed. So there might be superbugs lurking below the harsh Martian terrain.

Although today Mars is cold and dry, in the remote past it was warm and wet, and not unlike Earth. It had rivers and glaciers and possibly a large ocean. Life may well have spread to the planet's surface and briefly flourished in the Martian spring, before the atmosphere leaked away and the temperature plunged. It's possible these ancient organisms left fossils in the surface rocks, where they may be discovered by forthcoming missions to the Red Planet.

 

	The infamous "microbes from Mars". Magnified 100,000 times the interior of a meteorite from Mars (coloured red) reveals intriguing bug-like shapes (blue). Researchers are stilling arguing whether these are indeed fossilised bacteria.
	photo - NASA

A few years ago, some scientists claimed to find fossilised microbes within a Martian meteorite collected in Antarctica. Although the jury is still out on that, it no longer seems so fanciful to speculate that Mars has, or at least once had, some form of life.

If there is, or was, life on Mars, then it raises a fascinating possibility. Back in the early history of the Solar System, the heavy bombardment that made conditions on the planets so dangerous for surface life would also have kicked vast numbers of rocks out into space. Many Mars rocks must have reached Earth during our planet's history, and many ejected Earth rocks will have hit Mars. Could hardy superbugs dwelling within these rocks have hitched a ride through space and taken up residence on arrival?

Current evidence strongly suggests the answer is yes, and that Earth and Mars cross-contaminated each other billions of years ago. It's even conceivable that life began on Mars and travelled to Earth some time later in meteorites, colonising our planet when conditions eventually became favourable. If so, we are all descended from Martians!

The Garden of Eden - Revisited

Tantalising though these developments are, the problem remains of how the first living cell formed. What chemical magic triggered the vital spark? Whether the key process happened on Mars, Earth, or both, the puzzle of the chemical genesis of life is still unsolved. Armed with the new ideas, however, researchers are now focusing their efforts on the chemistry of hot rocks infused with sea water. Might the gases exuded by a planet's crust offer a more potent mix than those of the primordial atmosphere? Could the pores in ocean basalt play the role of tiny crucibles, in one of which was forged that microbial Adam long, long ago? Some scientists think so, and are increasingly hopeful that the key steps will soon be understood.

In many cultures the underworld has long been associated with the realm of the dead. For Christians, it is the traditional location of Hell, a place of fire and brimstone, and eternal torment. How ironical if the torrid, sulphurous depths actually harboured the cradle of life. Far from being Hell, the broiling bosom of our planet might well turn out to have been the true Garden of Eden.

Professor Paul Davies is a leading physicist with an international reputation for explaining scientific ideas in simple language. Born in London, but now based in South Australia, he has held academic positions at Cambridge, London, Newcastle upon Tyne, Adelaide and Queensland Universities. Davies's interests include the nature of time, high-energy particle physics, the foundations of quantum mechanics, the origin of life and the nature of consciousness. His books include The Cosmic Blueprint, The Mind of God, The Last Three Minutes, About Time and The Fifth Miracle, which examines the origin of life and the possibility of life on Mars.