Source: New Scientist (via UASR)
Date: Sept 12, 1998

Battered by meteorites, the early Earth was no place for life to take hold. But our most primitive ancestors could have found refuge on the planet next door, argues Paul Davies

ONE DAY, the Earth will probably be hit by an asteroid or comet large enough to wipe out most living creatures. It is not likely to be soon--but such events do happen. The comet that reputedly killed off the dinosaurs and many other species 65 million years ago left a crater 180 kilometres in diameter and plunged the whole world into years of cold and darkness.

But even events of this magnitude pale into insignificance compared to the ones that happened back when the Earth was newly formed. Until about 3·8 billion years ago, vast hunks of rock--many measuring 100 kilometres or more across--frequently crashed into the planet, typically at speeds of around 30 kilometres per second. The effects of such a collision would be truly awesome. It would excavate a crater larger than the British Isles. The blast would strip away most of the atmosphere, replacing it with vaporised rock from the impacting object. This incandescent material would swathe the planet, creating a global furnace with a temperature of 3000 •C. In the intense heat the oceans would boil away, and the exposed land would be thoroughly sterilised. A lethal pulse of heat would travel as much as a kilometre into the ground.

The Earth would be a pretty inhospitable place for life after such a cataclysm. Yet, paradoxically, scientists are beginning to suspect that the life forms from which we are descended survived just such conditions. Fossil microbes are known that date back 3·6 billion years, while hints of life have been found in rocks as old as 3·85 billion years.

This has led to some fascinating speculation. These organisms might have survived the cataclysms by cowering deep within the Earth. Or they might have been thrown into space inside fragments of rock, where they could safely wait for an opportunity to put down roots again. And once in space, they could even have found refuge on another planet, and then returned to Earth cocooned in more rocks. Stranger still, this could all just as easily have happened to Mars: and if it did, all living things on the Earth's surface may have come originally from the planet next door.

Everybody agrees that the sort of life now found on Earth could not have originated without two basic raw materials: liquid water and a supply of organic substances--carbon-based molecules that typically include hydrogen, oxygen and perhaps nitrogen. So the first question is: where did these raw materials come from? The answer, many now believe, is that they probably did not originate on Earth.

Astronomers have built up a blow-by-blow account of how the Solar System formed. First, a collapsing cloud of hydrogen turned into a glowing blob--the proto-Sun--surrounded by a swirling disc of gas and dust. From this, the planets condensed. Substances that can survive high temperatures without melting--iron, silicon and the like--solidified relatively close to the Sun and clumped together to make the innermost planets Mercury, Venus, Earth and Mars. More volatile substances such as water and hydrocarbons condensed much farther out. There, sticky snowflakes snowballed to form the cores of the gas giants, including Jupiter and Saturn, plus numerous minor icy bodies laced with organic molecules that remained free to wander the frigid outer Solar System. Some of these dirty snowballs were flung out by Jupiter's gravitational field to form the Oort Cloud of comets. Others remained lurking beyond Neptune's orbit, where they make up the Kuiper Belt.

During this initial period of aggregation, collisions between partly formed planets and hurtling debris were common. At some point a Mars-sized body smashed into the Earth, stripping away its mantle before ploughing on to become the Earth's core. The material thrown out by the crash eventually came together to form the Moon. This cataclysmic encounter would have baked the Earth bone dry and driven off any trace of organic substances that might have survived the fierce heat of the solar nebula.

While all this was happening, about 4·5 billion years ago, the Earth was scarcely a congenial place for life. And the violence didn't stop then. Over the following 700 million years, gravitational perturbations from the newly formed giant planets disturbed many of the large comets and asteroids that were milling around the periphery of the Solar System, and sent some of them plunging our way. Most of these interlopers fell into the Sun, broke up or were flung back out again. But many smashed into the planets.

From the point of view of the prospects of life on a planet, a collision with a comet has both pros and cons. Comets are packed full of ice and life-encouraging organic substances. On the other hand, the impact itself releases a huge amount of energy, which may blast this material--or anything similar already on the planet--into space, thinning the atmosphere and depleting the oceans. In the words of the late Carl Sagan, "Comets giveth and comets taketh away."

Whether a planet is a net winner or loser in these encounters depends on circumstances. As a rule, the bigger the planet the more likely it is to gain rather than lose material. Thus Mercury and our Moon, which bear conspicuous scars of this early bombardment on their cratered surfaces, lost out badly. With the exception of tiny amounts of ice located near the permanently shadowed poles, both these bodies have been left with virtually no atmosphere or water.

Mars was a borderline case. It did acquire moderate amounts of water, and it once had a thick atmosphere too, but its gravity was not strong enough to hold onto it, and the Red Planet is now an arid desert.

Earth did well out of the bombardment, and emerged with plentiful water and air. It has been estimated by Chris Chyba, now at the University of Arizona, that enough comets hit the Earth to supply the world's oceans many times over. Our planet also received a veneer of organic substances. Exactly which chemical process transformed a mixture of lifeless substances into the first living thing has yet to be established. But it is clear that neither liquid water nor a supply of organic molecules--the two key ingredients--existed on the newly formed Earth. So the biosphere must have been constructed, at least in part, from the raw material that comets and asteroids brought to Earth more than 4 billion years ago.

The craters on the Moon suggest that the cosmic barrage was especially intense between 4 and 3·8 billion years ago, after which it gradually abated as the Solar System was swept clean of debris. Towards the end of this period there must have been many huge impacts, and at first sight this would seem to rule out any possibility of life.

Recently, however, some dramatic discoveries have put a new spin on the subject. For one thing, we now know that life on Earth is not restricted to the planet's surface. Microorganisms have been discovered dwelling happily several kilometres under the ground, existing on a diet of minerals and gases (see "The intra-terrestrials", New Scientist, 28 March, p 28). Similarly, the dark ocean floor is home to many exotic microbial species, and the international Ocean Drilling Program has discovered that the submarine biosphere extends deep into the rock of the seabed itself. Evidently the Earth's crust is teeming with tiny life forms.

This gives a clue to how life on the early Earth could have endured repeated impacts from space debris. Organisms for which the comfort zone extended into the hot crust by a kilometre or more could have survived a major impact event, so long as they were well away from ground zero. In effect, the deep strata could have provided shelters against the ferocious bombardment, as long as the organisms could tolerate the naturally high temperatures. Many of today's deep-living microbes would have managed this feat with ease. They thrive near volcanic vents, or in geothermal rocks, in some cases enduring temperatures well above the normal boiling point of water.

Tree of life

Microbiologists have been studying these "hyperthermophiles" in the hope that they will cast light on Earth's earliest life forms. One of the key tools is gene and protein sequencing, which can help determine the evolutionary distances between different species. By comparing sequence data from many organisms, biologists have reconstructed a plausible tree of life, showing which species branched from which. The technique was pioneered by Carl Woese of the University of Illinois twenty years ago, and over the past few years Karl Stetter of the University of Regensburg and Susan Barns and Norman Pace, both at Indiana University, have applied it to hyperthermophiles with amazing success. It turns out that all the oldest and deepest branches of the tree of life are occupied by heat-loving superbugs. In effect, they are living fossils, having remained largely unchanged for billions of years.

Some scientists now believe that the earliest organisms on Earth were deep-living hyperthermophiles, and that we and the rest of surface life are later adaptations. At first sight it appears difficult to see how life could begin in solid rock, which severely restricts the movement of the different chemicals that would have to be brought together. But it is possible that fissures or pores in the rock could have acted as tiny crucibles that concentrated the necessary substances. Life might have started deep in the hot crust of the planet, and ventured up only when it was safe to do so. If this is right, life didn't so much crawl out of the slime as ascend from Hades.

Underground colonies

Unfortunately the evolutionary record cannot yet confirm this. It is possible that the Earth's first life forms started out on the surface and then colonised the torrid subterranean zone. Come the next big impact, only the microbes that had evolved to live hot and deep survived.

A variant of this idea was proposed a decade ago by Kevin Maher and David Stevenson of Caltech, and elaborated by Norman Sleep of Stanford University and his co-workers. Suppose, as many scientists maintain, that life emerged rapidly from lifeless chemicals once physical conditions were suitable. There were probably gaps of a few million years between really big impacts, during which time life could have got under way, only to be zapped when the next large asteroid plunged home. The early history of life on Earth might then have been an extended series of false starts, as sterilising impacts destroyed successive attempts by primitive organisms to establish themselves. Life as we know it would then be descended from the first microbial colony that just managed to survive the bombardment.

If this theory is right, we may yet find fossilised traces of these earlier organisms. As they would be completely unrelated to life on Earth today, they would, by most definitions, constitute an alien form of life. It is even possible that an isolated colony of these "alien" superbugs survived, and is still lurking in an unexplored niche somewhere, awaiting the prospector's drill.

But there is another even more intriguing possibility. The early bombardment of Earth would have displaced prodigious quantities of rocks into space. So rather than surviving underground, could microorganisms have escaped destruction by going into space? Jay Melosh of the University of Arizona has shown that several per cent of the ejected material produced by a large impact may be flung into orbit without the rocks being severely heated or shocked. Since we know that Earth rocks provide a congenial home for life, it appears inevitable that some viable microbes will have been sent into space.

Ensconced cosily within an orbiting chunk of rock, shielded from radiation, and freeze-dried by the vacuum of space, a microbial spore could survive virtually indefinitely (see "Superbug survival"). And some of the material thrown into Earth orbit would eventually fall back to Earth--not all of it burning up on re-entry. So the planet may have been recolonised from space once the aftermath of a sterilising impact subsided.

Earthly messenger

It is only a simple extension of this scenario to imagine that a rock from Earth harbouring live organisms might travel to one of our neighbours in the Solar System. During the heavy bombardment there was no lack of cosmic encounters that packed enough punch to achieve this. Transport of such microbes to Mars might have been particularly significant. Although the surface of Mars is too harsh for life today, things were very different in the past. The Mars Global Surveyor, now orbiting the Red Planet, has revealed a landscape deeply etched with dried-up river valleys and embellished by extinct volcanoes. It seems that about 3·6 billion years ago, Mars was warm and wet--not unlike Earth. Life existed on Earth before this time, so terrestrial microbes could have reached Mars when conditions there were quite congenial. This, I believe, makes it virtually certain that there was once life on Mars.

And if Earth organisms colonised Mars, why not the other way round, as well? Indeed, as a cradle for primeval life, Mars offers some distinct advantages over Earth. Being smaller, it suffered fewer impacts. It also cooled more quickly, enabling any hyperthermophiles to burrow deeper and so be better protected from the effects of the bombardment. Mars's lower gravity would have helped too, by reducing the speed of impacts. And the lower escape velocity means that material can be kicked into space by a less violent blast, giving microbes a better chance of survival in the ejected rocks. Most importantly, the surface of Mars may have been hospitable to life more than 4 billion years ago, when our own planet was still a barren cauldron.

Meteorites originating from Mars have been found on Earth, so if life did get going on Mars first, it becomes a distinct possibility that it was transferred to Earth inside such a meteorite. Computer simulations published two years ago by Bret Gladman of Cornell University and his collaborators suggest that 7·5 per cent of Mars ejecta eventually reaches Earth, a third of it within ten million years. This is easily a short enough time for a microbial spore to remain viable; on Earth, some spores have been preserved in salt and amber for much longer.

The last Martians

The theory that Earth was seeded with Martian microorganisms would explain why life established itself here so early, in the most marginal of circumstances. A steady supply of fecund Martian debris raining down on Earth throughout the bombardment period would have given Martian bugs a good opportunity to colonise Earth as soon as conditions permitted.

Of course, we can also imagine that life started independently on both planets. In this case, any incoming Martian microbes would have found themselves in competition with Earth life. Would one form have destroyed the other, or might they have been similar enough to join forces in a kind of interplanetary symbiosis? Another possibility is that they found separate ecological niches, and continued on a path of peaceful coexistence and parallel evolution. Who knows, exotic Martian microbes may still be lying undetected all around us.

Mars ceased to be a good abode for life when surface conditions there began to deteriorate about 3·6 billion years ago. Volcanism slowed, the atmosphere leaked away, oceans and lakes either evaporated or froze, and the planet turned into the hostile desiccated wasteland that we see today. It seems increasingly likely that Mars is now a totally dead world. So, it would be an ironic twist if life on Earth did originate on Mars. You and I, and all the other life forms we share this planet with could actually be the last Martians.

Paul Davies is a physicist and writer living in Adelaide. His new book The Fifth Miracle: the search for theorigin of life will be published next week by Penguin

Superbug survival

THE amazing resilience of some microbes has earned them the tag superbugs or extremophiles. Their survival feats include being able to withstand temperatures ranging from near absolute zero to 120 C or possibly higher, from near vacuum conditions to pressures of hundreds of atmospheres, accelerations of 10 000g, immersion in saturated brine and in acid strong enough to dissolve metal. Some bacteria even thrive in the waste pools of nuclear reactors and swallow plutonium without ill effects.

Others can hibernate almost indefinitely by forming spores: they shrivel up and surround themselves with a thick wall, and their metabolism slows almost to a halt. From this state of suspended animation they can be revived if water and nutrients eventually become available. Nobody knows if there is a limit to how long bacterial spores can survive, but when freeze-dried at ultra-low temperatures it could be many millions of years.

The main hazard to microbes is radiation, especially ultraviolet. Once an organism's DNA is irreparably fractured, it is effectively dead. Fred Hoyle and Chandra Wickramasinghe of Cardiff University in Wales, long-standing supporters of the theory that microbes can journey through space, have stressed the remarkable radiation resistance of some bacteria. Experiments suggest that a few species of microorganism can stay alive for a time even when completely unshielded from the harshness of outer space. For example, Peter Weber and Mayo Greenberg of the University of Leiden cooled bacterial spores to within 10 •C of absolute zero and shone an intense ultraviolet beam on them to mimic the effects of 2500 years in interstellar space. Even then, one in a thousand of the organisms survived.

Given such toughness, some micro-organisms have every chance of surviving an interplanetary journey, especially if they lie protected within rocks thrown into space by an impacting comet or asteroid. A thin covering of rock is enough to block ultraviolet rays: a metre would shield most cosmic radiation. An asteroid of the sort that may have killed off the dinosaurs might kick a billion one-tonne rock fragments into solar orbit, providing ample opportunity for live microbes to hitch a ride from Earth to Mars, or vice versa. The temperature within ejected material travelling between Mars and Earth is typically -50 C, which is perfect for preservation.

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