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鈥攂ut 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鈥攎any measuring 100 kilometres or more
across鈥攆requently 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 掳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.
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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鈥檚 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鈥攃arbon-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鈥攖he proto-Sun鈥攕urrounded by a swirling disc of gas and dust.
From this, the planets condensed. Substances that can survive high temperatures
without melting鈥攊ron, silicon and the like鈥攕olidified 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鈥檚
gravitational field to form the Oort Cloud of comets. Others remained lurking
beyond Neptune鈥檚 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鈥檚 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鈥檛 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鈥攐r
anything similar already on the planet鈥攊nto space, thinning the atmosphere
and depleting the oceans. In the words of the late Carl Sagan, 鈥淐omets giveth
and comets taketh away.鈥�
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鈥檚 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鈥攖he two key ingredients鈥攅xisted 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鈥檚 surface. Microorganisms have been discovered dwelling happily several
kilometres under the ground, existing on a diet of minerals and gases
(see 鈥淭he intra-terrestrials鈥�, 快猫短视频, 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鈥檚 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鈥檚
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 鈥渉yperthermophiles鈥� in the hope that
they will cast light on Earth鈥檚 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鈥檛 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鈥檚 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 鈥渁lien鈥� superbugs survived, and is
still lurking in an unexplored niche somewhere, awaiting the prospector鈥檚
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 鈥淪uperbug survival鈥�). And some of the material
thrown into Earth orbit would eventually fall back to Earth鈥攏ot 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鈥攏ot 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鈥檚 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.
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 掳C or possibly higher, from near
vacuum conditions to pressures of hundreds of atmospheres, accelerations of
10 000 g, 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鈥檚 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
掳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
掳C, which is perfect for preservation.