Is this
how life on Earth began?
Far from being just red and dead,
Mars may have nurtured our bacterial ancestors in its warmer,
liquid depths.Paul Davies reports. 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 HG 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 coupde 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 oflakes
and even a small ocean.All are dried upnow, 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 b 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 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 Earths 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 4 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 dence 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 mates 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 hitch ing 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 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 b 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 forAstrobiology 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.
Mars, the appearance of aliens and fate of the human race will
be discussed at the Cheltenham Festival of Science which starts
on June 4. To order a brochure, call 01242 237377, email [email protected]
or visit www.cheltenhamfestivals.co.uk
22 Jan
2003


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