ARE WE ALL MARTIAN SCUM?
or
BLACK SMOKERS AND BUCKYBALLS
New discoveries about the origin of life on Earth suggest we might all be
descendants of Martian pond scum.
All life on earth had a single origin, according to the orthodox view. The
proof is the remarkable consistency of DNA, the genetic blueprint
common to all algae, bacteria, plants and animals. But how did life on
earth start and when?
Scientists studying the origins of terrestrial life are generally agreed that
the fossil evidence of organic carbon dates back some 3.8 billion years to
a time when the earth was just half a billion years old. In 1991, biologists
Thomas Cech (of the University of Colorado) and Sidney Altman (of
Yale University) shared a Nobel Prize for discovering how RNA
molecules, the predecessor of DNA, could assemble themselves from a
chemical soup. Since then, other scientists have successfully created
cell-like structures containing a primitive form of RNA.
David Deamer of University of California, for example, who studies scum
in the tidal pools at Monterey Bay, has demonstrated how compounds
forming an oily scum can spontaneously curl to form tiny bubbles.
Sheltering inside that outer membrane, organic chemistry can take place
to some extent independently of the more chaotic environment. Thus,
says Dr Deamer, the creation of membranes "is an essential step in the
evolution of the first cells." Stuart Kauffman, a biophysicist studying
biological complexity at the Santa Fe Institute, believes "life is a natural
property of complex chemical systems." He theorises that chemical soups
have "a certain threshold at which a self-sustaining network of reactions
will suddenly appear."
Precisely how early life made that leap across the threshold from
"chemicals to biology" is still a mystery. Gerald Joyce, a molecular
biologist at the Scripps Institution of Oceanography in San Diego, argues
that the gap must have been bridged by a succession of several distinct,
self-replicating entities. "Life did not start with RNA," he said. "RNA
followed the evolutionary footsteps of some other replicating molecule,
just as DNA followed RNA."
WHERE THERE'S SMOKE
Scientists like Dr Deamer have been content until now to peer into tidal
pools for signs of pre-cellular life. However, exciting new discoveries
suggest that it might be more profitable to look elsewhere.
One direction -- into the depths of our oceans, where sunlight never
penetrates -- has proved very rewarding. Projecting from the floor of the
mid-Atlantic Ridge -- the crack created as Africa and America are
pushed apart -- some 3,000 metres down and adjacent to the Azores, is
a forest of tall, thin volcano-like structures called 'black smokers'. They
pour out dark clouds of chemicals dissolved from fissures deep in the
crust below them, propelled upwards by superheated steam. When this
hot caustic smoke hits the cold ocean waters the minerals freeze out,
creating chimneys as tall as three-storey buildings.
The environment surrounding the black smokers is a highly toxic cocktail
of hot water, gas, sulphur and heavy metals, yet life has evolved here in a
surprising variety of unusual forms, usually eyeless and anaemic. Smokers
in the Pacific are home to giant red worms that live in tube-like structures
three metres long. "These volcanoes are like oases in the desert," says Dr
Bromley Murton of the Oceanography Centre at Southampton
University. "The surrounding area is completely barren. These animals are
only here because of the volcanoes. They are totally interlinked to the
geological situation."
Unlike life near or on the surface of the planet, these strange ecosystems
show an adaptation to life in darkness, great pressure, intense heat and
poisonous gases. More important, for our topic, is that their biochemistry
is driven, not by sunlight, but by bacteria that thrive on heat and chemical
actions based on sulphur. Studies began recently of a similar ecosystem
in a cave in Romania which was cut off from the surface about 5.5 million
years ago [see Karl Shuker's column, last issue].
Still other bizarre bacteria-based ecosystems are coming to light. In
September last year, a novel bacterium was discovered in an oil reservoir
below Paris. The following month, another previously unknown
ecosystem was found in deep aquifers near the Columbia River,
Washington State. Neither ecosysyem is dependent on sunlight,
geothermal heat, or organic matter but the chemical interaction between
water and basalt rock. Todd Stevens and Jim McKinley of the US
government's Pacific Northwest Laboratory in Richmond, Washington,
have dubbed these rock-based ecosystems SLiMEs (subsurface
lithoautotrophic microbial ecosystems).
ROCKS OFF
The implication is that terrestrial life has other strings to its bow, so to
speak, than the familiar and successful action of photosynthesis. Stevens
and McKinley believe that life on Mars, if it exists, is more likely to be a
SLiME or similar bacterial ecosystem, sheltering deep below the
inhospitable Martian surface. The implication is that we, ourselves, may
well be the descendants of Martian SLiME brought to earth on a
meteorite.
Earlier this year, astronomers from Manchester University and scientists
from the American Museum of Natural History announced that they had
found the oldest Martian meteorite to date, named 'Allan Hills 84001', in
Antarctica. At around four billion years old, it dates to the time-frame of
the 'Lunar Cataclysm', a period between 3.9 and 4.1 billion years ago in
which the moon underwent massive meteoric bombardment. During this
period -- when Mars had liquid water on its surface -- an impact had
been powerful enough to propel the 'Allan Hills' rock fragment towards
earth. If it, or any other meteorite from Mars, carried traces of organic
matter, it could have stirred the pre-cellular soup into protolife. The age
of the meteorite also falls comfortably within the period during which life
is thought to have begun on earth -- 3.5 to 4.1 billion years ago.
The most serious objection to the idea of 'seeds of life' hitching rides on
interplanetary meteors is that the extremely intense temperatures of the
impact explosion would destroy any organic matter that survived the
heating during entry (passage through our atmosphere). This notion was
challenged in a recent discussion on the 'forteana' mailing list and I am
indebted to the participants for the following ideas.
Incoming life 'seeds' would have a better chance of survival if the object
falling from space does not hit full on. Those hitting deep waters or
glancing slopes or with a shallow trajectory may well generate less
damaging explosions on impact. While the majority of meteorites are
ferrous in nature, there are alternatives. Charles Fort recorded falls of ice
chunks, slushy ice (possibly cometary debris), gooey masses and other
organic matter. Then there are meteoric objects -- eg. the Dhurmsala
meteorite (1860) discussed by Charles Fort -- which were found to be
cold or even frosted over after their fall and impact. We know so little
about these controversial meteoric objects, but they deserve
consideration as they would be perfect vehicles for infecting this earth
with proto-lifeforms.
Naturally, heating is related to the speed of fall and size of object; the
heat being generated, not by friction, but by compression of the relatively
thick atmosphere it travels through. Smaller particles of dust and crystals
of ice, therefore, may well survive the fall to earth because of their low
mass. Indeed, some astronomers believe there is a constant and almost
invisible rain of microscopic detritus, any fragment of which may carry
germs or organic material. The really interesting implication of this notion
is that the genesis events are still happening. In this view life could be in
continual creation and not restricted to a single ancient origin.
Even so, it would only take a single bacterium or virus -- perhaps only a
few lengths of primitive RNA or amino acids -- to introduce life to Earth.
It needs only to survived the journey and drop into an environment -- a
primordial pizza, some now think, rather than a soup -- in which it could
multiply and adapt. Some epidemiologists have attempted to link the
major waves of world-wide illnesses to infections from a meteor shower
or passing comet. What we call 'influenza' may well have been, literally,
the 'influence of the stars' in more than just the astrological sense.
BUCKYBALLS
Recently scientists found confirmation that some carbon-based material
might survive the intense heat of entry from space and shock of impact. It
stems from a study of the huge Sudbury 'impact structure' in central
Ontario where this earth was struck, around two billion years ago, by a
comet or meteorite about the size of a large mountain. Around the
perimeter of this site -- the second largest known on this planet -- Jeff
Bada and Luann Becker of the Scripps Institution found large deposits of
'buckyballs' -- a hollow spherical arrangement of carbon-60, called
Buckminsterfullerene after the late philosopher-engineer and member of
the Fortean Society Buckminster Fuller and because they resemble the
football-like geodesic dome structures he invented.
As the Sudbury crater has the world's largest known concentration of
soot-like buckyballs Doctors Bada and Becker knew there was some
connection with the visitor from space. At the time, they believed the
buckyballs must have been made in the impact explosion and asked
Robert Poreda of the University of Rochester to ascertain what atoms or
molecules may be trapped within the spheres. To their delight, Dr Poreda
discovered molecules of helium-3 and helium-4 in a ratio that does not
exist within our solar system. This form of helium was almost certainly
forged within a red giant star. At some point, around five billion years
ago, they were trapped within the forming buckyballs which, in turn,
became part of a comet and wandered in space for billions of years
before colliding with Canada.
The temperature of the Sudbury impact is estimated to have been over
5000*C. As carbon molecules begin disintegrating at temperatures
above 1000*C, scientists have been searching for scenarios that would
allow some buckyballs to survive the intense heat of the impact explosion:
these include fragments flying off during entry and the explosion itself
being uneven in temperature.
Whatever the answer, here is a scientifically plausible mechanism for
complex carbon-based molecules to arrive here from deep space. "I had
believed," said Bada, "as most people did, that these [impacts] were just
too energetic for the stuff to survive. Now, all of a sudden, I have a
different view." It may be that archaic notions of the fertilizing forces of
the sky -- eg. tornadoes as the penis of the sky god, according to Dr
Terence Meaden -- may have to be, er, enlarged to include the cosmos
itself.
In answer to the original question, it is now looking more likely that life on
earth was kick-started by some external source than that it began
spontaneously. This external stimulus -- dirty meteorites from Mars, rains
of cosmic dust or buckyballs from the galactic core -- is a constant
process and four billion years ago the chemical conditions on this earth
happened to be just right, the so-called 'Goldilocks' prescription.
However, this 'splash' theory of life, in which is propagated through the
universe by being blasted and wafted from world to world, only displaces
the ultimate origin of life further into the unknown galactic deep. The idea
is not new; indeed Sir Fred Hoyle, in the 1980s, speculated about a
dying alien race seeding the universe with the microscopic building blocks
of life.
The launch this December of the Mars Pathfinder mission, which includes
the Sojourner lander, makes all of this quite topical. The Viking lander of
1976 found no life signs on the surface -- this time around, when
Sojourner touches down on Mars in July 1997, we know enough to drill
deeper into the planetary crust. The first earthman to set foot on Mars
may actually be returning home. |
