By Stephen Hart
NASA Astrobiology Institute
In early 1977, John B. Corliss and John M. Edmond rode the deep-sea
submersible Alvin to a discovery that would revolutionize biology. Gliding
just above a moonlike landscape nearly devoid of life, they crested a slope.
Suddenly, Alvin’s headlights illuminated a thriving multi-hued community of
organisms biologists never suspected – six-foot, red-tipped tube worms;
large, white clams; yellow mussels and pale crabs – all living thousands of
feet below the surface of the sea. Scientists later dubbed the site The Rose
Garden.
The most significant color, however, was the one the two scientists didn’t
see: green. This community, occupying a 100-meter-wide area where
volcanically heated water seeped up through the sea floor, relied not on
sunlight for its energy but on decomposition of sulfur compounds pumped from
deep underground by the rising hot water. The organisms that carry on this
process are no ordinary bacteria, but members of a vast group of
single-celled organisms even more primitive than bacteria, called archaea.
The discovery began a dramatic shift in the thinking of biologists and
geochemists studying the origin of life. Within three years, Corliss and
colleagues had published a suggestion that life on Earth may have arisen in
or near hydrothermal systems such as The Rose Garden.
The Soup versus the factory
The suggestion that life may have arisen in hydrothermal systems raises a
number of questions: What about the warm, prebiotic soup we all learned
about in school? What about experiments by Stanley Miller and others in the
50s that replicated an early Earth atmosphere, zapped it with simulated
lightning, and succeeded in forming some of the building blocks of the
molecules of life?
Shock points out, however, that “the starting conditions for those
experiments are now thought to be highly inappropriate, unlikely, virtually
implausible for the early Earth. Actually, geologists and geochemists all
along have been saying that, but it has taken decades to get the point
across.”
The experiments began with a supposed early Earth atmosphere containing
ammonia and methane. But researchers now know that these compounds,
irradiated by the sun and reacting with water vapor in the atmosphere, would
quickly have broken down into relatively inert carbon dioxide and nitrogen
gas. And ammonia and methane likely didn’t enter the atmosphere from
volcanic process on Earth either, Shock adds. “Volcanic gases do not have
methane and ammonia in them to any appreciable extent.”
With no ammonia and no methane there would be no prebiotic soup, no
inventory of organic molecules for early cells to arise from and to eat.
“It’s like everything we knew was wrong,” says Shock.
If life did arise near the surface of the ocean, many researchers believe,
it probably didn’t survive there without interruption. Continued violent
collisions with asteroids and even small planetoids, similar to those that
formed the Earth, continued after the planet’s formation. The largest of
these, although rare, would have heated surface waters to boiling, wiping
out any cells or prebiotic molecules that had gained a foothold.
Hydrothermal systems lying under tons of water, on the other hand, may have
provided a continuous haven for emerging life.
Furthermore many biologists think that the microbes (both bacteria and
archaea) that convert sulfate to hydrogen sulfide in hydrothermal
communities use biochemical processes similar to those used by the planet’s
first organisms.
Finally, hydrothermal systems as the location for the emergence of life
require no “special case” arguments. “They are probably one of the most
common features throughout the entire history of the Earth,” Shock says.
Tiny Bubbles
A few years ago Shock turned from theoretical work to field work at the
hydrothermal systems in Yellowstone National Park. He hopes that by studying
present-day hydrothermal life, he can determine what geochemical signatures
to look for in the most ancient rocks.
An international group of researchers, including Hugh Rollinson, of the
Cheltenham and Gloucester College of Higher Education in the UK, Peter
Appel, of Geological Survey of Denmark, and Jacques Touret, of Vrije
Universiteit, in Amsterdam, claims to have found what looks like just such a
signature.
Isua, on the on the edge of Greenland’s ice cap, holds an outcrop of rock
that appears to be 3.7 to 3.9 billion years old, only about half a billion
years younger than the Earth itself. “When basalt erupts under water,”
Rollinson says, “it forms what we call a pillow lava, a rounded blob of
basalt about half a meter across.” Greenstone results from geochemical
changes occurring to basalt over eons. The geologists combed the Isua
greenstone belt for undistorted pillow formations, blobs that had remained
undisturbed for billions of years.
After slicing the rocks with a diamond saw, Touret studied the slices using
a specially fitted microscope. He saw tiny “bubbles” in the quartz crystals,
and the bubbles, like minute glass globes, contained a fluid, Rollinson
says.
“You see this half-filled inclusion, half water vapor, half liquid water,
sort of wobbling under the effect of heating it up with the microscope
lamp,” Rollinson says.
By systematically heating or cooling the tiny vessels and zapping them with
a laser, the team worked out the composition of the fluid and gas inside.
“These things froze at a very, very low temperature, around minus 90
[degrees Celsius], indicating that they had methane in them,” Rollinson
says. The fluid turned out to be salt water, similar to the water found
spewing from hydrothermal vents.
Other researchers have studied tiny carbon grains embedded in different
rocks found in the Isua formations. By measuring the ratio of carbon
isotopes, these researchers determined that the grains contained carbon of
biological origin. Two common and stable isotopes of carbon, carbon 12 and
carbon 13, occur in a mixture in the Earth’s atmosphere. Biological
processes build organic molecules with a higher carbon 12:13 ratio than
abiotic processes. And because both isotopes are stable, the ratio remains
in all of life’s products, even after billions of years.
Rollinson and his colleagues cannot yet perform carbon 12:13 analyses on the
contents of their bubbles, however; they’re just too small.
He admits that using his Isua evidence alone as evidence that life
originated in hydrothermal systems – or even that their evidence points to
early life – is a leap. “We’re making quite a big leap in linking what we
find with early life. But there are other arguments from other geological
localities that make it look a sensible thing to say.”
“Quite a big leap” may be putting it mildly, asserts Christopher M. Fedo, a
geologist at The George Washington University in Washington, DC. Fedo has
extensively studied the rocks of the Isua formation. “Saline fluid
inclusions are common in geology and not all are from sea water…. There is
absolutely no proof that the rocks in question formed at a hydrothermal
vent, near a hydrothermal vent, or in association with a hydrothermal vent.
Further, there has been no evidence presented that these rocks host life of
any sort, nor can previous reports of potential life be correlated with the
studied samples. Lastly, the extreme deformation state of the samples
studied [changes induced by approximately one billion years of subsequent
intense tectonic activity], coupled with major geochemical changes, renders
any conclusions about sea water or life entirely speculation.”
Rollinson, agrees, to a point. “We agree that our rocks may have been much
modified, but still argue that the inclusions we describe are primary. We
have shown, from the textures preserved in our quartz grains, that some
grains preserve some primary features. Further, the inclusions we describe
are very rare, implying, perhaps, that the original inclusion population was
almost entirely obliterated, but not quite.” Rollinson realizes the
challenges to his team’s ideas are strong. “We have to be very careful that
we distinguish between later water that got into these rocks when they were
reheated and original water that was there when they first formed. And we
think that we can do this.”
The group’s findings are in press and expected to be available soon in
Precambrian Research.
What’s Next
“I think what will happen when our paper comes out and other people get
interested, is that people with access to probably more sophisticated
techniques than we have had will want samples,” Rollinson says. “And we’ll
probably collaborate with people in trying to get some of these gasses
extracted so that we can actually look at them isotopically. That would be
very important…. The discovery of life at Isua implies, I think, that life
was existing on Earth long before Isua times. But we don’t have any
geological record, so we can’t go back any farther to look for that. We may
be able to use biological fingerprinting.”
These fingerprints, the chemical signs left by life, may exist in the
abundant rocks from in or near hydrothermal systems, Shock says, and his
current field research is aimed at learning how to dust for these prints.
Shock hopes to find chemical traces left by archaea and bacteria living in
extremely hot environments, such as Yellowstone. Similar heat-loving
organisms also live in deep-sea hydrothermal vent systems.
Shock thinks these organisms may have left a detectable fossil record. But
in this case, the fossils are chemical traces. “What are you going to look
for? This is the question right now. By studying the active systems that are
supporting hyperthermophiles [heat-loving organisms] you might have a better
idea of what to go after in a fossil record that’s hydrothermal.
“It’s quite plausible to me, I’ve made this argument,” Shock continues,
“that there’s another fossil record of microbial life that’s going to be
perhaps preserved in some of these hydrothermically altered rocks.” This
fossil record may hold more clues to how life arose on Earth.
