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Frontiers
Greenland
Rocks Tell New Tale of Life on Earth
March 1997
It may not have had
fingers and toes, nonetheless it was life, according to
NSF-supported geochemist Mark Harrison of the University
of California, Los Angeles (UCLA). He is describing evidence
of ancient life found in a rock formation on Akilia Island
in West Greenland. Harrison and colleagues recently made
a ground-breaking discovery, one that pushed the age of
the oldest known life on Earth back almost 400 million
years, to some 3.86 billion years ago.
The scientists -- from UCLA, University of California,
San Diego's Scripps Institution of Oceanography and
other universities -- say their research has provocative
implications. "Our evidence establishes beyond reasonable
doubt that life emerged on Earth at least 3.86 billion
years ago," says Stephen Mojzsis, a graduate student
in geochemistry at Scripps and lead author of a recent
paper on the subject in the scientific journal Nature. "This
is not the end of the story. We may well find that
life on Earth existed even earlier."
Explains Harrison, "We look in rocks like the ones
from Akilia Island for a chemical suggestion of life;
in this case, we found it. Although it would be wonderful
to see a head and toes, while we don't have those,
we have found very strong evidence for ancient life
in these rocks."
Scripps senior scientist Gustaf Arrhenius is the principal
investigator for the project and Mojzsis' research
advisor. He adds, "We don't necessarily need fingers
and toes. As long as there is no other known way for
nature to achieve the strong carbon isotope change
found in the rock from Akilia Island, this change provides
the best evidence for the existence of life."
The life, according to Arrhenius and colleagues, was
located in an unusual rock formation in what is now
a desolate part of Greenland. The site, known to be
3.85 billion years old, is the oldest known collection
of rocks on Earth.
The team, which included scientists from the Australian
National University and Oxford Brooks University in
England, as well as Scripps and UCLA, used funding
from the National Aeronautics and Space Administration
to collect the samples.
Back in the United States, carbon in the Akilia Island
rock was analyzed with UCLA's high-resolution ion microprobe,
an instrument that enables scientists to determine
the exact composition of samples. Mojzsis describes
the instrument, developed with support from NSF, the
W. M. Keck Foundation and UCLA, as "the world's best
instrument for this kind of research." The microprobe
directs a beam of ions -- charged atoms -- at a sample.
The sample then releases its own ions, which are in
turn analyzed in an instrument called a mass spectrometer.
Scientists can aim the beam of ions at specific microscope
areas of a sample and analyze the results.
Harrison believes the recent research was in part
the result of a jump in technology. "It was a scientific
problem awaiting a new-generation microprobe with a
high level of resolution. No other instrument would
have been sensitive enough to reveal the exact isotopic
composition of carbon in the rock."
"Major advances in comprehending Earth and the solar
system have followed development of new analytical
tools like this one," adds Dan Weill, program director
in NSF's Division of Earth Sciences, which supports
the ion microprobe facility. The advent of high-precision
mass spectrometry, says Weill, led to new research
on isotope variations. "The results of that research
have forever altered our views of the origin and evolution
of Earth, its moon and the solar system. Similar advances
are now occurring as conventional high-precision mass
spectrometry is augmented by new-generation ion microprobes
capable of working on microscales."
The ion microprobe facility at UCLA provides access
to scientists conducting research in many earth sciences
fields, "allowing NSF, the Keck Foundation and UCLA
to leverage their various contributions to this facility
into a whole with much more to offer than any one of
these three funders could have supported alone," says
Weill.
To the Akilia Island rock researchers, the ion microprobe
provided one of the most important pieces of evidence:
a high ratio of one isotope, or type of carbon, to
another. Carbon atoms come in two varieties: carbon-12,
with six protons and six neutrons, and carbon-13, with
six protons and seven neutrons. Since living organisms
use the lighter carbon-12, rather than the heavier
carbon-13, a lump of carbon that has been processed
by a living organism has more carbon-12 atoms than
one found in other places in nature.
The researchers found that the ratio of carbon-12
to carbon-13 was three percent higher than would be
expected if life were not present, says Mojzsis, "and
that three percent is a very large amount."
They also found carbon in a phosphate mineral called
apatite, the material that makes up bones and teeth.
Apatite is often formed through interactions with microorganisms,
but can also be formed inorganically. The association
of the carbon with the apatite is "suggestive," says
Arrhenius, "but does not in itself establish the existence
of life." However, when linked to the evidence of excessive
carbon-12, the researchers are confident that these
rocks hold the remnants of ancient life.
The form of life discovered was once likely a simple
microorganism, although its actual shape or nature
cannot be ascertained. Over time, heat and pressure
have destroyed its original physical structure, "which
was likely a 'blob' even then," says Mojzsis.
It's unknown exactly when life first appeared on Earth, but it's thought
it was about 4.5 billion years ago. The previous earliest evidence for life
showed that, on the basis of bacteria-like fossils, primitive plant life
that looks much like modern "pond scum," existed on Earth some
3.46 billion years ago.
"The evolution of lifeless matter into primitive life forms, and
their organization into the complex structure of cells, represents an enormous
leap, one that happened long before the Akilia sediments came into being,"
says Arrhenius.
The residues of ancient life the scientists discovered existed prior
to the end of the heavy bombardment of the moon by large objects, approximately
3.8 billion years ago. The implication, says Harrison, is that the often
assumed simultaneous bombardment of Earth did not lead to the extinction
of life, if it existed prior to that time in the Akilia Island sediments
and likely elsewhere.
The new research claims that life on Earth began during the first 700
million years of the planet's existence. This places an upper limit on the
time needed for the creation of life, or on the time period available for
it to arrive from elsewhere.
The claims are meeting both encouragement and caution from other parts
of the scientific community. William Schopf, discoverer of the earliest
form of physical fossils and director of the UCLA Center for the Study of
Evolution and Origin of Life, told the Los Angeles Times that the findings
were "very significant."
John Hayes, a leading researcher in the chemistry of early Earth who
works at Woods Hole Oceanographic Institute, told the New York Times that
the Nature paper was "good science," but adds, "There are
potential processes other than life that could account for the Arrhenius
isotope results, and even though they are long shots, we cannot entirely
rule them out."
However, Mojzsis says life is everywhere. "Life is tenacious, and
it completely permeates the surface layer of the planet. We find life beneath
the deepest ocean, on the highest mountain, in the driest desert, on the
coldest glacier, and deep within rocks and sediments.
"Since we don't know for sure what conditions are needed for the
emergence of life, it's only possible to speculate about exactly when it
first began on earth, or its existence elsewhere in the universe."
An important contribution to the answer to this latter question, he says,
could come from exploration of the surface of Mars for traces there of extinct
life.
An equally interesting question being studied here on Earth, says Arrhenius,
is how life could originally arise from lifeless molecules and evolve into
the forms recorded in the Akilia Island rocks. They may have been blob-like,
and not have had fingers and toes, but, says the scientist, "they were
pretty sophisticated nonetheless."
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