LIFE ON EARTH BEGAN AT LEAST 3.85 BILLION YEARS AGO
Wed, 6 Nov 1996 16:50:29 -0500
Source: NASA HQ Public Affairs Office
Donald Savage
NASA Headquarters, Washington, DC November 6, 1996
(Phone: 202/358-1547)
Stuart Wolpert
UCLA, Department of Earth and Space Sciences
(Phone: 310/206-0511
Cindy Clark
Scripps Oceanographic Institute, San Diego, CA
(Phone: 619/534-1294)
Cheryl Dybas
National Science Foundation, Arlington, VA
(Phone: 703/306-1070)
RELEASE: 96-230
LIFE ON EARTH BEGAN AT LEAST 3.85 BILLION YEARS AGO, 400
MILLION YEARS EARLIER THAN PREVIOUSLY THOUGHT, SCIENTISTS SAY
Life on Earth began at least 3.85 billion years ago,
an international team of scientists reports in the cover
story of the Nov. 7 issue of the journal Nature.
The scientists, from UC San Diego's Scripps
Institution of Oceanography, UCLA's Department of Earth and
Space Sciences, the Australian National University and
England's Oxford Brookes University, present evidence that
pushes back the emergence of life on Earth by 400 million years.
The evidence comes from a rock formation discovered on
Akilia Island in southern West Greenland that is at least
3.85 billion years old. The research -- funded primarily by
the National Science Foundation and NASA -- has provocative
implications.
"Our evidence establishes beyond reasonable doubt that
life emerged on Earth at least 3.85 billion years ago, and
this is not the end of the story," said Stephen J. Mojzsis, a
graduate student in geochemistry at Scripps and the lead
author of the article. "We may well find that life existed
even earlier."
"We look in rocks like this for chemical suggestions
and isotopic evidence, and we found both," said T. Mark
Harrison, professor of geochemistry at UCLA and director of
UCLA's W.M. Keck Foundation Center for Isotope Geochemistry.
"It would be wonderful to see a head and toes, and while we
don't have those, we have found very strong isotopic evidence
for ancient life."
"But in the cases of Earth's most ancient rocks and
minerals, we are actually better off relying on this type of
isotopic evidence -- chemofossils -- rather than on the shape
of life-like objects with which nature has often been
deceiving the unwary," said Gustaf Arrhenius, professor of
oceanography at UC San Diego and principal investigator for
the research project.
The carbon inclusions in the rock were analyzed with
UCLA's high-resolution ion microprobe -- an instrument that
enables scientists to learn the exact composition of samples
-- which Mojzsis described as the "world's best instrument"
for this research. The microprobe shoots a beam of ions --
charged atoms -- at a sample, releasing from the sample its
own ions that are analyzed in a mass spectrometer. Scientists
can aim the beam of ions at specific microscopic areas of a
sample and analyze them.
The team of scientists, Mojzsis; Arrhenius, who is his
research adviser; Harrison; Kevin McKeegan, a researcher in
UCLA's Department of Earth and Space Sciences; Allen Nutman,
a research fellow at the Australian National University; and
Clark Friend, a geologist at Oxford Brookes University,
presents the following evidence for the ancient life:
· Most importantly, a high ratio of one form -- an isotope
-- of carbon to another, which provides a "signature of
life," Mojzsis said. The carbon aggregates in the rock
have a ratio of about 100 to one of 12C (the most common
isotope form of carbon, containing six protons and six
neutrons) to 13C (a rarer isotopic form of carbon,
containing six protons and seven neutrons). "The light
carbon, 12C, is more than three percent more abundant than
scientists would expect to find if life were not present,
and three percent is, in this case, a very large amount,"
Arrhenius said;
· The inclusion of the carbon in a phosphate mineral called
apatite, which is also the material of which bones and
teeth are made. Apatite is often formed by microorganics,
but it can also be formed inorganically. The association
of the carbon with the apatite is "suggestive, and not
surprising, but does not in itself establish life,"
Arrhenius said.
The form of life discovered was probably a simple
micro-organism, although its actual shape or nature cannot be
ascertained, Mojzsis said, because heat and pressure over
time have destroyed any original physical structure of the
organisms.
Harrison, who directs UCLA's ion microprobe, said of
the research, "This was a scientific problem that was waiting
for a new generation microprobe of this resolution. The
individual samples are very small, and no other instrument
would have been sensitive enough to reveal precisely the
isotopic composition and location of the carbon inclusions in
the rock."
It is unknown when life first appeared on Earth, which
is approximately 4.5 billion years old. The previous earliest
evidence for life was presented by UCLA paleobiologist J.
William Schopf, who showed that on the basis of bacteria-like
fossils, primitive life, much like modern "pond scum,"
existed on Earth 3.46 billion years ago. "The evolution of
lifeless matter into primitive life forms, and their
organization into the complex structure of cells like those
found by Schopf, represent an enormous development in the
earliest history before the deposition of the Akilia
sediments," Arrhenius said.
The residues of ancient life that the scientists have
discovered existed prior to the end of the "late heavy
bombardment" of the Moon by large objects, which ended
approximately 3.8 billion years ago, Harrison said. The
implication, he added, is that the often assumed simultaneous
bombardment of Earth did not lead to the extinction of life.
This research shows that life on Earth began during
the first approximately 700 million years after the formation
of the planet, placing an upper limit on the time needed for
the creation of life on Earth, or on the time period
available for it to arrive here from elsewhere, the
scientists said.
"Life is tenacious, and it completely permeates the
surface layer of the planet," Mojzsis said. "We find life
beneath the deepest ocean, on the highest mountain, in the
driest desert and the coldest glacier, and deep down in the
crustal rocks and sediments. Not knowing what conditions are
needed for the emergence of life, it is only possible to
speculate about its existence elsewhere in the universe. An
important contribution to the solution of this problem could
come from exploration of the surface of Mars for traces there
of extinct life."
An equally interesting question, the scientists
agreed, that is currently studied in laboratories on Earth is
how life originally could have arisen from lifeless
molecules, and evolved into the already sophisticated isotope
fractioning life forms recorded in the Akilia rocks.
Mojzsis' research is supported by a graduate
fellowship from the NASA Specialized Center for Organized
Research and Training (NSCORT) in Exobiology, which is
located at Scripps Institution of Oceanography. Arrhenius has
received support from NASA's Exobiology Office, from NASA
NSCORT and from the NSF (Earth Sciences). Harrison's ion
microprobe research is supported by a grant from the NSF's
Instrument and Facilities Program. Nutman's research has been
supported by the Carlsberg Foundation and the Danish Natural
Science Research Council. Friend's research has been
supported by the Oxford Brookes University and the Royal
Society of London.
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