Canberra,
AUSTRALIA (4 August 1999)- The discovery of bacterial fossil remnants
more than two billions years old, and of a new form of carbon never before
seen in nature, may both provide clues to the origins of life on our planet.
The Earth has many unique features, among which is an abundance of oxygen.
This element upon which we depend was not abundant here until the appearance
of microorganisms known as cyanobacteria that produce oxygen by photosynthesis.
An Australian research team now reports the discovery of what may be the oldest
remnant of life on Earth, in the form of the biomarker 2-methylhopane. These
compounds, considered a marker for cyanobacteria, were found in 2.5-billion-year-old
sedimentary rocks from the Mount McRae shale in Western Australia.
Above:
Photomicrograph of Cyanidium caldarium, a modern day cyanobacteria found in
the acid hot springs of Yellowstone.
These biomarkers are leftover parts of lipid molecules that once formed the
cell walls of the ancient cyanobacteria. The molecular structure of this particular
biomarker is unique to cyanobacteria. The geological circumstances are such
that the researchers feel certain there could have been no cross contamination
from a later source. These biomarkers are at least 700 million years older
than any previously reported.
"These biomarkers may help constrain the age of the oldest cyanobacteria
and the advent of oxygenic photosynthesis. They could also be used to quantify
the ecological importance of cyanobacteria through geological time,"
notes Roger E. Summons, Australian Geological Survey Organization, Canberra,
Australia.
There is reason to believe that cyanobacteria may go back even farther than
2.5 billion years. Isotopic analysis of the rocks in which the biomarkers
were found indicates they could be as old as 2.8 billion years. Moreover,
it would have taken the cyanobacteria some time to evolve into the form that
left the current fossils. Even after the cyanobacteria appeared, geological
evidence of oxidative weathering on land suggests that there was not a lot
of oxygen in he atmosphere for another 300 million years after the appearance
of the cyanobacteria.
"Many key events in bacterial evolution and the oxidation of the Earth's
surface are still unknown. Biomarker studies such as that of Summons and colleagues
hold great promise in unraveling the timing of evolutionary milestones and
in deducing the structure of ancient ecosystems," said researcher Donald
Canfield of Odense University in Denmark, in a commentary in Nature (Vol 400,
August 5, 1999).
New carbon form found in meteorite
In what may be related research, a University of Hawaii researcher reported
finding in nature a new form of carbon that had previously only been seen
in the laboratory.
Luann Becker and colleagues discovered the presence of fullerenes, one of
three pure forms of carbon, in deposits at the site of a meteroite impact
crater in Ontario, Canada. Unlike diamonds or graphite, the other pure forms
of carbon, fullerenes can be extracted in an organic solvent. Becker crushed
a piece of the Allende meteorite, demineralized the sample with acids, and
used the organic solvent to extract fullerenes from the residue.
Right: Artist's
rendering of a fullerene
Fullerenes, named after science innovator Buckminster Fuller, form unique
molecular clusters. Exobiologists hypothesize that fullerene molecules may
have trapped stellar gasses, and then delivered them to the early Earth where
both the carbon and the gases they contained may have made essential contributions
to the prebiotic planetary atmosphere
"It's not every day that you discover a new carbon molecule in nature; that's
what makes this interesting. If it played a role in how the earth evolved,
that would be important," said Becker.
The cyanobacteria study appears in the August 5, 1999 issue of Nature. The
fullerene research appeared in the July 15, 1999 issue of the British journal
Nature.
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