The Gunflint Chert - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 12

flourished. Such Gunflint-type micro-
biotas have since been recorded from
coeval formations in other parts of the
world (for example, the Duck Creek
Dolomite of Western Australia; Knoll and
Barghoorn, 1975), suggesting that they
were cosmopolitan and representative of
the level of evolution that had been
reached 2 billion years ago, rather than
being just a local phenomenon.
However, soon after this time the
Gunflint-type bacteria almost became
extinct, not due to competition with
cyanobacteria, but due simply to the loss
of their habitat once the ferrous iron had
been flushed from the early oceans by the
process of oxidization. Never again would
organisms that neither use nor produce
oxygen be able to dominate Earth's
ecosystems in the manner of the Gunflint
microbes.

Oxford University has recently cast doubt
on the microfossils (Brasier et al ., 2002),
suggesting that they are merely chains of
crystals formed in a hydrothermal vein.
Schopf still insists that the structures
contain organic matter, but evidence of
life at Warrawoona must remain doubtful
unless further, more convincing
microfossils are found from these rocks.
However, as Knoll (2003) points out,
carbon and sulfur isotopes do suggest that
photosynthesizing and sulfate-reducing
bacteria did live in the Warrawoona

lagoon.

Barberton Greenstone Belt, near
Kruger Park, South Africa
This is the only other place in the
world where rocks approaching 3,500
million years old show fossil evidence of
early life. The Fig Tree Chert (dated
between 3,200 and 3,400 million years
old) contains stromatolites of uncertain
origin, and spherical and filamentous
microstructures, 2-4 microns in diameter
within cherts. This is the correct size for
cyanobacteria, the structures are made of
organic matter and possess an outer wall.
However, nonbiological processes can
produce similar structures and, like
Warrawoona, these presently remain in
doubt.

C OMPARISON OF THE G UNFLINT
C HERT WITH OTHER
P RECAMBRIAN MICROBIAL BIOTAS
North Pole, Western Australia
The 3,500 million year old Apex Chert of
the Warrawoona Group, which outcrops in
the Marble Bar region of Western
Australia, contains the earliest fossil
evidence of life on this planet. Domed
stromatolites were first discovered at
Warrawoona in 1980 by Don Lowe (of
Stanford University) and by Roger Buick
and his colleagues (then of Harvard);
then in 1987 more convincing evidence
was found when Bill Schopf of UCLA
discovered tiny microfossils in the cherts
of Chinaman's Creek, near North Pole.
These fossils, 1-20 microns in diameter
and a few hundred microns long, looked
like simple cyanobacterial filaments
(Schopf and Packer, 1987; Schopf, 1993)
and for a decade these were accepted as
the earliest evidence of life on Earth.
However, doubt has since been cast
on both the stromatolites and
microfossils. The stromatolites have been
reinterpreted as chemically deposited
layers, which have been deformed
tectonically. Moreover, Martin Brasier of

Somerset Island, Arctic Canada
The cherts of the Hunting Formation
of Somerset Island, arctic Canada are
much younger than Gunflint, dated at
1,200 million years old. These stratified
cherts within intertidal to supratidal
carbonates contain fossils of a well-
preserved bangiophyte red alga (sea-
weed) belonging to the extant genus
Bangia . This is the first undisputed
eukaryote to be recorded in the fossil
record and shows us that multicellular
algae diversified well before the Ediacaran
radiation of animals (Chapter 2). Found
with the cyanobacterium, Polybessurus ,
and with stratiform stromatolites, the
algae have a mean diameter of 26 microns
and form unbranched filaments with