Page 207 - Introduction to Paleobiology and The Fossil Record

194 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
Figure 8.7 The oldest fossils on Earth? A mass
of thin thread-like filaments found in a massive
sulfide deposit in Western Australia dated at
3.2 Ga. The fact the threads occur in loose
groups and in tight masses, and that they are not
oriented in one direction, suggests they are
organic. The filaments are lined with minute
specks of pyrite, showing black, encased in chert.
Field of view is 250 μm across. (Courtesy of
Birger Rasmussen.)
a massive sulfide deposit produced in an envi-
ronment like a modern deep-water black
smoker, with temperatures up to 300°C. The
fossils show evidence of recrystallization by
the influx of hydrothermal fl uids, and then
progressive replacement by later sulfi des. The
fossils are thread-like filaments (Fig. 8.7) that
may be straight, sinuous or sharply curved,
and even tightly intertwined in some areas.
The overall shape, uniform width and lack of
orientation all tend to confi rm that these
might really be fossils, and not merely inor-
ganic structures. If so, they confi rm that some
of the earliest life may have been thermophilic
(“heat-loving”) bacteria. Other tubes and
filaments of similar age have been reported,
but many of these are highly controversial.
There is then a long gap in time until the
next generally accepted fossils. These are
diverse fossils of cyanobacteria from the
Campbellrand Supergroup of South Africa,
dated at 2.5 Ga (Altermann & Kazmierczak
2003). The fossils include cell sheaths and
capsules that can be identifi ed with modern
orders of cyanobacteria. There is then a
further long time gap before the next assem-
blage of prokaryote fossils, from the Gunfl int
Chert of Ontario, Canada, dated at 1.9 Ga.
The Gunflint microorganisms include six dis-
tinctive forms, some shaped like fi laments,
others spherical, and some branched or
bearing an umbrella-like structure (Fig. 8.8).
These Precambrian unicells resemble in shape
various modern prokaryotes, and some were
found within stromatolites. Most unusual is
Kakabekia, the umbrella-shaped microfossil
(Fig. 8.8b); it is most like rare prokaryotes
found today at the foot of the walls of Harlech
Castle in Wales. These modern forms are tol-
erant of ammonia (NH 3 ), produced by ancient
Britons urinating against the castle walls; so
were conditions in Gunflint Chert times also
rich in ammonia?
Biomarkers
Even if the oldest fossils are controversial,
paleontologists have been able to identify
another source of information on early life.
These are so-called biomarkers, organic chem-
ical indicators of life in general, and of par-
ticular sectors of life. Most biomarkers are
lipids, fatty and waxy compounds found in
living cells. For a long time, the oldest accepted
biomarkers dated from 1.7 Ga, but Brocks et
al. (1999) reported convincing examples from
organic-rich shales in Australia dated at
2.7 Ga. The biomarkers they identifi ed were
not only 1 billion years older than previous
examples, they also proved a wider diversity
of life at that time than anyone had
suspected.
The 2.7 Ga biomarkers were of two types.
First were indicators of cyanobacteria, as
might be expected. Brocks and collea-
gues identifi ed 2-methylhopanes, which
are known to be breakdown products of
2-methylbacteriohopanepolyols, specialized
lipids that are only found in the membranes
of cyanobacteria. The investigators also,
unexpectedly, identifi ed C28–C30 steranes,
which are sedimentary molecules derived
from sterols. Such large-ring sterols are syn-
thesized only by eukaryotes, and not by pro-
karyotes. Moreover, the biochemical synthesis
of such large sterols requires molecular
oxygen, so that the eukaryotes likely lived in
proximity to oxygen-producing cyanobacte-
ria, strengthening the interpretation of the 2-
methylhopanes. So, this biomarker evidence
confirms the existence of cyanobacteria at