Geoscience Reference
In-Depth Information
4
inorganic carbon
graphite
3
2
1
0
-25
-20
-15
-10
-5
0
δ
13
C
Figure 6.3. Isotopic composition of graphite and inorganic carbon from the Isua sedimen-
tary rocks. (the frequency distribution of analyses only applies to the graphite.)
with life, as is the geologic setting. Organic matter settled from the upper
waters of the ocean as we would expect from organisms fixing CO
2
in
the upper ocean. Indeed, if not life, it's hard to imagine where the or-
ganic matter would have come from.
11
Though we didn't get details
about the specific organisms, we have pretty good evidence that by Isua
times, some form of life was making organic matter from CO
2
in the
upper ocean.
Can we turn elsewhere in our search for cyanobacteria? How about
fossils? Do we see anything in ancient fossils that look like cyanobacte-
ria? We used to think so. Bill Schopf of UCLA is most famous for de-
scribing fossil-like structures from the approximately 3.5-billion-year-old
Apex Chert of Western Australia. Examples of these are shown in
igure
6.4.
Although not terribly well preserved, many of these structures
appear to consist of multiple cells arranged in filaments (so-called tri-
chomes), and the filaments are big (by microbial standards), ranging up
to over 50 microns (0.05 millimeters) in length. Bill has made several
compilations of the sizes of different microbial groups, and cyanobacte-
ria tend to be bigger than most. Indeed, they dominate in a range of
sizes that match those observed for these Apex fossils. Assembling this
information, Bill concluded that these fossils represent “probable” cya-
nobacteria; though not proof of cyanobacteria, the fossils were taken
as pretty compelling evidence. I say not proof for at least two reasons.
