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in diamond. It also characterizes the degree
of crystalline order in solids. However, it
does not provide a list of elemental compo-
nents or even ratios of carbon, hydrogen
and oxygen, which might help to establish
the biogenicity of a compound.
Most pertinent to the analysis of putative
microfossils is the fact that the Raman spec-
trum of carbonaceous (that is, carbon-dom-
inated) materials is sensitive to the degree of
ordering of the carbon they contain (distin-
guishing, for example, between amorphous
carbon, poorly ordered graphitic material
and highly crystalline graphite)3–10. The
Raman spectra of Schopf et al.1confirm that
their samples consist of highly disordered
carbonaceous material and are consistent
with the spectra of kerogens3,4. However,
their spectra are indistinguishable from
those of many other types of structurally
disordered carbonaceous matter generated
from a wide range of starting materials by a
wide variety of processes. Those processes
(including high-temperature heating of
organic or inorganic compounds4–7, inor-
ganic deposition from high-temperature
synthetic fluids3–5 and geological deposition
from hydrothermal solutions8) and materi-
als (for example, ion-bombarded graphite9
and graphite-intercalation compounds10)
may be strictly non-biogenic. There are no
distinctive features in the spectra shown by
Schopf et al.1that directly and unambigu-
ously link them to kerogens.
Raman microprobe spectroscopy is useful
for investigating the molecular structure of
micrometre-sized features, such as putative
microfossils, in rock. Showing that fossil-
like objects consist of highly disordered
carbonaceous material by Raman spec-
troscopy provides necessary, but not suffi-
cient, evidence that the objects are biogenic.
Although the microscopic objects analysed
by Schopf et al.1may indeed be biogenic, we
see nothing in their spectra that indicates
the origin of their disordered carbonaceous
material. The basic question remains unans-
wered: which measurable chemical and/or
physical properties of a fossilized and/or
altered material will unambiguously identify
it as biological in origin?
Jill Dill Pasteris, Brigitte Wopenka
Department of Earth and Planetary Sciences,
Washington University, St Louis,
Missouri 63130-4899, USA
e-mail: pasteris@levee.wustl.edu
1. Schopf, J.W., Kudryavtsev, A. B., Agresti, D. G., Wdowiak, T. J.
& Czaja, A. D. Nature 416, 73–76 (2002).
2. Brasier, M. D. et al. Nature 416, 76–81 (2002).
3. Wopenka, B. & Pasteris, J. D. Am. Mineral. 78, 533–557 (1993).
4. Beny-Bassez, C. & Rouzaud, J. N. Scan. Electr. Microsc.
119–132 (1985).
5. Vidano, R. & Fischbach, D. B. J. Am. Ceram. Soc. 61,
13–17 (1978).
6. Sato, Y., Kamo, M. & Setaka, N. Carbon 16, 279–280 (1978).
7. Lespade, P., Al-Jishi, R. & Dresselhaus, M. S. Carbon 20,
427–431 (1982).
8. Pasteris, J. D. in Applications of Microanalytical Techniques to
Understanding Mineralizing Processes (eds McKibben, M. A.,
Shanks, W. C. & Ridley, W. I.) 233–250 (Soc. Econ. Geol.,
Littleton, Colorado, 1998).
9. Elman, B. S., Shayegan, M., Dresselhaus, M. S., Mazurek, H. &
Dresselhaus, G. Phys. Rev. B 25, 4142–4156 (1982).
10.Dresselhaus, M. S. & Dresselhaus, G. in Light Scattering in Solids
III 2–57 (Springer, New York, 1982).
Schopf et al. reply — The criticism by
Pasteris and Wopenka of our use of
laser–Raman imagery to investigate the
carbonaceous make-up of extremely
ancient fossils1focuses only on their Raman
signature; however, our interpretation that
the carbonaceous matter that makes up
these specimens is biogenic is based on
several lines of evidence, of which Raman
spectroscopy is only one.
We did not state, nor did we imply, that
Raman spectroscopic analysis can by itself
be used to establish the biological origin of
geochemically highly altered carbonaceous
matter present in ancient sediments. We
believe that the biogenicity of such matter,
whether in fossil-like objects or sapropel-like
detritus, should be demonstrated by a com-
bination of data drawn from independent
but mutually reinforcing lines of evidence.
For fossils in each of the four geological
units we analysed1— including those of
the roughly 3,375-million-year (Myr)-old
Kromberg Formation and 3,465-Myr-old
Apex Chert, which are among the oldest fos-
sils known — three lines of evidence are most
compelling. These are their cellular mor-
phology2,3, their carbonaceous molecular-
structural make-up1–4, and the carbon isotope
composition of such fossils5and/or of
co-existing particulate kerogen6,7, which have
been shown by replicate analyses5–7 to be well
within the range established for Precambrian
biological organic matter on the basis of over
1,200 measurements from hundreds of fossil-
bearing units7.
Our study1, which focuses on the first
two of these lines of evidence, is centred
on the use of laser–Raman imagery (rather
than on more conventional single-point
measurements), a technique new to palaeo-
biology1,4. We showed that there is a one-
to-one correlation of cellular morphology
and carbonaceous make-up in individual
microscopic fossils from each of the four
units investigated. Our claim is that such
an analysis based on a combination of
morphology and chemistry together provides
a powerful new means to investigate the
biogenicity of putative fossil-like objects, a
problem that for many decades has plagued
the search for evidence of early life8.
J. William Schopf*, Anatoliy B. Kudryavtsev†,
David G. Agresti‡, Thomas J. Wdowiak‡,
Andrew D. Czaja*
*Department of Earth & Space Sciences, and
Institute of Geophysics & Planetary Physics,
University of California, Los Angeles,
California 90095-1567, USA
e-mail: schopf@ess.ucla.edu
brief communications
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†Department of Physics, and ‡Astro and Solar
System Physics Program, Department of Physics,
University of Alabama, Birmingham,
Alabama 35294-1170, USA
1. Schopf, J. W., Kudryavtsev, A. B., Agresti, D. G., Wdowiak, T. J.
& Czaja, A. D. Nature 416, 73–76 (2002).
2. Schopf, J. W. in The Proterozoic Biosphere, a Multidisciplinary
Study (eds Schopf, J. W. & Klein, C.) 25–39, 1055–1117
(Cambridge Univ. Press, New York, 1992).
3. Schopf, J. W. Science 260, 640–646 (1993).
4. Kudryavtsev, A. B., Schopf, J. W., Agresti, D. G. & Wdowiak, T. J.
Proc. Natl Acad. Sci. USA 416, 73–76 (2002).
5. House, C. H. et al. Geology 28, 707–710 (2000).
6. Strauss, H. & Moore, T. B. in The Proterozoic Biosphere, a
Multidisciplinary Study (eds Schopf, J. W. & Klein, C.) 709–798
(Cambridge Univ. Press, New York, 1992).
7. Strauss, H., Des Marais, D. J., Hayes, J. M. & Summons, R. E.
in The Proterozoic Biosphere, a Multidisciplinary Study
(eds Schopf, J. W. & Klein, C.) 117–127 (Cambridge Univ.
Press, New York, 1992).
8. Schopf, J. W. & Walter, M. R. in Earth’s Earliest Biosphere, its
Origin and Evolution (ed. Schopf, J. W.) 214–239 (Princeton
Univ. Press, Princeton, New Jersey, 1983).
COMMUNICATIONSARISING
Palaeontology
Thermal alteration of
the Earth’s oldest fossils
Microscopic carbonaceous structures
found in ancient rocks could
provide clues to early life on Earth if
they turn out to be genuine fossil micro-
organisms. Here we show that thermal
alteration of microbial remains embedded
in a mineral matrix may significantly change
their original morphology and produce
structures that resemble those of what are
claimed to be the Earth’s oldest fossils1.
These observations may shed light on the
controversy2,3 that surrounds these micro-
fossils from the 3,465-Myr-old Apex Chert
of the early Archaean Warrawoona Group
in northwestern Australia.
The biogenicity of these fossils has been
called into question3on the basis of
suggestions that the Apex Chert structures
were formed from amorphous graphite
within multiple generations of metalliferous
hydrothermal-vein chert and volcanic glass,
and that the carbonaceous composition and
characteristically biological carbon-isotope
make-up of the carbonaceous filaments
could have been products of non-biological
(Fischer–Tropsch) organic synthesis3.
We have investigated structures that are
present in silicified (chertified) cyano-
bacterial mats from the Bardo Mountains
(Z
•danów locality4) in southwestern Poland,
which date to the early Silurian period
(about 440 million years ago). The fossil
mats occur in black, laminated radiolarian
cherts, which have been interpreted as
sediments that formed at moderate depths
within the photic zone5. The mats are
composed of cyanobacteria that are closely
related to representatives of modern
colonial chroococcaleans (particularly the
© 2002 Nature Publishing Group
families Entophysalidaceae and Xenococ-
caceae6). Living colonies of these cyanobac-
teria are composed of globular subcolonies
surrounded by thick mucous envelopes
(Fig. 1a). The subcolonies are composed of
minute cells (which in some species are less
than 2 m in diameter).
Post mortem degradation processes in
modern mats composed of coccoid
cyanobacteria7indicate that the components
that are most resistant to decay are the thicker
outer mucous envelopes that surround
groups of cells, subcolonies and entire
colonies. After burial, these partially bio-
degraded envelopes often remain preserved
as a cobweb-like polysaccharide material.
With time and progressive diagenesis, this
material may undergo kerogenization and
be transformed from a more-or-less struc-
tured biological material into amorphous
organic matter.
The early-Silurian cyanobacterial mats
we describe represent a kerogenized stage
in which the outlines of the subcolonies and
of even smaller groups of densely packed,
minute cells are still recognizable in the
chert matrix (Fig. 1e). Owing to com-
paction, the circular outlines of the
subcolonies are best seen in petrographic
thin sections made parallel to the bedding.
The subcolonies reach 60–90 m in
diameter and occur as blackish, cobweb-like
structures (Fig. 1b; and see Fig. 1c in supple-
mentary information) or as yellow–brown
circular or semicircular areas, which are
bordered by blackish, continuous or discon-
tinuous, irregularly segmented or porous
zones (Fig. 1c, d; and see Fig. 1a, b in
supplementary information).
The densely packed masses of small bod-
ies that fill the interiors of many subcolonies
(Fig. 1e) are likely to be remnants of cells
that created the original colonies. In vertical
thin sections, the blackish material is usually
present as slightly curved or undulated,
often segmented, filamentous structures
(Fig. 1f) which are locally branched (see Fig.
2 in supplementary information).
All of these features make the blackish
structures almost identical in appearance to
the filamentous structures described from
the Apex Chert. This similarity is particu-
larly striking when the shapes of Archaean
structures are compared to the porous and
irregularly segmented structures from the
peripheries of the Silurian cell aggregates.
Their quasi-circular, C- and J-shaped out-
lines fit almost perfectly the morphologies
described for the Apex structures1,3. The
same is true for the size classes of both
groups of structures.
The blackish, pseudo-filamentous Silu-
rian structures probably represent thermally
altered, kerogenized remains of coccoid
cyanobacterial mats. The Silurian deposits
in the Bardo Mountains were influenced by
Caledonian and/or Variscic thermal events8,
as evidenced by generations of microcracks
filled with hydrothermally precipitated
microcrystalline chalcedony and quartz.
The cyanobacterial remains distributed in
the vicinity of these cracks have been
markedly altered, but still preserve traces of
their primary biological structure.
Thermal alteration was apparently more
advanced in the Apex Chert samples,
leaving only isolated fragments in the
chert background of much-changed
(‘carbonized’) kerogen, and preserved
pseudo-filamentous ghosts of the original
biostructures. The generation of gaseous
and bituminous hydrocarbons associated
with thermal maturation and the conver-
sion of kerogenous materials9could have
been responsible for the apparent reduction
in volume and partial relocation of the
cyanobacterial material.
Several inferences can be drawn from
our observations of chertified and slightly
thermally altered early-Silurian cyano-
bacterial mats. First, the early Archaean
Apex Chert filaments may have originated
through late diagenetic (thermal or thermo-
baric) in situ alteration of kerogenized
remnants of mucilage sheaths, capsules
and extracellular polymer substances that
originally enveloped groups of coccoid cells.
Second, the Apex Chert microfossil-like
filamentous structures could therefore be
biogenic but may represent diagenetic
ghosts of benthic mats composed of colonial
microorganisms resembling some modern
chroococcalean cyanobacteria. Third, what
have been described from the Apex Chert as
11 filamentous microbial taxa1may rather
represent remnants of a homogeneous (and
most probably monospecific) microbial
community, similar to modern benthic coc-
coid cyanobacteria, that is also known from
later Precambrian and Phanerozoic strata.
Józef Ka´zmierczak, Barbara Kremer
Institute of Paleobiology, Polish Academy of
Sciences, 00818 Warszawa, Poland
e-mail: jkaz@twarda.pan.pl
1. Schopf, J. W. Science 260, 640–646 (1993).
2. Dalton, R. Nature 417, 782–784 (2002).
3. Brasier, M. D. et al. Nature 416, 76–81 (2002).
4. Wyz
•ga, B. Geologia Sudetica 22, 119–145 (1987).
5. Kremer, B. in Early Palaeozoic Palaeogeographies and
Biogeographies of Western Europe and North Africa (eds Alvaro,
J. J. & Servais, T.) 36 (Univ. Sci. Technol., Lille, 2001).
6. Komárek, J. & Anagnostidis, K. Arch. Hydrobiol./Algolog. Stud.
43, 157–226 (1986).
7. Horodyski, R. J. & Vonder Haar, S. J. Sedim. Petrol. 45,
894–906 (1975).
8. Aleksandrowski, P., Kryza, R., Mazur S., Pin, C. & Zalasiewicz,
J. A. Trans. R. Soc. Edinb. 90, 127–146 (2000).
9. Tissot, B. P., Pelet, R. & Ungerer, Ph. Am. Assoc. Petrol. Geol.
Bull. 71, 1445–1466 (1987).
Supplementary information accompanies this communication on
Nature’s website.
brief communications
478 NATURE
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Figure 1 Light micrographs of mats of modern and early Silurian coccoid cyanobacteria. a, Modern mat of benthic coccoid cyanobacteria
from Sulejów Dam, central Poland; the mat is composed of globular colonies surrounded by thick mucous envelopes (arrow).
b–f, Examples of variously thermally altered colonies of Silurian coccoid cyanobacteria from Z
•danów in the Bardo Mountains, Poland;
these were originally composed of minute cells (magnified in e). Note the quasi-regular, segmented, blackish structures of thermal origin,
which are strikingly similar to the purported Archaean Apex Chert microfossils. Scale bars: a, 50 m; b–d, f, 20 m; e, 5 m.
© 2002 Nature Publishing Group
