Nora Noffke
Old Dominion University, Norfolk, Virginia
An astrobiologist considers life's oldest oxygen.
The presence of atmospheric oxygen would have been necessary for the evolution of eukaryotes — organisms that group their genetic material into a membrane-bounded nucleus — so the question of when oxygen first became available is important in dating their rise. The availability of such oxygen is linked to the evolution of cyanobacteria, oxygen-producing microbes that appeared early in Earth's history and exist to this day.
Fossil microbial mats preserved in the Pongola Supergroup, a rock succession in South Africa, suggest that cyanobacteria were already highly diverse 2.9 billion years ago. But conclusive proof of their presence can be provided only by the presence of hydrocarbon biomarkers — stable chemical compounds found in the walls of single-celled organisms.
Work by Jacob Waldbauer at the Woods Hole Oceanographic Institution in Massachusetts and his colleagues focuses on biomarkers from shallow-marine deposits in the younger, 2.6-billion-year-old sedimentary rocks preserved in South Africa's Transvaal Supergroup. Detailed laboratory analyses extracted biomarkers called hopanes, possibly attributable to cyanobacteria, as well as steranes, biomolecules typically found in eukaryotes (J. R. Waldbauer et al. Precamb. Res.10.1016/j.precamres.2008.10.011; 2008). The biosynthesis of steranes requires free oxygen; therefore, the fossil steranes imply that oxygen was readily available 2.6 billion years ago. This is at least 200 million years before a persistent oxygen-containing atmosphere is thought to have arisen.
Waldbauer et al. show that cyanobacteria had colonized the floor of Earth's ancient oceans by 2.6 billion years ago at the latest. Free oxygen has been available in the atmosphere ever since, and set the stage for the evolution of more complex organisms.
Comments
Nora Noffke is wrong; the presence of atmospheric oxygen may predate the evolution of eukaryotes by hundreds of millions of years. It is the result of the evolution of cyanobacteria. The evolution of eukaryotes has rather followed the rise of oxygen, as the eukaryotes adapted to oxygen which increasingly competed with CO2 for binding sites in the Rubisco molecule (Knoll, A.H., 2003. The geologic consequences of evolution. Geobiology 1, 3-14). The stem group eukaryotes must have been anaerobes, as argued already by Woese, C.R. and Fox, G.E., 1977 (The concept of cellular evolution. Journal of Molecular Evolution 10, 1-6).
As to the paper by J. R. Waldbauer et al., Precambrian Research;10.1016/j.precamres.2008.10.011; 2008, it is an interesting, detailed and meticulous work. Unfortunately, it completely fails in terms of scientific fairness, painstakingly avoiding to cite the first reports of cyanobacteria from the Transvaal Supergroup. These reports are in chronological order:
Lanier, W.P., 1986. Approximate growth rates of Early Proterozoic microstromatolites as deduced by biomass productivity. Palaios 1, 525-542.
Klein, C., Beukes, N.J. & Schopf, J.W., 1987. Filamentous microfossils in the early Proterozoic Transvaal Supergroup: their morphology, significance, and palaeoenvironmental setting. Precambrian Research 36, 81-94.
Altermann, W. & Schopf J.W. 1995: Microfossils from the Neoarchean Campbell Group, Griqualand West Sequence of the Transvaal Supergroup, and their paleoenvironmental and evolutionary implications.- Precambrian Research 75, 65-90.
Wright, D.T. & Altermann, W. 2000: Microfacies development in Late Archaean stromatolites and oolites of the Campbellrand Subgroup, South Africa. In: Insalco, E., Skelton, P.W. & Palmer, T.J. (Eds.): Carbonate Platform Systems. Components and interactions. Geol. Soc. London, Spec. Publ., 178. 51-70.
Kazmierczak, J. & Altermann, W. 2002: Neoarchean biomineralisation by benthic cyanobacteria. Science 298, 2351.
Altermann, W. & Kazmierczak, J. 2003: Archean microfossils: A reappraisal of early life on Earth. Res. Microbiology, 154, 611-617.
Kazmierczak, J. Kempe, S. and Altermann, W. 2004: Chapter 6.4: Microbial origin of Precambrian carbonates: Lessons from modern analogues. In: Eriksson, P.G., Altermann, W., Nelson, D.R., Mueller, W. & Catuneanu, O. (Eds.): The Precambrian Earth: Tempos and Events. Developments in Precambrian Geology, Elsevier, 545-563.
The complete age of the discussed rock series, firstly reported from SHRIMP Zircon analyses, were reported in:
Altermann, W. & Nelson, D.R. 1998: Sedimentation rates, basin analysis, and regional correlations of three Neoarchean and Paleoproterozoic sub-basins of the Kaapvaal Craton as inferred from precise U-Pb zircon ages from volcaniclastic sediments. Sedimentary Geology 120, 225-256.
And the first full stratigraphic description of these rocks from an over 3000 m deep borehole, in:
Altermann, W. & Siegfried, H.P. 1997: Sedimentology and facies development of an Archean shelf - carbonate platform transition in the Kaapvaal Craton, as deduced from a deep borehole at Kathu, South Africa. J. African Earth Sci., 24/3, 391-410.
Posted by: Wladyslaw Altermann | February 19, 2009 11:58 AM
Thank you, Wlady, for your thoughtful comment. We have described sedimentary structures that have been caused by possible cyanobacteria in sandy tidal flats of the 2.9 Ga Pongola Supergroup, South Africa. The microbial structures in sandstones differ significantly from stromatolites and cannot be mimicked by abiotic processes. Interestingly, the structures in the Pongola SG point to the existence of an already highly diverse (cyano?-)bacterial population by 2.9 Ga. If so, the rise of cyanobacteria must have taken place already much earlier than in general assumed. Of course we do not know, if those microbiota have been oxygenic or anoxigenic photoautotroph.However, the geometries of the structures as well as their paleogeographic distribution correspond exactly to that of the structures caused by microbial mats made by benthic cyanobacteria in the same tidal facies settings. The paper is Noffke, N., Beukes, N., Bower, D., Hazen, R.M., and Swift, D.J.P., 2008, An actualistic perspective into Archean worlds - (cyano)bacterially induced sedimentary structures in the siliciclastic Nhlazatse Section, 2.9 Ga Pongola Supergroup, South Africa. Geobiology, 6, 5-20.
Posted by: Nora Noffke | February 19, 2009 07:22 PM