Nature's Journal Club

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.

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