Pass the Sulfur, Please

Science  14 Sep 2007:
Vol. 317, Issue 5844, pp. 1465e
DOI: 10.1126/science.317.5844.1465e

Carbon and sulfur isotopic signatures provide the main evidence for the identification of the earliest life on Earth. Details in the signatures of the several sulfur isotopes can now be used to track metabolism. It was previously suggested that a large fractionation in the 34S versus 32S isotopes implies that sulfate-reducing bacteria were present in rocks dated to about 3.5 billion years ago. Philippot et al. (p. 1534; see the Perspective by Thamdrup), making use of 33S data, show these rocks record the presence of organisms that metabolized and disproportionated elemental sulfur. Several such organisms are present near the base of the phylogenetic tree.

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