Sulphide-rich sediments, stromatolitic limestones and tidal-flat deposits in the late Archaean (2.7 Ga) Manjeri and Cheshire Formations, Belingwe greenstone belt, Zimbabwe show evidence for complex and extensive prokaryotic mat communities, including (1) shallow-water coastal sulphur mats; (2) mats, probably in somewhat deeper water; (3) nearby stromatolites that lived by oxygenic photosynthesis in shallow coastal settings. Petrological and geochemical (rare earth element; REE) evidence, coupled with high-resolution stable isotope results, identifies several complex interdependent metabolic consortia of bacteria and archaea. These microbial consortia would have exchanged nutrients and products both locally within prokaryotic mats and more widely via the waters of the Belingwe basin. This isotopic, sedimentological and REE evidence for a complex ecology of bacteria and archaea is consistent with metabolic inferences from rRNA phylogeny and is direct evidence that a diverse prokaryotic community, managing carbon on a global scale, had evolved by the late Archaean.

Sulphur and carbon isotopic analyses on small samples of kerogens and sulphide minerals from biogenic and non-biogenic sediments of the 2.7Â10 9 years(Ga)-old Belingwe Greenstone Belt (Zimbabwe) imply that a complex biological sulphur cycle was in operation. Sulphur isotopic compositions display a wider range of biological fractionation than hitherto reported from the Archaean. Carbon isotopic values in kerogen record fractionations characteristic of rubisco activity, methanogenesis and methylotrophy, and possibly anoxygenic photosynthesis. Carbon and sulphur isotopic fractionations have been interpreted in terms of metabolic processes in 2.7 Ga prokaryote mat communities, and indicate the operation of a diverse array of metabolic processes. The results are consistent with models of early molecular evolution derived from ribosomal RNA.

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