Abstract
The currently available sedimentary carbon isotope record goes back to 3.85 Ga and conveys a remarkably consistent isotopic signal of biological carbon fixation based on the bias for light carbon (<SUP>12</SUP>C) exercised by common photosynthetic pathways. This holds particularly for the time segment < 3.5 Ga, whereas the older (Isua) record is blurred by a metamorphic overprint. In spite of the marked impairment of the oldest evidence by isotopic reequilibration between organic and carbonate carbon in the wake of the amphibolite-grade metamorphism suffered by the host rock, a coagent case can be built for the emergence of (photo)autotrophic carbon fixation and the start of a biogeochemical carbon cycle as from at least 3.85 Ga ago. This would imply that microbial (prokaryotic) ecosystems had been prolific on the Archaean Earth not long after the formation of the planet.
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