HOW OLD IS AEROBIC PHOTOSYNTHESIS? A FRESH LOOK AT THE FOSSIL EVIDENCE

While molecular evidence from bacterial rRNA sequencing confirms that cyanobacteria are likely to have evolved after the emergence of hyperthermophile methanogenic archaebacteria it gives no evidence about timing. Comparison of d¹³C ratios in organic matter and marine carbonates can be argued to provide an indirect proxy for the existence of photosynthesis >3.7 Ga. Such values are in broad agreement with values from modern cyanobacteria but also lie within the range of anaerobic methanogens.There is, as yet, little biomarker evidence for the presence of cyanobacteria until the late Archaean at 2.7 Ga. Stromatolites have been described from various units in the Archaean but their cyanobacterial origin is hard to prove and even their biogenic origin is often controversial - many could also have been formed abiogenically by rapid marine precipitation of aragonite or by hydrothermal precipitation of barite and chalecdonic quartz. The description and interpretation of morphological remains in the form of microfossils preserved in sedimentary rocks must therefore play a crucial role in underpinning any claims for the early emergence of oxygenic photosynthesis. Relatively few such assemblages have been described to date, of which those from the c. 3.46 Ga old cherts of the Apex Basalt, Warrawoona Group in Western Australia hold a key position. Eleven reported species of microfossils (incuding cyanobacteria-like forms), were interpreted by Schopf (1993) to occur in water-worn carbonate grains that had been transported a long distance before redeposition in a bedded grainstone conglomerate prior to silicification. Major aspects of the preservation and context of this potentially important evolutionary benchmark have received little critical scrutiny.We have re-examined the type thin sections of microfossil-bearing type material deposited by Schopf (1993) at the Natural History Museum (NHM) in London, and compared them with samples collected from the same horizon during recent field work. The new study involves Automontage image analysis, SEM with microprobe and EDX, and ToF-SIMS.