Modelling suggests that the UV radiation environment of the early Earth, with DNA weighted irradiances of about three orders of magnitude greater than those at present, was hostile to life forms at the surface, unless they lived in specific protected habitats. However, we present empirical evidence that challenges this commonly held view. We describe a well-developed microbial mat that formed on the surface of volcanic littoral sediments in an evaporitic environment in a 3.5–3.3 Ga-old formation from the Barberton greenstone belt. Using a multiscale, multidisciplinary approach designed to strongly test the biogenicity of potential microbial structures, we show that the mat was constructed under flowing water by 0.25 μm filaments that produced copious quantities of extracellular polymeric substances, representing probably anoxygenic photosynthesizers. Associated with the mat is a small colony of rods–vibroids that probably represent sulphur-reducing bacteria. An embedded suite of evaporite minerals and desiccation cracks in the surface of the mat demonstrates that it was periodically exposed to the air in an evaporitic environment. We conclude that DNA-damaging UV radiation fluxes at the surface of the Earth at this period must either have been low (absorbed by CO2, H2O, a thin organic haze from photo-dissociated CH4, or SO2 from volcanic outgassing; scattered by volcanic, and periodically, meteorite dust, as well as by the upper layers of the microbial mat) and/or that the micro-organisms exhibited efficient gene repair/survival strategies.
↵1 This sample was one of 16 different Barberton rock samples (age range 3.472–3.2 Ga) analysed for their δ13C values. Most of the samples contained very little carbon, ranging from 0.01 to 0.16 wt%. Between 14 and 158 μg of C were extracted from the samples for analysis. Three blanks runs were also done, which had C concentrations of 1.3–8 μg and μ13C values between −21.8 and −22.4%. Samples with less than 10 times background (approx.80 μg) were considered likely to need a correction for the estimated blank value, and those with less than five times background (approx. 40 μm) were initially ignored as being unreliable. Thus, corrections assuming a blank contribution of 6 μg C with a δ13C of −227% were made to the samples having between 40 and 80 μg C. The errors of these analyses are probably around ±0.5%. The sample analysed in this study contained 16 μg of C and using the above criteria, should be considered unreliable. The C may represent rock C or there could be traces of contamination during the rock preparation or adsorbed on the rock powder, although this is not likely as acid (HCl) digestion indicated no carbonate C and the sample was prepared in such as way as to eliminate the possibility of contamination. Moreover, the result is consistent with those of other, similar cherts containing reliable concentrations of C and measured at the same time, e.g. sample 96–SA–01 which had 48 μg of C and a corrected μ13C value of −27.0%, and others with >40 mg of C with δ13C having values between −24.1 and −28.3% (i.e. similar to cherts analysed by de Ronde & Ebbesen 1996).
- © 2006 The Royal Society