Large and robust lenticular microorganisms on the young Earth

In recent years, remarkable organic microfossils have been reported from Archean deposits in the Pilbara craton of Australia. The structures are set apart from other ancient microfossils by their complex lenticular morphology combined with their large size and robust, unusually thick walls. Potentially similar forms were reported in 1992 from the ∼3.4 Ga Kromberg Formation (KF) of the Kaapvaal craton, South Africa, but their origin has remained uncertain. Here we report the first determination of in situ carbon isotopic composition (δ¹³C) of the lenticular structures in the KF (obtained with Secondary Ion Mass Spectrometry [SIMS]) as well as the first comparison of these structures to those from the Pilbara, using morphological, isotopic, and sedimentological criteria. Our results support interpretations that the KF forms are bona fide, organic Archean microfossils and represent some of the oldest morphologically preserved organisms on Earth. The combination of morphology, occurrence, and δ¹³C values argues that the lenticular forms represent microbes that had planktonic stages to their life cycles. The similarity in morphology, δ¹³C, and facies associations among specimens from Australia and South Africa suggests that the lenticular microfossils on the two continents represent related organisms. The biological success of these organisms is demonstrated by their abundance, widespread distribution, and the fact that, as a group, they appear to have been present at least 400 million years. This success may be due in part to their robust structure and planktonic habit, features that may have contributed to survival on a young planet. Isotopic results further suggest that the lenticular organisms were autotrophs, an interpretation supporting the view that autotrophic metabolisms developed early on the young Earth.