Speciation of Paleoarchean Life Demonstrated by Analysis of the Morphological Variation of Lenticular Microfossils from the Pilbara Craton, Australia.

The ca 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton, Australia, represents a Paleoarchean sedimentary succession preserving well-described and morphologically diverse biosignatures such as stromatolites and cellularly preserved microfossils. The SPF microfossil assemblage identified from three greenstone belts includes relatively large (20-80 µm in width), acid-resistant, organic-walled lenticular microfossils, which can be extracted using a palynological technique. In this study, we present results of measurements of over 800 palynomorphic specimens of SPF lenticular microfossils from 2 remote (∼80 km apart) localities that represent different depositional environments and thus different habitats, as evidenced by their distinct lithostratigraphic association and trace element geochemistry. We demonstrate statistically that the two populations are distinct in oblateness from a polar view and furthermore that each population comprises subpopulations defined by different areas and oblateness. This study may provide the earliest morphological evidence for speciation of unicellular organisms, which could have been allopatric (geographic) and adaptive. It can also be suggested that SPF lenticular microbes had highly organized cytoskeleton indispensable for strict control of the cell morphology of large and robust microbes, which in turn were likely advantageous to their prosperity and diversification.