| Summary: | Biomarker molecular fossils in 2770 Ma shales suggest that the Eucarya diverged from other principal domains early in Earth history. Nonetheless, at present, the oldest fossils that can be assigned to an extant eukaryotic clade are filamentous red algae preserved in ca. 1200 Ma cherts from Arctic Canada. Between these records lies a rich assortment of potentially protistan microfossils. New microscopic study of late Paleoproterozoic shales from China (1800–1625 Ma Chuanlinggou Formation) and Australia (1650 Ma Mallapunyah Fm) permits the significant extension of the stratigraphic range of fossil evidence for early eukaryotes. Combined light microscopy, scanning electron microscopy, and transmission electron microscopy on fossils from the 1500–1400 Ma Roper Group, Australia, and broadly coeval rocks from China show that these intermediate assemblages do indeed include a moderate diversity of eukaryotic remains, although the observed diversity remains well below Neoproterozoic levels. In particular, preserved cell wall ultrastructure, observed using transmission electron microscopy (TEM), can help to bridge the current stratigraphic gap between the unambiguous eukaryotic morphologies of mid-Proterozoic assemblages and molecular biomarkers in much older rocks. Ongoing microchemical analyses (Micro-FTIR spectroscopy, Laser micro-Raman spectroscopy and Laser micro-pyrolysis-GC/MS) of individual microfossil walls may help us to place firmer constraints on phylogenetic interpretation.
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