THE HUNT FOR EARLY EUKARYOTES IN ARCHEAN AND PROTEROZOIC OCEANS

Biomarker molecular fossils in 2.7 Ga shales suggest that the Eucarya diverged early in Earth history. However, the oldest currently known fossils that can be assigned to an extant eukaryotic clade are ca.1.2 Ga bangiophyte red algae from Arctic Canada. Between these records lies a rich assortment of potentially protistan microfossils, but determining their biological affinities requires paleontological exploration for interpretable eukaryotic phenotypes, be they morphological, ultrastructural, or chemical. Combined light microscopy, scanning electron microscopy, and transmission electron microscopy on 1.65-1.4 Ga microfossils from the McArthur and the Roper Groups, Australia, and from the Ruyang Group, China, show that these intermediate assemblages do indeed include a moderate diversity of eukaryotic cells, although the observed diversity remains well below Neoproterozoic levels.

Our samples contain eight different populations identified as eukaryotic. At least two (Tappania plana and Shuiyousphaeridium macroreticulatum) can be recognized as eukaryotic based on morphological characters observable by light microscopy. SEM imaging of cell wall microstructure marks two additional forms (Satka favosa and Valeria lophostriata) as protists. But for the four remaining taxa (three species of Leiosphaeridia and unnamed tubular microfossils), TEM imaging is necessary to reveal eukaryotic characters. Preserved wall ultrastructures of spheroidal acritarchs range from single, homogeneous, electron-dense layers of variable thickness - and variably ornamented - to multilayered walls differentiated by electron density and texture.

Despite our confidence that eukaryotic fossils occur in 1.65 Ga rocks, we do not, as yet, know how to place these fossils into eukaryotic phylogenies. Ongoing microchemical analyses (Micro-FTIR spectroscopy, Laser micro-Raman spectroscopy and Laser micro-pyrolysis-GC/MS) of individual microfossils may help us to place firmer constraints on phylogenetic interpretation. Our TEM observations, however, confirm that ultrastructure can be used to probe the biological relationships of simple microfossils in older rocks and to bridge the current gap between the unambiguous eukaryotic morphologies of mid-Proterozoic assemblages and Archean molecular biomarkers.