Abstract
Unequivocal evidence for Archean eukaryotic life has been long sought for and is a matter of lively debate. In the absence of unambiguous fossils this debate has focused on biogeochemical signatures and molecular phylogenies. Most researchers agree that fossil forms comparable with modern eukaryotic cells can be credibly identified only in Proterozoic (∼1.8–1.6Ga) and younger rocks. Herein, we report for the first time, Neoarchean mineralized tubular microfossils from ∼2.8 to 2.7Ga lacustrine deposits of South Africa. The exceptional preservation of these microfossils allows recognition of important morphological details in petrographic thin section and in HF-macerates that links them to modern siphonous (coenocytic) green or yellow-green microalgae (Chlorophyta and Xanthophyta). The microfossil identification is supported by Raman spectroscopic analyses, EPMA, SEM/BSE and SEM/EDS microprobe analytical results, NanoSIMS elemental mapping and micro-tomographic sectioning of the thalli. All results point to indigenous, bona fide eukaryotic microfossils of algal affinity. These Neoarchean microalgae-like remains and their assumingly combined in vivo and early post-mortem precipitated mineral envelopes greatly improve our knowledge of early life and its habitats and may have far-reaching consequences for the studies of the evolution of life.
Highlights
The presence of Archean prokaryotic life, including photosynthesizing cyanobacteria, is almost uniformly accepted today
Small (15x15x15 mm) cubes of rock samples SA-31 and SA2-178 were macerated for traces of organic materials with 40% hydrofluoric acid (HF)
The presence of siphonous microalgae-like fossils in the Neoarchean biosphere corroborates with morphological (Niklas, 2000) and conceptual (Egel, 2012) models assuming siphonous/coenocytic body plan as one of the earliest cellular systems in the evolution of life
Summary
The presence of Archean prokaryotic life, including photosynthesizing cyanobacteria, is almost uniformly accepted today (for review see: Awramik, 1992; Nisbet and Sleep, 2001; Altermann and Kazmierczak, 2003; Brasier et al, 2006; Schopf et al, 2007; Knoll, 2015). Attempts to establish a time-scale of eukaryote evolution based on molecular phylogeny have led to contradictory results: from not earlier as 1100 Myr to as old as 3970 Myr (e.g., Dickerson, 1971; Feng et al, 1997; Hedges et al, 2001, 2004; Douzery et al, 2004; Roger & Hug, 2011) These estimations were recently reviewed (Eme et al, 2014), restudied and constrained using taxon-rich multigene data and a relaxed molecular clock framework (Parfrey et al, 2011). Based on rich and well-preserved cellular remnants of 2.8-2.7 Ga-old tabular microfossils from South Africa, reminiscent of modern and geologically much younger siphonous microalgae, we argue that these opinions may be significantly changed
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