2 Billion Year Phosphatic Rock Hosts Microfossils With Possible Eukaryotic Affinities

The search for the earliest sign of life on Earth is made difficult by the reworking of the rock that harbors life’s earliest vestiges. Nonetheless, rare evidence remains for prokaryotes within Earth’s first billion years. Acritarchs and other cryptic structures begin to appear in the rock record around 2 billion years (Ga) ago, and arguably may comprise the earliest evidence of eukaryotes, but more clearly eukaryotic organisms do not appear for another ~800 million years, in the form of red algae. We have identified filamentous microfossils in 2 Ga phosphate rock with characteristics suggestive of eukaryotic origins including apparent trichome branching and a distinctive morphology reminiscent of algal holdfasts. We sought first to establish their authenticity as microfossils, and then to determine whether they are eukaryotic. Toward this end we have obtained Raman spectral evidence of kerogen, a biologically-produced material, substantiating the authenticity of the microfossils. Additional Raman 2D/3D mapping, corroborated by Imaris 3D imaging, is expected to clarify the presence of any branching trichomes, holdfasts, or multicellular morphology distinct to eukaryotes. Additional support may be established via follow-up antibody testing of the kerogen for steranes, a product of eukaryotic metabolism. Our results support a biogenic interpretation of microfossils contained in 2 Ga stromatolitic phosphate rock, and observable morphological characteristics raise the possibility that they may be among the oldest definitive eukaryotic microfossils thus far discovered.