Doctor of Philosophy
Planetary Science and Exploration
The presence of eukaryotic life during the early Paleoproterozoic has been a matter of debate because well-preserved fossils older than 1.8 Ga rarely exhibit eukaryotic cellular microstructures. In this study, microfossils from the 1.9 Ga Gunflint Chert were studied using the extended-focal-depth imaging technique, combined with scanning electron microscopy, resulting in recognition of three types of large (10–35 μm diameter) complex unicellular bodies (CUBs) and one type of “multicellular body” (< 50 μm diameter). The CUBs show the following eukaryotic cyst-like structures: (1) radially arranged internal strands similar to those in some acritarchs and dinoflagellates; (2) regularly spaced long tubular processes, stubby pustules, and/or robust podia on the cell surface; (3) reticulate cell-wall sculpturing such as pits, ridges, and scale-like ornaments; and (4) internal bodies that may represent membrane-bounded organelles. These morphological features provide strong evidence for the presence of protists in the late Paleoproterozoic.
Among the three types of CUBs from the Gunflint microbiota, a new species, Germinosphaera gunflinta sp. nov., was recognized. This species has the diagnostic characteristics of Germinosphaera, such as a subrounded to an ellipsoidal cyst, a robust main podium (up to 15 μm), multiple smaller processes, and scale-like ornaments on the surface. Within a broadly continuous lineage of Germinosphaera from the Paleoproterozoic to the early Cambrian, there is a clear increase in cell size from the Paleoproterozoic to the Mesoproterozoic, with the Gunflint species being the smallest and oldest with complex, eukaryote-like, surface ornaments that are well preserved.
Summary for Lay Audience
The oldest fossilized life forms on Earth, dating to ~3.5 Ga, are represented by bacteria (prokaryotes). These cells, lacking a true nucleus or other organelles, are among the simplest and ubiquitous forms of life on our planet. Prokaryotic organisms dominated the Earth from the Archean to Paleoproterozoic (3.5–1.6 Ga) and spread unopposed across the planet until the appearance of eukaryotes - more complex organisms with cells containing a well-defined nucleus.
Fossil evidence indicates that eukaryotic cells, which make up all complex life, have existed on Earth since at least 1.7 Ga. Considering that complex life is made up of this class of cells – from giant whales to microscopic unicellular algae – the existence, origin, and evolution of eukaryotes are vital to understanding the development of complex life on Earth and other planets that have the necessary conditions to support complex life forms. Due to the relevance of the origin and evolution of these organisms to the understanding of how early life developed on Earth, it is important to establish their morphological characteristics and range of occurrence in the fossil record. This study focuses on chert samples from the Gunflint Formation (1.9 Ga) in Canada. These samples present fossil remains of simple, prokaryotic organisms such as cyanobacteria that are expected for this age. However, co-occurring with these remains are more complex fossil cells with structures that have remained unidentified until now.
Three different unidentified organisms were recognized: 1) star-shaped, 2) irregularly shaped organisms with multiple spikes, and 3) organisms with the main body and one or more podium/podia. Our data suggest eukaryotic affinities for these organisms. Furthermore, through these studies, we recognized a new species, Germinosphaera gunflinta, with morphological characteristics typical of eukaryotes.
González Flores, Ana L., "Complex Unicellular Microfossils from the 1.9 Ga Gunflint Chert, Canada" (2022). Electronic Thesis and Dissertation Repository. 8796.
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