The evolution of eukaryotes is one of the most important issues in the history of life. Paleontological studies discovered the oldest eukaryotic fossil from the Paleoproterozoic Francevillian Group in Gabon. To clarify specific features of ancient sedimentary basins in Gabon, geologic evidence for the Francevillian Group is summarized and a new geotectonic model is proposed. The model gives much weight to the upwelling of mantle plume, which can explain why the Francevillian basin only hosted natural reactors. The Great Oxidation Event in the Paleoproterozoic played an important role in not only the evolution of eukaryotes but also in the formation of natural reactors. Reductive weathering of the continental crust, which was affected by plume-related volcanisms, transported Uranium-rich minerals into sediments of the Francevillian Group without dissolution of Uranium. The subsequent oxidation event enabled uranyl ions to accumulate within an oxic failed rift basin, and settled large quantities of organic matter on the seafloor. Hydrothermal circulation within the Francevillian Group precipitated highly Uranium-rich ores, which became natural reactors at approximately 2.0 Ga, and might have influenced the evolution of eukaryotes in this basin. In a large sense, the degree of oxidation of the ocean-atmosphere system has been linked to the amount of sedimentary rocks. In that way, mantle overturn in 2.7 Ga, which created a large continental crust, was one of the crucial events that affected the evolution of eukaryotes.