Around 2.4 billion years ago, the Great Oxidation Event caused fundamental changes to the chemistry of Earth’s surface environments. However, the effect of these changes on the biosphere is unknown, due to a worldwide lack of well-preserved fossils from this time. In new research, paleontologists examined exceptionally preserved, large spherical microfossils permineralized in chert from the lower part of the 2.4-billion-year-old Turee Creek Group...