Oxygen availability is widely regarded as a crucial factor in the early evolution of eukaryotes. Specifically, it has been proposed that persistently low oxygen levels may explain the delay between the origin of stem-group eukaryotes in the late Paleoproterozoic and the crown-group eukaryotes that likely emerged in the Meso-Neoproterozoic transition. However, the temporal relationship between the rise of atmospheric oxygen levels (pO₂) and the radiation of crown-group eukaryotes remains uncertain. Here, we quantified the pO₂ on the basis of Rare Earth Elements plus Yttrium (REY) composition of shallow water carbonate from the middle Mesoproterozoic to early Neoproterozoic. Our results suggest that the pO₂ was ~5-10% of the present atmospheric level (PAL) in the middle Mesoproterozoic, which declined to ~1% PAL during the Meso-Neoproterozoic transition. The temporal variations of pO₂ were inconsistent with the evolution of eukaryotes, which experienced an abrupt radiation in the Meso-Neoproterozoic transition, i.e., multiple eukaryotic lineages appeared in this time period, including red algae, green algae, and fungi. Thus, we propose that low pO₂ in the Proterozoic was unlikely a critical factor inhibiting the evolution of early eukaryotes.
 

eukaryote, atmospheric oxygen levels, Mesoproterozoic, Neoproterozoic, Ce/Ce*