Early fossil eukaryotes were benthic aerobes

All complex life on Earth is descended from a single-celled eukaryotic ancestor that lived more than a billion years ago. Although the timing and environmental context of eukaryogenesis, the prolonged origin and evolution of the eukaryotic cell, are poorly constrained, the geological record offers considerable insight. Through integrated paleontological, sedimentological, and geochemical analyses, we reconstruct the habitats of the oldest known (ca. 1.75–1.4 billion-year-old; Ga) fossil eukaryotes. These data show that although eukaryote microfossils are found in samples deposited in a range of marine environments from coastal to offshore, they are almost entirely restricted to settings with oxygenated bottom waters. This distribution suggests that these organisms were aerobic and, given their size and morphological complexity, likely possessed mitochondria by this time. Further, the absence of eukaryotes from otherwise fossiliferous samples that record bottom water anoxia suggests a benthic habit, since planktonic eukaryotes would be expected to have been deposited in those settings.