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.