2.1 billion years ago, laid down during the Proterozoic eon.
It is of interest that the oldest fossil euakaryotes are found some 400-million years following attainment of relatively modern atmospheric concentrations of molecular oxygen. Why is this of interest? Because in addition to a nucleus, the defining feature of eukaryotic cells are mitochondria, which are the organelles associated with eukaryotes that actually make use of molecular oxygen.
These fossils are present in rock that is nearly one and one-half-billion years younger than the first fossil cells, which are of prokaryotic cells. These fossil eukaryotes are also approximately 2.5 billion years younger than the Earth itself.
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