3.5 billion years ago, laid down during the Archaean eon.
The oldest visually recognizable, though by necessity microscopic scale fossils are of prokaryotic cells.
These fossils are present in rock that is about one-billion years younger than the Earth itself and present in rock that is only 300-million years short of the oldest known rock on Earth.
It would be over one-billion years later that the first fossils of eukaryotic cells were laid down.
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