Because bacteria do not have a robust fossil record, attempts to infer the timing of events in their evolutionary history requires comparisons of molecular sequences.
In the Archean eon, biomarker compounds characteristic of possibly extinct stem eukaryotes are found ≈2,700 million years ago (Mya) (2).
The molecular clock is a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged.
The biomolecular data used for such calculations are usually nucleotide sequences for DNA or amino acid sequences for proteins.
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