What uses mutation rates to measure evolutionary time?

Answer 1

"Molecular clocks" can be used to estimate the time at which two lineages diverged. Often, the calibration of the clock uses fossil evidence to gauge a timeframe for the existence of a common ancestor of a number of lineages. From this, an average mutation rate can be calculated. This average mutation rate can then be used to estimate the timeframe for further branching points in the genealogy of the lineages in question.

For instance, fossil evidence could be used to establish a timeframe for the existence of a common ancestor to all great apes. By dividing the sum of all differences in a particular genomic sequence for all great apes by the time between then and now, we get an average rate. We can then take the differences between that genomic sequence in men and Neanderthals to estimate when Neanderthals diverged from us.