Relaxed molecular clock dating
Each branch in a phylogeny (subtending a clade) is a possible location for a change of rate from one local clock to a new one. For example,the gene that codes for the protein alpha-globin (a component of hemoglobin) experiences base changes at a rate of .56 changes per base pair per billion years*.If this rate is reliable, the gene could be used as a molecular clock.BEAST estimates that the former has a high evolutionary rate and the latter has a low evolutionary rate, with those rates being constant within each clade.
The shift in evolutionary rate occurs at the (T)MRCA of those defined clades, and hence no shifts are allowed to occur at random times on a branch.
That means that the two DNA versions differ by 100 million years of evolution and that their common ancestor lived 50 million years ago.