Without getting into excessive detail: today we genotype or sequence DNA from many many people. We can cluster people into groups that have a higher than expected number of shared alleles (genetic variations). This happens when people share common ancestry because at some point their ancestors were isolated to an area, or had moved to a new area and founded a new settlement that grew, establishing a large group with common genetic history. This is effectively what the ancestry DNA companies do. You can plot that information on a map and to show where those people groups are most prevalent.

That same information can be used to make something like a pedigree of people groups, tracing the divergence and convergence of populations in time and geography.

Given that information, you can then ask where in that tree is the allele most common? Can you trace a line between people groups that carry the specific mutation? Sometimes you can, and you can estimate when and where the mutation first occurred. In some cases, you simply can’t — perhaps because the mutation is very common and doesn’t seem to have a strong correlation with any people group.