I found this interesting...
A person’s maternal ancestry is traced by mitochondrial DNA or mtDNA for short. Both men and women possess mtDNA, but only women pass it on to their children.
So we all inherit our mtDNAs from our mothers, but not from our fathers. Your mother inherited it from her mother, who inherited it from hers, and so on back through time. Therefore, mtDNA traces an unbroken maternal line back through time for generation upon generation far further back than any written record.
Researchers over many years has shown that all of our maternal lines are connected at some time in the past and that these connections can be traced by reading mtDNA. One striking finding was that people tended to cluster into a small number of groups, which could be defined by the precise sequence of their mtDNA. In native Europeans, for example, there were seven such groups, among Native Americans there were four, among Japanese people there were nine, and so on. Each of these groups, by an astounding yet inescapable logic, traced back to just one woman, the common maternal ancestor of everyone in her group, or clan.
we read a section of your mtDNA, about 400 base pairs long, and compare its precise sequence to the many thousands of others from all over the world that we have in our database. That way we can not only give you an exact readout of your DNA sequence, but also discover to which of the clans you belong, and from which ancestral mother you are descended. For many of the ancestral mothers, and there are about 36 world-wide, we know whereabouts they lived and how many ten of thousands of years ago. DNA changes very slowly over time and this is what we use to calculate how long ago the clan mothers lived. By studying features of the geographical distribution of their present-day descendants, we can work out where they lived as well. To emphasise that they were real individuals, we have given them all names and, using archaeological and other evidence, we have reconstructed their imagined lives.
Everyone in the same clan is a direct maternal descendant of one of these clan mothers and carries her DNA within every cell of their body. Your mtDNA actually helps cells use oxygen – so you are using your clan mother’s mtDNA every time you breathe. However, not everyone in the same clan has exactly the same mtDNA, because DNA changes gradually over the generations. From your precise DNA result, we are able to assign you a place within the genealogy of the clan, which will be shown on your “Seven Daughters of Eve” or “World Clans” certificate.