Thank you chrisnice, yes that is one of the studies I was referring to.
As to anyone else who is interested, it is a study of midochondrial DNA, which is only passed down through the maternal line and is never changed because it is not part of the genes invoved in the "mix" of mother and father. The only changes are slight mutations which take about 10,000(?) years to occur. It also involves the y chromozome which comes from the male.
For you religious zealots you will be happy to know that science has figured out that every human today is genetically linked to one man, and one of three women. (Does the arc come to mind?)
Furthermore the only way a gene is washed away, or wiped out, dissapears is if the person or persons who have the particular gene are not capable of procreation or if the products of their procreation are not viable. This is called natural selection.
As to the question of how far back. From the beginnings of the species know as homosapiens.