I doubt oral history would provide the dates and
places that are needed to document an ancestor's birth
in Africa during the 17th or 18th centuries.  The
original poster questioned how one could do it, given
that birth records were probably not kept on Africans
during the slave trade.  If the records exist, then we
haven't mined them to the extent that we can track
individuals in Africa and America.  

Certainly the field of African American genealogy has
not developed to the extent that a significant portion
of the population can even trace their ancestry to the
beginning of the 19th century.  Anita Wills is an
exception.  She cited an example of some research
findings in her family, but this ancestor of hers
landed in South Carolina at a much later date within
the trade.  South Carolina and Georgia were states
where the slave trade still operated despite it's
being illegal to do so.  Terms such as "fresh water
Africans" referred to these men and women who arrived
in the south long after the legal trade ended.