Having traveled genealogical trails down through American history until the paper trail ran out, in episode four Dr. Gates visits scientists around the country who are using DNA analysis to trace ancestral roots. Among them is Dr. Rick Kittles, an associate professor at Ohio State University and co-founder of the ancestry-tracing firm African Ancestry, Inc., who is building a DNA database of present-day African populations, against which he compares the genetic signature of the series' participants. With their DNA results and genealogical research in hand, Dr. Gates meets with leading historians on the slave trade, who derive dramatic "crossing stories" about how the participants' ancestors may have been taken into slavery and brought to America. And along the way, Dr. Gates learns several stunning facts about his own ancestry. Finally, Dr. Gates and one participant make the last leg of the journey - back to Africa. They visit the West African port from which the participant's patrilineal ancestor was most likely shipped into slavery, and meet local tribal elders whose DNA suggests they are the participant's long-lost cousins.
