At one point in the PBS series Finding Your Roots, Harvard genetics professor and founder of the Personal Genome Project George Church is asked about the difference between written genealogical records and DNA.
“Written records go back to the dawn of written history,” Church said. “DNA goes back to the dawn of human existence.”
This is the power of genetics — it can tell us not just about ourselves, but also about human history and our place in it.
We can map the story of human migration out of Africa not just from archaeological evidence — the bones and artifacts our ancient ancestors left behind — but also using the DNA we inherit only from our mothers or only from our fathers.
These bits of DNA — the Y chromosome (y-DNA) passed from father to son, or mitochondrial-DNA (mtDNA) passed from mother to child — are inherited almost identically in each new generation. The key word here is “almost.”
Over thousands of years there are occasionally small changes in the mtDNA and the y-DNA that become a kind of timestamp for a person’s deep ancestry. Because geneticists can calculate the rate of those changes over time, the changes act as markers in time that conveniently match up with points in the history of human migration.
As people migrated, groups of people also became more isolated from one another. The small changes concentrated over generations within these isolated populations set the stage for the pattern of genetic diversity we see today. Specific y-DNA sequences and specific mt-DNA sequences are associated with certain groups that were culturally or geographically more isolated, passing certain mutations from one generation to the next until they were prevalent.
People with similar geographic ancestry share these small differences in their mtDNA or their y-DNA and they are grouped together in what are called haplogroups— specific clans or branches of the human family tree. There are hundreds of different haplogroups on both the maternal and paternal sides. Some of them can indicate things like whether you have ancestry in a Nordic country or the Middle East
In a family tree, the maternal line (like mitochondrial DNA) traces back through a string of mothers; the paternal line (like the Y chromosome) traces back through a string of fathers.
This doesn’t mean that if you (or your father if you are a woman) have the same paternal haplogroup as Thomas Jefferson (T), or Genghis Khan (C3), or Bono (V) or Albert Einstein (E1b1b1*), that you are “related” to them, at least not in a strict sense. It does mean that you have a common ancestor dating back thousands of years. You could find that common ancestor if you followed your father’s, father’s, father’s line back each generation and the other person did the same. Eventually those two paternal lines would meet with a shared ancestor.
By finding out his haplogroup, Prof. Gates now knows that he is likely to have Irish ancestry on his paternal side. He, and a majority of men in Ireland, share the paternal haplogroup, R1b1b2a1a2f2. It is thought to come from an Irish King named Niall of the Nine Hostages. Prof. Gates is one of the very few African Americans, about 1 percent, who traces his ancestry on both his maternal and paternal side to Europe.
By tracking their maternal DNA, Prof. Gates was able to inform the actresses Kyra Sedgwick and Maggie Gyllenhaal not just of their Jewish ancestry, which isn’t so surprising, but in Gyllenhaal’s case that her Jewish ancestry traces back to one of four Jewish founding mothers who lived thousands of years ago. In the case of Brown University President Ruth Simmons, Prof. Gates was able to look at her maternal line and see that she has Native American ancestry. (Women do not get y-DNA from their fathers, but their paternal ancestry can be traced by testing a close male relative.)
With the help of geneticist Rick Kittles, Prof. Gates was able to help his guests track their ancestry to regions of Africa. He did this by comparing the guest’s mt-DNA with databases of people from certain locations in Africa. This is how Prof. Gates was able to tell former Secretary of State Condoleezza Rice that her maternal DNA shows she has ancestry in what is now modern day Cameroon and Samuel Jackson that his y-DNA indicates he has ancestry from what is now modern day Gabon.
Although this information doesn’t always tell the guests much about close relatives, it gives them a connection to their deep ancestral roots in a way that resonates. Gates took his guests comedian Margaret Cho, entrepreneur Martha Stewart and CNN medical correspondent Sanjay Gupta on a similar journey tracing back deep ancestry by looking at their maternal haplogroups. Cho’s D5a haplogroup is rare among Koreans, and indicated ancestry in China, while Stewart’s maternal haplogroup W6 connects her eastern European roots to the Tatars people of the near east. In the case of Gupta, his maternal haplogroup U2c only confirmed his deep roots in India going back thousands of years.
In each case, the guests said the information not only helped them complete the picture of their families, but also the picture of themselves.
About the Authors:
Joanna Mountain is a geneticist who is a consultant to the PBS series “Finding Your Roots,” and consulted previously on the PBS series “Faces of America.” Dr. Mountain completed her PhD in Genetics at Stanford University and has spent over 20 years studying human genetic diversity. Currently, she is Senior Director of Research at 23andMe, Inc.
Scott Hadly is a writer with extensive experience as an investigative reporter. Currently Mr. Hadly is Content Editor at 23andMe, Inc.