You share no DNA with the vast majority of your ancestors.
You have more ancestors — hundreds a few generations back, thousands in just a millennium — than you have sections of DNA.
You have 64 great-great-great-great-grandparents — but if you are a man, you share your Y-chromosome with only one of them.
The amount of DNA you pass on to your descendants roughly halves with each generation. It is a matter of chance which of your descendants actually carry any of your DNA.
And then, there's this tidbit, courtesy of Mark G. Thomas and his friends at University College London: It can be demonstrated that 5,000 years ago everybody alive was either the common ancestor of everyone alive today, or the common ancestor of no one. Thomas captures the startling upshot of this fact: "At this point in history we all share exactly the same set of ancestors."
What can we learn from this? This is a question I hope to explore with you in coming months.
A couple of things seem reasonably clear. We care about family and we care about our family history. Millions of us watch Henry Louis Gates Jr.'s Finding Your Roots on PBS with fascination. And when Jimmy Kimmel or Shonda Rhimes weeps in the face of information about the trials and accomplishments of great-great-great-great-grandparents, we get it. These stories matter.
Here's something else that's pretty clear: These are potentially unsettling stories. I get why Ben Affleck didn't want to the world to know that he descended from slaveholders. He didn't want to know himself. How would you feel to learn that one of your parent's parent's parents had done something really bad? How should you feel?
But the really interesting upshot of the facts mentioned above seems to be that family and family history are one thing, and DNA-based ancestry is another. You just can't map these beautiful, defining, important family stories onto a DNA tree. The facts above seem to show that you literally can't. DNA draws the boundaries in the wrong place. According to some estimates, after all, as recently as 3,500 years ago a person lived who is the common genetic ancestor of everyone alive now.
This is salutary. As a culture, we like simple solutions. And the idea that who, and what, we really are is written in the language of the genome, that it is inside us — and that we need only send away to have it decoded — is almost irresistible. But to judge by the example of Gates' television show, the stories that matter, the ones that bring his guests and his viewers to tears, are sagas of marriage and migration, of childrearing, hard work and love. It is family that matters — and family is relationship, not DNA. Family is not to be found inside us. The DNA story is a good one, and no doubt important for certain purposes, e.g., medical. But when what we want to know is who we are, it won't deliver the answers.
This, at least, is Thomas' view. He attacks the consumer DNA ancestry industry as peddling just-so stories and serving up "genetic astrology." For a detailed statement of his view, and a nice tutorial on genetics and ancestral analysis, listen to this lecture that he delivered last April at the genealogy enthusiasts conference Who Do You Think You Are Live in Birmingham, England.
Alva Noë is a philosopher at the University of California, Berkeley, where he writes and teaches about perception, consciousness and art. He is the author of several books, including his latest, Strange Tools: Art and Human Nature (Farrar Straus and Giroux, 2015). You can keep up with more of what Alva is thinking on Facebook and on Twitter: @alvanoe