Everyone Is Related. The (Big) Data Proves It

by Kasia Bryc

Earlier this month, I was at the South by Southwest conference in Austin to speak on a panel with the writer AJ Jacobs, about how interconnected we all are.Kasia @ SXSW

But walking around the conference with such a diverse mix of people — many with their rock star cool outfits and denim jackets — you’d be forgiven if you questioned whether they could possibly come from the same gene pool as me, a former nerdy high school kid.

And yet, we really are all one big family. In June, AJ will host the Global Family Reunion in New York City, what he’s described as the world’s largest family reunion.

AJ spoke about the many cousins he has found in his year of research. He was very proud of his family connection to Gwyneth Paltrow, and would welcome her to join his family for a kosher Thanksgiving. He also noted that he was related to Judge Judy, which caused him to revise his opinion of her — as a person — in a more favorable light.

Also with us on the panel was Joanna Mountain, 23andMe’s Senior Director of Research. Joanna spoke about the incredible stories and discoveries that people have made by connecting to their DNA cousins. Such stories are regularly highlighted in this very blog.

But I was most excited to show just how many cousins we all have. The typical 23andMe customer has over two thousand DNA cousins, and that’s just within the 23andMe database.

It all comes down to the math: the further back — in generations — you go, the more genealogical ancestors you have. Two parents, four grandparents… all the way up to a 128 fifth-great grandparents. And that only takes you back about two hundred years. Imagine if you were looking for your ancestors from the sixteenth century, you’d have 131,072 genealogical ancestors to hunt down!

So, as special as you are, all these many ancestors passed on their DNA not just to create you, but also your many cousins. And, as you can imagine, 130 thousand people have a lot of descendants, so it’s unsurprising that the average 23andMe customers can find so many cousins.Connections

It easily follows that, with each of us having so many DNA relatives, any pair of individuals are connected through their cousins. To illustrate this point, I showed that each of Tuesday’s panelists, together with Prof. Henry Louis Gates Jr. (host of “Finding Your Roots” on PBS who kindly agreed to participate in my experiment), are all just one or two “cousin” connections apart.

In fact, almost everyone in the database is connected through one, two, or three DNA cousins. Which should make you rethink how you treat anyone you meet on the street. Regardless of what they may look like, you never know who you might be related to!

  • wes

    131,000 ancestors for one person. 1000 trillion ancestors for the world population of 7.5 billion. Is the earth’s population growing or shrinking?

    • 23blog

      Hi Wes,
      I think the issue you’ve got is that you would be counting the same people multiple times.

      • wes

        I’m still thinking about that one.  But Thank You.

        Sent from my T-Mobile 4G LTE Device

        ——– Original message ——–

        • 23blog

          Hi Wes,
          The individuals that you or I are related to, may be some of the same individuals. Ditto with any other person, so there’s a lot of overlap.

        • wes

          Got it!

          With so much overlap, aka inbreeding, and if inbreeding generally results in “decreased biological fitness of a population” how do you account for the proliferation (apparent success) of the human population?

        • Oliver Pereira

          Who says it does? There is an optimal amount of inbreeding for any given population. Too much inbreeding may be bad for the individuals in a population (look up “inbreeding depression”), and too little inbreeding may be bad for the individuals in a population, too (look up “outbreeding depression”).

        • wes

          On the other hand with so many cousins marrying cousins how do you account for so much racial diversity in our population?

  • Oliver Pereira

    With every generation that you go back in time, your total number of ancestors grows geometrically, for a while, and then the growth slows somewhat as it gets closer to the total population size of the Earth, because of course your total number of ancestors cannot exceed the population of the planet. Your ancestor set grows as you go back in time, and it merges with everyone else’s. However, your number of matrilineal ancestors – i.e., the ancestors from whom you inherited your mitochondrial DNA – remains resolutely at one per generation.

    Although your have two parents, only one out of those two is your matrilineal parent: your mother. Although you (probably) have four grandparents, only one out of those four is a matrilineal grandparent: your mother’s mother. And so on, going back in time. One out of eight, then one out of sixteen, then one out of thirty-two, and so on. Your total number of ancestors roughly doubles at each step, but your number of matrilineal ancestors stays at just one, so the proportion of your ancestry covered by your matrilineal ancestors dwindles very quickly to a minuscule fraction. You can think of it as a thin thread of cotton weaving its way up an abundantly branching tree, picking out just one route through the branches, out of millions of possibilities.

    This continues right back to whichever of the so-called “seven daughters of Eve” you are, if you are of ordinary European ancestry, matrilineally descended from. In her generation, you may well have had hundreds of thousands of ancestors over all, but you still only had one matrilineal ancestor, and that was your “Eve”.

    The same is true of every other person in Europe. They would have hundreds of thousands of ancestors over all, too, but each is descended from only one “Eve”. (Ignore, for the sake of simplicity, the fact that the various Eves lived in different time periods!) There is no “shrinking down” of the set of ancestors to just seven individuals. The total set of ancestors is still huge. It is only the sets of matrilineal ancestors – those narrow, almost invisible threads at the very edges of the ancestral trees – that converge upon that set of seven individuals.

    Does that help at all?

  • musings2

    Although I found a lot of famous white cousins, one of my most treasured discoveries was Ruby Dee. Oh, and also Walt Whitman, who when he was doing his thing, was a fourth cousin of a multi-great grandfather. I think it would have been great to share Greenwich Village with both parties at the same time. Remember Steven Allen’s “Meeting of the Minds”?

  • 23blog

    The short answer is yes.

  • Skeptic

    The short answer is that no one actually has 130K UNIQUE ancestors from the 16th century. Instead, the farther you go back in time, the more likely your family tree is to merge branches, i.e. you are related to the same ancestor via more than one line of descent. Consider the fact that until quite recently, most people never traveled more than 10-20 miles from where they were born. So they married cousins.

    Statisticians tell us that if you are European, you are almost 100% certain to be descended from Charlemagne (8th century I believe). WDYTYA? made a big deal about finding out that one of its show’s subjects had Charlemagne as an ancestor, when in fact the truly amazing thing would be to find a European who does not.

    I think the most recent common ancestor for ALL northern Europeans is thought to have lived around 1000 years ago. MRCAs would be farther back for two people from more far-flung populations, but the MRCA for ALL OF US humans alive today lived no earlier than around 300 BCE. This is based on genetic drift.

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