Charlemagne | Wikimedia Commons.
I recently discovered that my 10-times-great-grandfather bought a good chunk of Brooklyn from the Lenape Indians. He was one of the first Dutch landowners on this continent, a man who had run a laundry bleaching business in Holland but had traveled under the auspices of the Dutch West India Company to become a farmer in the New World. The deed in question, written in Dutch in 1636, is the first record of any land being sold on Long Island.
Pretty neat, right? But as I realized almost immediately, this isn’t much of a distinction. According to a genealogy site maintained by a distant cousin, I share this Dutchman with more than a quarter million other descendants. (We can all be kings and queens of Brooklyn—it is called Kings County, after all.)
Chances are, if you have a famous ancestor far enough back that finding out about them is a surprise, you share them with a small city of other people. And the farther back you go, the truer that is. In 2004, statistician Joseph Chang, computer scientist Douglas Rohde, and writer Steve Olson used a computer model of human genetics to show that anyone who was alive 2,000-3,000 years ago is either the ancestor of everyone who’s now alive, or no one at all. Think about that: If a person alive in 1,000 BCE has any descendants alive today, they have all of us—even people from different continents and isolated populations. This line of thought led to the revelation that everyone of European heritage alive today is a descendant of Charlemagne, who ruled over much of Europe as the first Holy Roman Emperor. As science writer Carl Zimmer wrote a few years ago, it’s “Charlemagne for everyone!” (Zimmer’s excellent post covers a recent paper that looked at actual genomic data from European populations and came to a similar conclusion: All living Europeans, from Turkey to England, Spain to Finland, are related many times over.)
Now, there’s another important implication of these studies: Most of the people you are descended from are no more genetically related to you than strangers are.
It doesn’t get any less weird when you look at it from the other angle: While you more than likely have four distinct grandparents and eight distinct great-grandparents, past a certain number of generations back, your number of ancestors stops growing exponentially, because they start being the same people. By the time a couple who married in 1450 in Holland, has had a few hundred descendants over the span of several generations, those people are distantly related enough that some of them start marrying (and, yes, reproducing with) each other. That couple thus becomes the however-many-great-grandparents of the children of those unions along multiple branches of their family tree. (If your number of ancestors actually doubled every generation, by the time you counted back the 40 or so generations to Charlemagne, you’d have around a trillion ancestors. Scholars estimate the world population was only about 300 million at that point.) Stretch this back a few thousand years and you can see how you wind up being related to every other member of your species.
Now, there’s another important implication of genomic ancestry studies: Most of the people you are descended from are no more genetically related to you than strangers are. Or to put it another way, your genealogical family tree, which includes all the history of your family going back thousands of years, is much larger than your genetic family tree—the people whom genome sequencing would pinpoint as related to you. 99.9 percent of your genome is the same as that of every other human being (apart from the x and y chromosomes), and that .1 percent of variation in each person gets thinned out pretty quickly across the generations, as each child gets half of each of her parents’ genomes, passes on half to each of her children, and so on. Geneticist Luke Jostins did a nice mathematical analysis and estimated that you have only about a 12 percent chance of being genetically related to an ancestor 10 generations ago; by the time you get to a 14-generation ancestor, the probability is nearly zero.
A graph showing the probability that a person shares a chunk of genetic material with their ancestors of different generations. By Luke Jostins.
So computer models and analysis of real genomes show that everyone’s distant ancestors are the same people. Thanks to the dilution of specific people’s genetic contributions, we stop being identifiably descended from most of our ancestors relatively quickly. And the illustrious folks we might like to lay claim to have so many descendants that being descended from them is kind of a moot point.
Given that people—especially those in melting-pot countries with only a vague sense of where they came from—often search out their genealogies to find their special background, this information might be a bit disconcerting. Everyone’s genomes and families are not as enduringly specific as we tend to think.
But while genetics doesn’t reflect much of our imagined genealogical uniqueness, it’s shown that we’re more closely tied to our species as a whole than we might have realized. We’re all part of this enormous human fabric, full of fascinating tendencies and bizarre biochemistry. And research is revealing more and more about humanity as a whole and our incredibly beautiful, incredibly unlikely perch in the universe. That’s a tradition to be proud of.
Veronique Greenwood is a former staff writer at DISCOVER Magazine. Her work has appeared in Scientific American, Popular Science, and the sites of Time, The Atlantic, and The New Yorker. Follow her on Twitter here.