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Carl Zimmer writes the Matter column for The New York Times and teaches science writing at Yale University. His latest book is She Has Her Mother’s Laugh: The Powers, Perversions, and Potential of Heredity, from which this essay is adapted.

A chart for a family record with ovals for portraits and spaces for textual data, circa 1888.

Library of Congress/Library of Congress.

For four seasons, Harvard professor Henry Louis Gates has been hosting the genealogy television show Finding Your Roots. In each episode, Mr. Gates presents celebrities with a book full of research into their ancestry, drawing from genealogical records and genetic tests. On a recent show, Mr. Gates introduced the actor Ted Danson to one of his 18th-century ancestors, Oliver Smith of Connecticut. Mr. Danson learned how Mr. Smith defended his seaside town against the British during the Revolutionary War.

Mr. Danson proudly patted the page that chronicled Mr. Smith’s heroics. “Well done, well done,” he said. “I never imagined a one-of-me’s out there in the revolution.”

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But when Mr. Danson turned to another page in his ancestry book, his mood swiftly shifted. Now he discovered that Oliver Smith bought a slave named Venture.

Mr. Danson fell silent. Mr. Gates tried cheering him up by pointing out that Mr. Smith later allowed Venture to purchase his own freedom.

“What’s it like to find that out?” Mr. Gates asked.

“Complicated,” Mr. Danson said.

Mr. Danson is far from the first person to experience such complicated feelings. For centuries, people have been exploring their genealogy, hoping to find impressive ancestors and fearing the discovery of disreputable ones. Medieval kings in Europe showed off their family trees as justification for their power. In Spain, noble families hire professional genealogists to prove they didn’t have a trace of Jewish ancestry – which was considered a devastating stain that could lose them a government job or a lucrative marriage.

By the 1700s, the appeal of genealogies had spread beyond aristocratic circles. Prosperous merchants and farmers tried to find a link to nobility in the branches of their family tree. Thomas Jefferson, for example, tried to track down the Jefferson family’s coat of arms back in England. “I have what I have been told were the family arms, but on what authority I know not,” Jefferson complained to a friend in 1771.

Another Founding Father, Benjamin Franklin, took matters into his own hands. He travelled to the English village of Ecton, where the Franklin family had lived for centuries. Determined to uncover his genealogy, he perused the parish registers, inspected the moss‐covered gravestones of his ancestors, and chatted with the rector’s wife about the Franklin family. The rector later sent him a hand‐drawn family tree stretching back to 1563.

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“I am the youngest Son of the youngest Son of the youngest Son of the youngest Son for five Generations,” Franklin wrote to a cousin, “whereby I find that had there originally been any Estate in the Family none could have stood a worse Chance of it.” Yet Franklin also came away from his research proud that he inherited the virtuous temperament of his ancestors, “for which double Blessing I desire to be ever thankful.”

After gaining their independence, Americans grew even more fascinated with genealogy. It served as a way to link themselves back to Europe. Old colonial families preserved their high status by flaunting their origins across the Atlantic. They put coats of arms on their silverware, their hearses and their gravestones.

Bourgeois families used genealogy to buy respectability of their own. They hired the United States' new class of professional genealogists to uncover connections to aristocracy and provide coats of arms, despite the fact the new-found heraldry often turned out to be fake.

A blank family record surrounded by vignettes showing childhood days, a district school, our wedding day, our last resting place, other scenes and flowers, circa 1889.

Library of Congress

Families of lesser means kept track of their genealogy in less ostentatious ways. They sewed needlepoint family trees and recorded names through the generations in their family Bibles. If they couldn’t prove they inherited noble blood, at the very least they could feel some pride in virtuous blood.

In the early 1800s, a Massachusetts woman named Electa Fidelia Jones investigated her roots, celebrating the Puritan blood that ran through her like a “magnetic wire,” vibrating two centuries later with a message for anyone who could appreciate it. She was thrilled to discover some of her fourth cousins through her research; the find was a better inheritance than any ancient fortune, she said.

But other kin did not please Jones. She uncovered a female relative and her husband from the 1750s who were “so near idiocy that it was said at the time of their marriage that laws ought to be enacted to prevent the marriage of those so unfit to sustain the relations which they assumed.” Among the children this unfit couple had, Jones complained, some were “so low in the scale of being that I do not wish to make their acquaintance so far as to ask after their name & age.”

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As Jones drew her family tree, she left those branches hidden. She preferred to spend her time dreaming of visiting her Puritan ancestors. “I love to go back in imagination to those old firesides,” she said.

It also became popular to search for famous individuals in one’s ancestry. In Virginia, many prominent whites claimed to be descended from Pocahontas. Their obsession with her would later cause legal headaches.

In 1924, Virginia legislators passed the Racial Integrity Act, barring interracial marriages. The law defined whites as people “whose blood is entirely white, having no known, demonstrable or ascertainable admixture of the blood of another race.”

That language would render many of Virginia’s most prominent families as no longer white. So the state legislature tacked on a so‐called “Pocahontas exception.” Even if Virginians were up to one‐16th Native American, the revised law held, they would still be considered white. People who were one‐16th black, on the other hand, were still black.

The obsession with famous people endures today. The Order of the Crown of Charlemagne is an American organization of people descended from the medieval king. To become a member, you have to prove your link to Charlemagne. But the order makes the task easier by providing a list of “Gateway Ancestors” you can link yourself to. On its website, charlemagne.org, the order declares that its objective is “to maintain and promote the traditions of chivalry and knighthood.”

But many people could not follow such a clear paper trail into the past. Adopted children were often left wondering who their biological parents were. Some Holocaust survivors were left so traumatized by their time in concentration camps that they told their children little about their ancestors. And slavery robbed millions of Africans of much of their pasts.

A chart exhibiting two farms, contrasting slavery with freedom in connection with a family record, circa 1880.

Library of Congress

It wasn’t just their connections back to Africa that disappeared. As families were broken up to be sold off, those connections were eroded all over again. Slaves could not record genealogical information the way white Americans could. Instead of leaving wills, slaves were listed in them, alongside oxen and pewter. The erasure of African genealogy reached down all the way to their names. In 1679, a New York mariner named John Leggett bequeathed to his son “a negro boy … known by the name of ‘You-Boy.’” The abolitionist Frederick Douglass, born in 1818, knew nothing about his ancestry beyond his maternal grandparents, with whom he lived for his first seven years. “Genealogical trees do not flourish among slaves,” Douglass later wrote.

For generations, slavery’s erasure left many African-Americans with a longing to know their ancestors. In 1976, Alex Haley reconstructed his own genealogy in Roots. He inspired a generation of African-American genealogists to dig through old historical records for their own clues to their ancestry. African-American genealogists were among the first to take advantage of genetic testing in the early 2000s, when its price dropped low enough to make it a consumer product. They began discovering genetic links between African-Americans and their distant cousins in West Africa.

In the years since, genetic testing and online genealogy websites has proven to be an explosive combination. Ancestry.com has signed up more than 10 million customers, while 23andMe has more than five million. While today’s companies are using cutting-edge software to analyze DNA, they are playing on the old desires that drive us back deep into our ancestry. Some companies promise to reveal your inner Viking. “Who knew a kid from Queens is descended from royalty?” asks an ad for Ancestry.com.

As we flock to these services, we rarely ask what exactly we’re seeking. Our emotions run deep, and sometimes don’t make a lot of sense. Before appearing on Finding Your Roots, Mr. Danson had no idea who Oliver Smith was. But suddenly he felt a surge of pride, followed immediately by one of shame. Are we supposed to believe that virtue and vice could secretly make their way down through eight generations, from Oliver to Ted?

Before the 20th century, genealogists had no idea what descendants biologically inherited from their ancestors. They only spoke vaguely of blood. The discovery of DNA seemed to solve that mystery: We had a bond with the past made of genes. But heredity actually works in deceptive ways.

We inherit one copy of each chromosome from our mothers and the other copy from our fathers. They inherited their chromosomes in turn from our grandparents. Which copy of a given chromosome gets passed down from a parent to a child is mainly a matter of chance. That flip of the genetic coins has a remarkable consequence: As you work your way back through your ancestry, you’ll encounter an ancestor from whom you inherited no DNA at all. And the further back you go, the more of those ancestors you’ll meet. Graham Coop, a geneticist at the University of California, Davis, has estimated that if you look at your ancestors from 10 generations back, almost half of them will have no genetic connection to you.

Discovering that we are descended from someone famous makes us feel special, but that feeling is another genealogical illusion, based on a faulty picture we have of family trees. We picture them as forever branching in two – two parents, four grandparents, eight great-grandparents and so on. If that were forever true, your family tree over the past few thousand years would contain more people than have ever lived.

God bless our family, family record, Chicago, 19th century.

Library of Congress

In fact, our family trees don’t branch outward forever. Our ancestors are related to each other, either as close cousins or distant ones. The diverging branches of our ancestry eventually fold back in on themselves.

The merged geometry of our family trees has its own astonishing consequence: If you could pick two strangers and follow their ancestry back through time, it wouldn’t take all that long to find a common ancestor. In Europe, some of the people who lived a thousand years ago are the ancestor of every living European. Charlemagne is likely one of those common ancestors. If you go back a few thousand more years, you’d find people who are ancestors of all living people on Earth.

While hundreds of millions of Europeans may descend from Charlemagne, most of them likely don’t carry even a wisp of his DNA. But scientists can estimate the ancestry of our genes by comparing our DNA to that of people from different parts of the world. When I got my own DNA analyzed, for example, I learned that it’s 43-per-cent Ashkenazi Jewish, 25 per cent from northwestern Europe, 23 per cent from south-central Europe (Italy, in other words), 6 per cent from southwestern Europe and 2.2 per cent northern Slavic.

But it’s a mistake to look at these figures as a clue to our essential nature. Learning your ancestry percentages is not like learning the human body is 61-per-cent oxygen, 23-per-cent carbon. My father is Jewish, but that only means he descends from a community of Ashkenazi Jews that emerged in Europe in the Middle Ages. They descended in turn from other communities scattered across Europe and the Near East. My mother has traced some of her ancestors to England, but the English are not pure, either: Their ancestry traces back to waves of genetically distinct people who moved across Europe in the Stone Age and the Bronze Age from distant places such as Russia and Turkey.

None of this is to say that we can’t learn important lessons from genealogy and genetics. But the most important lesson is that we don’t inherit our essence from some particular ancestors. We inherit all of history, our lives shaped by the broad trends of the societies in which we and our ancestors lived. In the words of Ted Danson, it’s complicated.

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