April 13, 2024

Does DNA say you are related to a Viking, a medieval German Jew, or an African enslaved in the 18th century? What a genetic match really means

In 2022, we reported the DNA sequences of 33 medieval people buried in a Jewish cemetery in Germany. Not long after we made the data publicly available, people began comparing their own DNA to that of 14th-century German Jews, finding many “matches.” These medieval individuals had fragments of DNA shared with thousands of people who uploaded their DNA sequences to an online database, much like you share fragments of DNA with your relatives.

But what kind of relationship with a medieval person does a shared DNA fragment imply?

It turns out that there is not much that helps in researching family roots.

We are population geneticists who work with ancient DNA. We understand how exciting it can be to find a genetic link to specific people who lived many generations ago. But these DNA matches aren’t the close ties you might think. See how it works.

Sequencing DNA from those who lived a long time ago

Ancient DNA is a new and rapidly growing field, with the 2022 Nobel Prize awarded to Svante Pääbo for his fundamental work.

Using samples taken from skull bones or teeth, aDNA researchers can sequence the DNA of people who lived 100,000 years ago. More than 10,000 ancient DNA sequences, or genomes, are currently available. These genomes, coming from all corners of the world, have dramatically revolutionized scientists’ understanding of human origins.

A new trend in ancient DNA is the sequencing of the genomes of “historical” individuals: those who lived during the last millennium.

Examples include genomes from Sweden, Norway, Denmark, Iceland, Poland, southeast Europe and London, Cambridge and Norwich in the United Kingdom. Outside of Europe, scientists have sequenced historical genomes from East Asia, the Swahili coast, South Africa, the Canary Islands, Lebanon, Machu Picchu, the Caribbean, and the San Francisco Bay area. Genomes from enslaved Africans from Delaware, Maryland, South Carolina and St. Helena are also available.

Some historical genomes belong to named individuals, including Ludwig van Beethoven, the family of the last Russian czar, medieval Hungarian royalty, Lakota Sioux leader Sitting Bull, and King Richard III of England.

The remains of King Richard III were reinterred in 2015.
Christopher Furlong via Getty Images News

How could you compare your own DNA to that of these historical people?

Several direct-to-consumer genetic testing companies, such as 23andMe, MyHeritage, or Ancestry, make reading your own genomic sequence simple and affordable. They compare your DNA with that of other customers. They identify relatives who share long, continuous stretches of identical DNA with you and report those matches – from closest to most distant.

After initial deliberation, 23andMe now allows customers to compare their genomes to historical people. Other genetic testing companies don’t yet do this, but passionate genealogists can take matters into their own hands. For example, the GEDmatch service allows users to upload their own DNA data along with published DNA sequences of any historical person. Once uploaded, GEDmatch will identify any user with whom you share genetic material.

two lines representing chromosomes with green, yellow and red bands along their length
A comparison of the DNA sequence of a chromosome between a 14th century German Jew and two living people who uploaded their DNA to GEDmatch. Each thin vertical bar represents a letter in the DNA sequence and is color-coded based on the match. A shared DNA fragment appears between living person 1 and the medieval person.

So what does a genetic match with a medieval person mean for your genealogy?

Surprisingly, very little.

Where genealogy and genetics diverge

The first thing to understand is how many ancestors you had in each past generation. A generation ago, you have two ancestors. Two generations ago, that doubles to four. Then, eight and 16. 30 generations ago, around the 12th century, we already had more than a billion ancestors.

It is clear that, at this point, your ancestors include the majority of people in your population who lived at that time, excluding a small fraction who left no long-term descendants. This includes, if of European origins, notable people such as Charlemagne or Edward I, but equally people from all medieval social classes. Your family tree reaches each of these ancestors through countless lines.

a web of red lines becoming increasingly dense across the top of the image, with generations marked 0 to 15 running vertically upward
The red dot in generation 0 represents a current person in a simulated population of 100,000 people. Each little red dot represents a person, and the red lines connect people to their parents. Ancestors reached across multiple lines in the family tree are marked in black circles. The number of lineages becomes so large so quickly that, over 15 generations, most ancestors are reached through multiple lines.
Graham Cooper

Mathematical research demonstrates the following surprising fact. In any population, the number of lines in your family tree that reach back to any specific medieval person is approximately the same between you and everyone else who belongs to the same population as you. In other words, all people alive today are equally related, genealogically, to all medieval people in that population.

The next step is to understand how many ancestors you actually inherit your DNA from. Surprisingly, again, very few.

Despite your millions or more medieval ancestors, you inherit DNA from only a small fraction of them. So, sorry, you probably didn’t inherit any DNA from Charlemagne or Edward I. For example, you only have about 2,000 genetic ancestors from the 12th century. In other words, your DNA sequence is a mosaic of approximately 2,000 “fragments,” each dating back to a single person from the 12th century.

Who are the medieval people whose DNA you inherited? Each fragment of your DNA descends from a random line in your family tree – your mother’s father, your mother’s father, and so on – in each generation in the past, randomly selecting one of the two parents. The more lines in your family tree go back to a particular medieval person, the more likely you are to inherit that person’s DNA.

For someone alive today, the number of genealogical ancestors doubles with each generation. But each fragment of DNA (colored bars) is inherited through a random, zigzag path through the family tree, meaning that DNA is inherited from only a small fraction of a person’s ancestors.
Shai CarmiCC BY-ND

But remember, the number of family lines that reach a medieval person is approximately the same for all current individuals in a given population. Therefore, all individuals inherit the DNA of any medieval person with very similar probabilities. So sharing genetic material with a given medieval person is just a matter of luck, and everyone is playing the same game.

Here’s an analogy. Going to a casino and rolling a roulette ball for 24 doesn’t mean that 24 is your special number. Anyone else could have gotten 24 as well. Likewise, sharing a fragment of DNA with any of the millions of medieval genealogical ancestors does not signify any special relationship – beyond sharing a fragment of DNA.

And if you don’t have a shared thread, you’re just out of luck. This does not mean that you are any less genealogically related to that medieval person than anyone else in your population who has a common thread.

As a side note, a “population” is not always well defined, but these arguments are generally valid for people with similar backgrounds.

How to interpret a historical DNA match

Let us consider medieval German Jews again. Some present-day Ashkenazi (European) Jews will share DNA with a particular medieval Jew. Some will share with others. Some will not share with anyone. It’s a lottery draw. And given that most Ashkenazi Jews today are genealogically related in a very similar way to medieval German Jews, seeing that the shared DNA fragment does not imply any unique genealogical relationship.

On the other hand, if you’re willing to consider more recent ancestors, DNA matches can be informative. The same mathematical models show that the number of family lines that reach a given historical person who lived around 200 or 300 years ago will be very different among current people. Therefore, a DNA match with an 18th century person implies a more specific genealogical relationship, which most other modern-day individuals do not have.

This pattern was demonstrated in a recent study by 23andMe. Comparing the genomes of enslaved Africans from 18th century Maryland to more than 9 million of its customers, 23andMe discovered more than 41,000 living relatives, including some near-direct descendants.

3D models of enslaved African Americans: one a teenager and the other a woman in her 30s
Facial reconstructions based on skeletal remains of enslaved African Americans who worked at the Catoctin Furnace in Maryland, where scientists also sequenced ancient DNA.
Katherine Frey/The Washington Post via Getty Images

How long does a DNA match still have genealogical significance? For example, are DNA matches informative in the period between the late Middle Ages and the 17th century? We don `t know yet. Future research will be needed to clarify this issue, as well as deviations from the simple model of a single, freely admixed population.

However, as scientists rapidly accumulate more and more historical genome sequences, keep in mind the peculiar behavior of human genealogies when interpreting a DNA match.

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