“Where were my ancestors?” is a question that’s followed Andre Kearns for as long as he can remember. He still recalls his middle school geography assignment to color in a map of New York City with the different ethnic enclaves at the turn of the 20th century. When Kearns began to shade in a Black neighborhood, his teacher directed him to focus only on Irish, Greek, and Italian immigrants. Bewildered—yet curious—Kearns now recognizes this moment as an early call for him to dig deeper into his lineage.

Since then, Kearns has become one of the few Black people in the United States who has traced his family back to an early period of the trans-Atlantic slave trade. Using a combination of public records research and DNA testing, Kearns learned he is a maternal descendant of Emanuel and Joan Cumbo, who arrived enslaved on the shores of Virginia around 1628 from Angola, and whose son gained his freedom over 35 years later. Kearns attributes being able to climb that far back on his family tree to dogged persistence, but also, luck.

“Since they were free by name since the 1600s, they’re listed in tax records, land records, court records and the census,” says Kearns. But for the side of his family who were enslaved until emancipation, it’s a little bit of a different story: “I’m four generations in and I’m not even close to a ship manifest.” 

Kearns is not alone. Tracing the paths of enslaved ancestors is incredibly hard for descendants. Records are sparse, and while genetic testing can link an individual to broad geographic areas or new relatives, it can’t reveal where an enslaved ancestor was born and raised. 

(Read about why your family’s untold stories matter more than you think.)

A heavy metal called strontium could help shed light on these ancestral origins. Strontium isotopes are versions of the element with different chemical footprints; they work their way from the earth into the food chain, and from there, into human tissue like bones and teeth. By comparing strontium values found in a person's tooth to strontium values across a landscape, scientists can gauge where that person is from.

After more than a decade of development, anthropologist Vicky Delze and a team of colleagues have created a detailed map of the Earth's strontium signatures in sub-Saharan Africa.

These signatures hold clues to the origins of African-descended people who were enslaved around the world and could become a resource for genealogists like Kearns. "Individual histories are completely erased" by the slave trade, says Oelze, but strontium signatures can help us see "the invisible."

Genealogy research presents unique challenges for Black descendants of enslaved people. Surviving records from the trans-Atlantic slave trade may show the names of the ships and captains, but captives are documented by a simple count. Similarly, until 1870, people who were enslaved in the U.S. could only be found on the census as property, identified solely by age, sex, and color. 

Many descendants have turned to consumer genetic testing, through companies like AncestryDNA and 23andMe, but researchers emphasize the need for public education on the tests’ constraints.  

“The accuracy of these DNA tests is entirely dependent on the reference data set,” says Aja Lans, an anthropologist at Johns Hopkins University. Genetic testing databases lack enough samples from Africa to make detailed matches, and they also reflect where people are living now, not necessarily where their families lived 400 years ago.

Scholars worry that, without interpretive guidance, consumers might unwittingly conflate genetic and genealogical ancestors. “You could be genetically related to someone 2,000 years ago through a distant Y chromosome or distant mitochondria, but that doesn’t mean doesn’t mean they’re in your family,” says Raquel Fleskes, an ancient DNA researcher at Dartmouth University. Researchers like Fleskes use special techniques to analyze DNA from remains found at archaeological sites, but even they can run into similar limits with reference genomes.  

There’s also concern that when companies market their tests in racialized terms, it supports the misconception that racial or ethnic heritage can be determined by DNA. “To this day, humans are 99.9 percent identical genetically,” says Lans. “Anyone, anywhere on Earth can have the same gene as someone else. That’s the meaning of ‘race is a social construct.’”

Using DNA as part of a larger toolbox could help with these complexities and offer a different way into the question: What were the origins of the over 12 million people who were abducted from Africa during the largest forced migration in history?

Isotopes are variations of the same element that have the same number of protons but a different number of neutrons, giving them each a distinctive signature. 

Strontium exists in bedrock and leaves its signatures in the water and plants that grow in an area. Creatures that drink that water and eat those plants store those signatures in their teeth. Scientists can take a small sample of a tooth–or any organic material–into the lab and read its isotope values. A comparison of those values to a reference map can reveal where the test material originated. 

Strontium signatures are so specific that researchers have used them for a host of location-sensitive studies, including determining the origin of prehistoric populations, tracing bird migrations, and tracking illegal ivory trades. 

“While the concept of isotope testing isn’t new, it still isn’t that widely used,” says archaeologist Hannes Schroeder, who used the technique in Barbados. Schroeder’s study analyzed the bone, dentinal collagen, and tooth enamel of 25 people who were buried at Newton Plantation, and found that most were born on the island. But seven of the individuals tested yielded strontium and oxygen ratios that suggested they were captured in Africa. Samples from their teeth differed from their bones, further supporting the idea that they grew up in one place and spent the remainder of their lives in another.

To create the new map of strontium values, scientists added data from nearly 800 environmental samples sourced from 24 countries across sub-Saharan Africa to previously available data for the region. The samples range from termite soil to snail shells to dog teeth, and after analyzing them, the research team used machine learning to generate the predictive model that forms the basis of the map. 

So far, researchers have used the map to estimate where enslaved people buried in Charleston, South Carolina, and Brazil came from with higher resolution than ever before. “The power of this new map is that scientists are able to take the genomic data and strontium data and re-analyze it to get more specific about where these people were born and brought from,” says Fleskes, who had previously studied ancient DNA samples from the Charleston site and found that individuals had West and West-Central African ancestry.  

When Oelze’s team cross-referenced strontium data from enslaved people buried in Charleston with the new map, they found that two individuals likely came from Eastern Guinea or a sliver of Cote d’Ivoire and southern Ghana. The study also re-analyzed data from the teeth of enslaved people from Pretos Novos cemetery in Rio de Janeiro, Brazil. Using the map, scientists projected four individuals to be from different parts of Angola, which points toward the vastness of the intra-continental slave trading network and slavers’ practice of severing family and community ties.

Like ancient DNA analysis, strontium testing requires access to ancestral remains to reveal more about the trans-Atlantic slave trade. Researchers need small samples—typically less than 10 milligrams—from the teeth of people who were alive during that time. 

(Read how Black history is being told through a new lens in Charleston.)

“Archaeology can tell us a lot, especially when we don’t have information,” says Lans, but researchers are also mindful of anthropology’s colonial origins and the well-documented practices of Europeans and their descendants looting artifacts and treating Black and Indigenous bodies like property. “I strongly oppose taking ancestors out of the ground unless there’s no other option,” says Lans.

Archaeologists have called for lawmakers to pass the African American Graves Protection and Repatriation Act (AAGPRA), which would require universities and museums to account for any remains of Black Americans in their possession, halt research, and consult descendant communities about next steps.

Deference to the people who share kinship ties with the ancestors is part of “trying to heal global trauma in Black descendant communities,” says Alicia Odewale, an archaeologist at the University of Tulsa. It could also be a safeguard against scientists continuing the troubled legacy of conducting research on ancestors without the knowledge or consent of the people who are connected–by embodied experience, by slavery and by place–to those ancestors.

“There’s a growing list of burial grounds of African-descended people that we’re trying to learn more about,” says Andre Kearns. More information by way of strontium testing “opens up a possibility” to create a fuller picture of their lives—whether they arrived on sanctioned or outlawed ships, grew up in the interior of Africa or the coast, or if their families spent generations in the Caribbean. It’s all meaningful knowledge, embedded in a tooth, and intimately tied to stewardship and permission. 

“For me to know who I am, I need to know who came before me,” says Kearns. Learning more about the ancestors who “built a life and a family that extends to me and that I’m proud to be part of gives me an important perspective on my life, and inspiration. It reminds me that I stand on their shoulders,” he says. 

(Learn more about how the search for lost slave ships led this diver on an extraordinary journey.)

While the map is a promising tool, Oelze says it’s “not a magic bullet.” Gathering samples from the world’s second largest continent is a labor-intensive, yearslong effort. Also, strontium values can vary greatly within a single region, and cannot, in isolation, be relied on to pinpoint an historic person’s birthplace. 

Consulted alone, the map provides estimates based on probabilities. To increase those probabilities, scientists could collect more data per location and incorporate other elements, like oxygen, nitrogen, and carbon to develop a “multi-isotope” approach, says Alex Bentley, an anthropologist at the University of Tennessee, Knoxville. 

While the map’s most immediate use is for researchers to elucidate the realities of the trans-Atlantic slave trade, its potential also lies in how it can be used alongside other lines of evidence—–oral histories, genetics, historical records—–to reconstruct the personal stories of enslaved people with more texture, more precision, and more color.