A New Project Uses Isotopes to Pinpoint the Birthplaces of the Enslaved

While digging a trench for the renovation of a local performing arts center in 2013, construction workers in Charleston, South Carolina, made a startling discovery: human bones. The crew called the police and coroner’s office, unaware they had stumbled upon a late-18th-century burial ground. The site’s location—along with the coins, ceramics and beads that had been buried with the bodies—suggested that the people had been enslaved Africans.

For years after the initial discovery, scientists and community leaders worked to identify the people who had occupied the graves. The city enlisted Ade Ofunniyin, known as “Dr. O,” to helm these efforts. A cultural anthropologist, nonprofit director and grandson of a famed Charleston blacksmith, Ofunniyin ensured that the 36 individuals—who would come to be known as “the Ancestors”—had a modern-day champion. He sought to learn the Ancestors’ histories and honor their identities. He successfully lobbied the city to reinter the Ancestors. And in response to a high school student’s question—did the Ancestors have names?—Ofunniyin presided over a traditional Yoruba naming ceremony in 2019.

After Ofunniyin died unexpectedly in 2020, a group called the Anson Street African Burial Ground project took up the mantle. “He put together a beautiful team of people,” says La’Sheia Oubré, the group’s community education lead, “and he left us to continue the work.” That work included seeking permission from Charleston’s African American community to extract DNA samples from the remains of the Ancestors. With the community’s consent, the samples were analyzed, and the results shed some light on where the Ancestors had originated. Many hailed from West-Central Africa, West Africa or sub-Saharan Africa. One Ancestor was found to have mixed West African and Native American heritage.

But DNA can only tell you so much. “I was just getting so sick of the interpretation being, ‘We have an African individual, and our interpretation is this person is from sub-Saharan Africa,’” says Vicky Oelze, an anthropologist at the University of California, Santa Cruz who studies the archaeology of the trans-Atlantic slave trade. Being able to pinpoint where exactly a person is from, she says, “has implications for their culture, their language, their beliefs, their practices—which contributed to so much of the culture of the Americas and the African diaspora at large.”

To trace those origins with more precision, Oelze uses a tool called isotope mapping. Just as geographic regions vary in types of rocks and trees, they also have different proportions of elements. Oelze and her team focused on the isotopes strontium 86 and 87, which show up in the ancient bedrock of Angola. (A quick refresher from chemistry class: Isotopes are variations of an element that have the same number of protons—in the case of strontium, that’s 38—but different numbers of neutrons.)

The researchers created a map—or isoscape—of Angola showing where different concentrations of the strontium isotopes are found in the earth. Instead of sampling rocks, Oelze and her team analyzed the strontium in modern plants to get a sense of how the isotopes show up in living things. People growing up in Angola in the 18th century would have absorbed the area’s strontium isotopes in their bodies. The researchers looked at strontium isotope concentrations in teeth recovered from the Charleston burial ground as well as other African burial grounds in Rio de Janeiro; Campeche, Mexico; and Philipsburg, Sint Maarten. In a study published in June 2023, they reported that a handful of candidates from each site could have spent their formative years in Angola, and the researchers pinpointed the specific regions where they would have likely originated.

Oelze’s ambitions don’t end there. Her goal is to create maps using a variety of isotopes. For instance, levels of oxygen isotopes can tell scientists whether a person lived in a warm, dry region or a moist, tropical one. Sulfur isotopes can indicate a person’s proximity to the ocean. Isotopes of carbon and nitrogen, meanwhile, differ according to diet and can be particularly helpful in West-Central Africa, where different cultures ate different staple crops. Future isoscapes will reveal increasingly specific details of an enslaved person’s early life.

Over time, these findings will help form a more complete picture of the entire trans-Atlantic slave trade. Historians have long debated how deeply the trade penetrated into the interior of the continent. Soon researchers will be able to pair isotope data with other information like shipping records to trace a person’s likely path from the African interior to an Atlantic port, and then to destinations throughout the Americas. All of that will add up to a remarkably precise—and personal—view of who enslaved individuals really were.

Meanwhile, the Anson Street African Burial Ground project continues to work toward Ofunniyin’s vision. At the site where the Ancestors were reinterred, an artist will create bronze casts of the hands of 36 Charleston residents who roughly match the Ancestors’ heritage, sexes and ages at death. The hands will sit atop a fountain whose basin incorporates soil from other nearby African American burial grounds.

“Those that are forgotten, those that nobody knows about, those that no one can even find—this monument is going to be a symbol of everybody being in one place,” Oubré says. “With this data and information that’s now accessible, it’s like you’re dreaming a true dream.”