DNA study links ancestral ties of living people with enslaved, free African Americans in US

An analysis of historical DNA tied thousands of living people to enslaved and free African Americans who laboured at Catoctin Furnace, an iron forge in Maryland, soon after the founding of the United States.

The analysis revealed how 27 individuals buried at Catoctin Furnace were related to each other, the genetic conditions they may have had, where in Africa and Europe they or their ancestors likely came from, and where in the US they have descendants and other genetic relatives living today.

The analysis provided a new method to reconstruct the life histories of people omitted from written records and identify their present-day relatives, the collaborative research from Harvard University, the Smithsonian Institution, the genetic testing company 23andMe, and the Catoctin Furnace Historical Society, US, said.

Also Read | New research claims Leonardo da Vinci was son of a slave

"This work demonstrates the power of DNA to provide information about ancestral origins," said David Reich, professor of genetics at Harvard and co-senior author of the study published in the journal Science.

The method analysed DNA segments shared by two or more people, because of being inherited from a recent common ancestor. For example, cousins may have identical DNA segments inherited from a shared grandparent.

"The more identical DNA segments a person shares, and the longer those segments are, the more likely they are to be a very close relative," the authors explained about the method developed by Reich and team and first author Eadaoin Harney, a population geneticist at 23andMe.

This method was used in comparing sites of entire genome sequences of the 27 historical individuals with those of 9 million living people (research participants) through a database.

The team thus found that 41,799 of the research participants were genetically related to one or more of the 27 sequenced individuals from Catoctin Furnace.

Of them, 2,975 participants were found to share more than 0.4 per cent of their genomes (not counting sex chromosomes) and thus, were deemed close relatives of the Catoctin individuals, ranging from five degrees of separation - the same amount as a great-great-great-grandchild - to about nine degrees, or the equivalent of first cousins six times removed.

The research does not yet reveal which, if any, of these close relatives descended from the Catoctin individuals and which were related in less direct ways.

Because the 23andMe database included geographic information, the study team was able to determine Maryland to have the highest concentration of close relatives, revealing that some relatives did not move far from Catoctin Furnace as the generations passed.

Until now, genetic insights into the identities and ancestries of early African Americans were limited to what could be gleaned from mitochondrial DNA, passed down through mothers; from Y-chromosome DNA in males; and from comparisons to DNA sequences in moderately sized public databases that often lacked sufficient numbers of Black participants.

Therefore, this study, the first to link up ancient DNA technology with a personal ancestry testing database and to use the new algorithm, was an advance.

"Recovering African American individuals' direct genetic connections to ancestors heretofore buried in the slave past is a giant leap forward both scientifically and genealogically, opening new possibilities for those passionate about the search for their own family roots," said study co-author Henry Louis Gates Jr. and host of the genealogy and genetics TV show Finding Your Roots.

"Our study combines for the first time two transformative developments in genomics in the last decade: ancient DNA technology, which makes it possible to efficiently sequence whole-genome data from human remains, and direct-to-consumer genetic databases that contain data from millions of people who have consented to participate in research," said Reich.

The majority of participants, though, appear to be distant relatives. The authors suspect many of their connections to Catoctin individuals stretch back to shared ancestors who lived in Africa or Europe during or before the transatlantic slave trade.

"Following on a 2020 study looking at the genetic impact of the transatlantic slave trade, we are privileged to contribute to an increased understanding of the impact of slavery on those in bondage, their descendants, and their unacknowledged contributions to American history," said co-senior author Joanna Mountain, formerly of 23andMe.