Though upward of 12 million Africans were enslaved and shipped to the Americas between 1500 and 1850, tracing their roots back home has been famously difficult—with poor record-keeping and poorly-preserved DNA samples partly to blame.

Now researchers from Stanford University and the University of Copenhagen report in the Proceedings of the National Academy of Sciences that for the first time they've been able to use very old and very poor DNA samples, in this case extracted from the teeth roots of three skeletons on Saint Martin Island in the Caribbean, to identify with extreme precision where in Africa these slaves came from.

"We have long sought to use DNA to understand who they were, where they came from," says a researcher. The team learned "we cannot only get ancient DNA from tropical samples, we can reliably identify their ancestry." The remains were initially dated back to the late 1600s thanks to artifacts like pottery shards found at the site, reports CBS News.

And while the head researcher at first thought it "wasn't possible" to recover DNA from bones that had "essentially been lying on a Caribbean beach for 400 years or so," he and colleagues devised a "whole-genome capture" technique that allowed them to isolate and sequence the trace DNA of interest.

What they learned, reports Stanford University in a press release, is that one of the three skeletons belonged to a man from a Bantu-speaking group in northern Cameroon, while the other two share similarities with groups in what is today Nigeria and Ghana.

"This approach, along with identification of African names, are ways of getting around the thin documentary record," notes an outside researcher. (Researchers also sequenced the genome of the ice mummy Ötzi in 2013.)

This article originally appeared on Newser: How Slave Skeletons Were Finally Traced to Their Home

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