A modern human who lived in what is now Romania between 37,000 and 42,000 years ago had at least one Neanderthal ancestor as little as four generations back—which is to say, a great-great-grandparent.

Scientists have known for at least half a decade that living humans bear traces of Neanderthal blood—or more specifically, Neanderthal DNA . Just when and where our ancestors bred with their now-extinct cousins , however, has been tricky to pin down until now. A new study published Monday in the journal Nature has the highest percentage of Neanderthal DNA of any modern human ever studied.

“I could hardly believe that we were lucky enough to hit upon an individual like this,” says study co-author Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig.

The specimen, known as Oase 1, consists only of a male jawbone, and from the moment it was discovered in 2002 its shape suggested that it might belong to a hybrid between Homo sapiens and Neanderthal. Those claims have remained controversial, but the new analysis lays the controversy to rest. “It’s really stunning,” says Oxford’s Tom Higham , an expert on the Neanderthal-human transition who was not involved in this research.

Part of what stuns Higham is the genomic artistry it took to tease useful genetic information out of the tiny DNA samples lead author Qiaomei Fu of Harvard Medical School and her team were able to extract from the jawbone. “We tried to do this in 2009 and failed,” says Pääbo. His lab has been working since then to improve their techniques, with resounding success.

The genome they sequenced from the samples was incomplete, but it was enough for the scientists to conclude that between 6% and 9% of Oase 1’s genome is Neanderthal in origin. People living today have 4% at most.

That difference is more significant than it might seem. “We found seven huge pieces of chromosomes that seemed to be purely of Neanderthal origin,” says Pääbo. That means pieces had to come from a relatively recent ancestor, since they hadn’t yet been broken up by the reshuffling that happens in each generation as parents' chromosomes combine, he explains.

View Images This jawbone from a 40,000-year-old modern human shows some Neanderthal features—and DNA now confirms he had a Neanderthal ancestor as few as four generations back. Photograph by Svante Pääbo, Max Planck Institute for Evolutionary Anthropology

The non-Neanderthal genome sequences, meanwhile, show that Oase 1 isn’t related to humans living today. His genealogical line died out at some point.

This analysis represents a biotechnological tour de force, but it also puts paleoanthropologists a step closer to fully answering to what Higham calls the $64,000 question: What happened to wipe out the Neanderthals, and when? Genomic analysis of a 45,000-year-old human thighbone last year suggested that humans and Neanderthals interbred in what is now Siberia sometime between 50,000 and 60,000 years ago—an extremely imprecise number, and a very broad conclusion.

“The great breakthrough here,” Higham says, “is the ability to say ‘this specific person had a Neanderthal great-great-grandfather.’ That puts a human timescale on it.” If scientists can figure out when interbreeding took place in different parts of Europe and the Middle East, they’ll be able to say in detail just how rapidly humans spread across these regions, how long they were in contact with Neanderthals—and maybe tell us at last why our nearest relatives vanished.