Scientists have known for some time that all modern humans who live outside Africa have a small amount of Neanderthal DNA, but they haven't been able to flesh out many of the details regarding when, where and how often our early ancestors mated with members of that now-extinct branch of the human family.
A new discovery, announced Monday in the journal Nature, adds a piece to the puzzle. DNA testing of an ancient jawbone has confirmed that a man who lived in Romania about 40,000 years ago descended from a Neanderthal ancestor just four to six generations – less than 200 years – before him.
"To our amazement, this guy had three or four times more Neanderthal DNA than any modern human we had ever looked at," said Svante Paabo, director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and co-senior author of the Nature study. "This is the first time we can say it's dramatically more, and close in the family tree."
The DNA in question came from a modern human jawbone with some Neanderthal characteristics that was discovered in a Romanian cave in 2002. Paabo's lab had previously tried to sequence its DNA, with no success. But now, the researcher said, the team is armed with new procedures that help recover more DNA from the sample and sort out the bits that contain the most relevant information.
"With those tricks we can use DNA that we couldn't use at all in 2009," Paabo said.
Their analysis of the sample revealed that 6% to 9% of the man's DNA came from Neanderthals. The scientists were able to figure out that one of those ancestors had been very recent by examining the size of the chunks of Neanderthal DNA in the man's genome.
When the body produces an egg or a sperm, chromosomes – the structures that carry the genes we pass down from generation to generation – split up and reorganize themselves in a process called recombination. The fewer the generations that have passed, the fewer such splits and reorganizations will have occurred, and the longer the stretches of DNA contributed from an ancestor will be.
The Neanderthal DNA in the jawbone was "distributed in big, big pieces on the chromosome," Paabo said. "This indicates there has to be a Neanderthal relative very close in time."
Scientists believe that the Neanderthal genes in people living today were contributed early on, when modern humans ancestral to all non-Africans mated with Neanderthals in the Middle East.
The new discovery suggests that early Europeans also mated with Neanderthals later on in human history, and probably on their own home turf. The DNA also revealed that the jawbone's owner was part of an ancient population that is not related to later European groups (including people living today).
The study was the latest of several released in recent weeks that take advantage of improved analytical techniques to probe ancient DNA samples – including a pair of studies detailing the population history of Bronze Age Europe; another report determining that the famous Kennewick man from Washington state is related to Native Americans living today; and a discussion of the evolution of dogs generated by studying DNA from a Siberian wolf that lived 35,000 years ago.