The 40,000-year-old Oase jawbone found in Romania belonged to an individual whose DNA was 6-9% Neanderthal.
Image credit: Svante Pääbo
The jawbone was found in 2002 in Oase Cave in southwestern Romania.
Writing in Nature, the international team that extracted and sequenced the ancient DNA says the findings show the Oase individual's ancestors included Neanderthals only four to six generations earlier.
This implies some of the earliest Homo sapiens to settle in Europe from Africa interbred with local Neanderthals.
Previously, it was thought early humans migrating out of Africa mixed with Neanderthals in the Middle East some 55,000 years ago, before spreading to the rest of the world, and did not mix again with Neanderthals, who died out around 40,000 years ago.
Co-first author Qiaomei Fu, a research fellow in genetics at Harvard Medical School, says:
"The data from the jawbone imply that humans mixed with Neanderthals not just in the Middle East but in Europe as well."
The Oase jawbone contains "exceptionally large" segments of Neanderthal DNA
Segments of DNA from an ancestor become shorter with each subsequent generation.
The researchers found that the Oase jawbone DNA contains some "exceptionally large" segments of Neanderthal origin. Study leader Svante Pääbo, of the Max Planck Institute in Leipzig, Germany, says they could hardly believe it when they saw the result:
"It is such a lucky and unexpected thing to get DNA from a person who was so closely related to a Neanderthal."
The researchers believe that the Oase individual is from a population that did not contribute much or at all to today's European ancestry.
They suggest he belonged to a group of modern humans who were among the first to reach Europe, mixed with local Neanderthals and were then displaced by later migrations.
The DNA of today's humans whose roots lie outside sub-Saharan Africa is between 1% and 3% Neanderthal.
Co-first author Mateja Hajdinjak, a graduate student at the Max Planck Institute, says:
"We hope that DNA from other human fossils that predate the extinction of Neanderthals will help reconstruct the interactions between Neanderthals and modern humans in even more detail."
The study is in line with findings from other fields. For example, radiocarbon dating of remains from archaeological sites across Europe, and stone tools recovered in Austria, suggest Neanderthals overlapped with modern humans for thousands of years, giving plenty of opportunity for them to interbreed.