Around 45,000 years ago, the only humans who lived in Europe were Neanderthals.
Some 10,000 years later, if the sketchy fossil record is right, the Neanderthals had been replaced by our ancestors, and one of the greatest mysteries in anthropology was born.
Were the Neanderthals wiped out by Homo sapiens, the smart hominids who rose out of Africa and went on to conquer the planet?
Or did they peter out as a separate line, surviving as a genetic echo in DNA they bequeathed to us?
A new study, published in the journal Nature on Monday, delves into the controversial sex-with-Neanderthals theory.
The hanky-panky, it suggests, had deep roots, for it began soon after H. sapiens showed up in Europe.
Researchers extracted DNA from the 40,000-year-old jawbone, found in 2002 in the Pestera cu Oase cave system in southwestern Romania, which is claimed to come from the oldest modern human found in Europe.
"The sample is more closely related to Neanderthals than any other modern human we've ever looked at before," said David Reich of the Howard Hughes Medical Institute at Harvard Medical School, who co-led the probe.
"We estimate that six to nine percent of its genome is from Neanderthals. This is an unprecedented amount. Europeans and East Asians today have more like two percent."
The proportion is so big that in this individual's case, Neanderthal and sapiens got it on just 200 years earlier, or four to six generations previously, the scientists believe.
H. sapiens arrived in Europe from the Middle East, the first stopping point in our forerunners' emergence from Africa, according to a common theory.
Interbreeding between Neanderthals and H. sapiens in the Middle East occurred around 50,000-60,000 years ago, previous fossil research suggests.
But the new study says the mingling did certainly did not stop there.
"Mixture between modern humans and Neanderthals was not limited to the first ancestors of present-day people to leave Africa, or to people in the Near East," according to the paper.
"It occurred later as well, and probably in Europe."
Reich's co-leader in the investigation is Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Germany.
In 2010, Paabo made ground-breaking discoveries in the interbreeding story.
He published a sequence of the Neanderthal genome and compared it to the DNA of ethnic groups around the world today.
With the exception of people who have their roots in sub-Saharan Africa, all humans have a little of our enigmatic cousins in them, it said.
One intriguing find is that the Neanderthal traces found in the Oase remains are not found in direct European descendants today, it found.
"It may be that he was part of an early migration of modern humans to Europe that interacted closely with Neanderthals but eventually became extinct," said Reich.