Neanderthals shared Europe with modern humans for as long as five millennia until they died out 40,000 years ago - "ample time" for cultural exchanges and interbreeding, researchers said on Wednesday.
While there is no evidence that the two groups lived closely together, they did co-exist for anything from 25 to 250 generations, depending on the region, according to a paper published in the journal Nature.
"The results reveal a significant overlap of 2,600-5,400 years," wrote the researchers, who used improved technology to date about 200 samples of bone, charcoal and shell from 40 archaeological sites from Russia to Spain.
This was "ample time for interaction and interbreeding," said a press statement.
In the latest attempt to date our cousins' final moments on Earth, the team found that Neanderthals disappeared at different times from different parts of Europe instead of being replaced by humans at one fell swoop.
The question of how, why and when Neanderthals became extinct, leaving humans to take over, has long fascinated scientists. Some have postulated a much more recent disappearance.
Anatomically-modern humans, having originated in Africa, reached Europe between 50,000 and 30,000 years ago, finding Neanderthals there. Their brief interaction resulted in non-African people today carrying about 1.5-2.1 per cent Neanderthal DNA.
According to the new, six-year study, Europe 45,000 years ago was still occupied mainly by Neanderthals with small pockets of humans in between.
This balanced shifted over the following 5,000 years, until the Neanderthals eventually disappeared, the paper said.
Rather than modern humans abruptly replacing their distant cousins, there appears to have been a progressive change "characterised by a biological and cultural mosaic that lasted for several thousand years," the researchers wrote.
They said theirs was the most accurate dating yet of this period in history.
Reliable radiocarbon dating is often rendered difficult by the degradation of carbon in bone or rock samples older than 25,000 years.
"Previous radiocarbon dates have often underestimated the age of samples from sites associated with Neanderthals because the organic matter was contaminated with modern particles," said study leader Thomas Higham of Oxford University.
"We used ultrafiltration methods, which purify the extracted collagen from bone, to avoid the risk of contamination.
"This means we can say with more confidence that we have finally resolved the timing of the disappearance of our close cousins, the Neanderthals."
The study did not reach a conclusion on whether there had been a single human-Neanderthal interbreeding event, or several over time.
"Of course the Neanderthals are not completely extinct, because some of their genes are in most of us today," said Higham.