Neanderthals shared Europe with early modern humans for up to 5400 years before dying out, according to a new study.
The considerable overlap would have allowed for thousands of years of interbreeding, helping to explain why the genome of non-African people alive today is about two per cent Neanderthal DNA.
"They were living next door to each other for a long time, and it's possible that they interacted," co-author Dr Rachel Wood of Australian National University said.
"Just before 40,000 years ago, there was a complicated mosaic of different groups of Neanderthals and modern humans.
"It's much more complicated than we previously thought."
Dr Wood and her colleagues used radiocarbon dating techniques to re-examine about 200 samples of bones and tools from 40 archaeological sites in Europe.
Reporting in the journal Nature, they estimate that Neanderthals died out between 39,000 and 41,000 years ago, coexisting with humans for 2600 to 5400 years.
The results help clear up an argument that has raged for about five decades, during which scientists have come up with radically different estimates.
Some have said Neanderthals died out before humans arrived; others that the two species coexisted for up to 20,000 years.
"Different people would look at the same data set but regard different dates as reliable," Dr Wood said.
"The data wasn't good enough."
The main problem was that old bones and charcoal have often been contaminated with more recent forms of the radioactive carbon isotopes scientists analyse to determine age.
Even a one per cent contamination could sway the measured age of a 50,000-year-old artefact by more than 10,000 years, Dr Wood said.
She said her team had selected artefacts very carefully, using state-of-the-art cleaning techniques to eliminate contamination.
It remains unclear why Neanderthals, a species of human characterised by their short limbs, barrel chests and big noses, died out.
But Dr Wood said it might have been due to environmental change combined with competition and diseases brought by our early ancestors.
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