The conventional teaching has been that modern humans exterminated them. A recently found trove of 13 teeth offers an alternative view:
There is now “considerable DNA evidence that interbreeding happened, both from fossils and modern genomes,” Chris Stringer, a co-author of the new study and an archaeologist at the Natural History Museum in London, explained in an email. Indeed, most people with recent ancestry from outside of Africa have around 2% Neanderthal DNA in their genomes. That said, archaeologists “still don’t know the exact circumstances, nor how much this was a blending absorption of the Neanderthals into expanding modern human populations,” added Stringer.
That communities of mixed ancestry existed during the Middle Paleolithic, some 48,000 years ago, is potential evidence that “extinction” is probably not the best word to describe the fate of Neanderthals. Instead, these hominins, and their DNA, were absorbed by the increasingly dominant newcomers to Europe: modern humans (Homo sapiens). George Dvorsky, “More Evidence That Neanderthals Were ‘Absorbed’ by Humans, Not Wiped Out” at Gizmodo
Some of us have always doubted the extermination hypothesis, not because we think our ancestors were too nice but because the absorption thesis fits the circumstances better.
Neanderthal man was just not very numerous to begin with. So when young Neanderthals came of age, they probably mainly found partners among the modern humans, arriving in considerable numbers. And it shows.
See also: Neanderthal Man: The long-lost relative turns up again, this time with documents
A deep and abiding need for Neanderthals to be stupid. Why?