So, where are they?
Here's one being interviewed!
They are us.
Interbreeding of Neanderthals and modern humans without transmission of Neanderthal mitochondrial DNA: effect of mating preferences
If male Neanderthals mated with female modern humans the offspring would have inherited mitochondrial DNA only from the female modern humans.
This mating was more likely than the mating of male modern humans with female Neanderthals because of the preference of human males for gracile females. It was also more likely because of the preference of human females for robust males.
The mating of a female Neanderthal with a modern human male was unlikely because she would find him gracile and he would find her robust.
If the mating preferences of Neanderthals were similar to those of modern humans, which are well documented, the sequencing of mitochondrial DNA from Neanderthal fossil bone will not indicate whether Neanderthals and modern humans did or did not interbreed. The work of M. Krings et al, for example, shows only that female Neanderthals did not mate very often with male modern humans.
References: Krings, M. et al. Cell 90, 19-30 (1997).
Ward, R. & Stringer, C. Nature 388, 225-226 (1997).
Published 21 January 2000. © Andrew Gyles