It takes about 3 minutes to pro-create (hilarity ensues) but there seems to be massive doubt as to wether or not the early humans inter-bred with neanderthals who looked not very different from us. A little stockier, a little hairier, broader nose, ete, but by and large much like we, or we them, depending upon your cultural perspective.
New carbon dating has landed the two peoples living alongside each the other in Europe for about 5000 years. Roughly from 39000 BC to 44000 BC. We tend to get time all jumbled up in our brain pans and try to make ourselves think in geologic terms, which is okay if your talking about the weather or climate, but in talking about fauna or flora it isn't really okay at all. Alot can happen in 5000 years. ALOT. Some evidence exists that a species can go from building mud buildings to walking on the moon in 5000 years.
So the idea that we didn't interbreed with our cousins . . . hmmm that didn't come out right . . . is about as believable as there isn't microbial life on Mars.
Of course the real question is could the two species have successfully interbred?