My understanding was that, based on DNA analysis of Neanderthal remains, humans couldn't successfully interbreed with Neanderthals because of a chromosome misma

My understanding was that, based on DNA analysis of Neanderthal remains, humans couldn't successfully interbreed with Neanderthals because of a chromosome mismatch, in the same way a Mule can't get fancy with another mule. So while a Neanderthal/Homo Sapien baby might exist, it was essentially sterile. But I'm happy to be told otherwise, that's just my knowledge as at the last update on the subject, which admittedly was a while ago. Personally I think the 'we all came from Africa' story is a little too neat. I think parallell evolution of Homo Sapiens in separate continents leading to eventual interbreeding is a distinct possibility.