is that Neanderthals and Homo Sapiens were not separate species. I don't think homo sapiens eliminated them. I think they were assimilated.
I don't pretend to be an expert on this. But I read a lot, and there is a whole bunch of recent genetic research that seems to support this interpretation of events. Probably same is true with Denisovans. It is becoming clearer that human history involved many forays out from Africa, with returns and new forays discovering those left behind 20,000 years prior.
I am not a biologist, but I think speciation requires the inability to produce fertile offspring. Shitsu's and Great Danes and wolves can still produce fertile offspring, but in 250,000 years that may no longer be the case.