Just because there were different kinds of ancient humans doesn’t mean they were separate from each other. The Neanderthals (Homo neanderthalensis) coexisted with Homo sapiens for a long time. But there was interbreeding among the different species. A recent article in Nature talks about genetic analysis of different groups of proto-humans, showing that these groups interbred with each other. In fact, much of the diversity in our variations came about due to the interbreeding of H. sapiens to H. neanderthalensis and H. Denisovan. Depending on where you’re from, up to about 4% of your genome can come from one of these other species.
However, there is some controversy about these results. David Reich has better quality genome sequences that should be analyzed in light of this work.